Edition 3 Isolated current and voltage transducers

Edition 3 Isolated current and voltage transducers
3rd E
Isolated current
and voltage transducers
Characteristics - Applications - Calculations
LEM provides multidisciplinary know-how
and solutions for power electronic
measurement in key economic segments
such as Energy, Transportation, Industry,
Automotive, R&D, Engineering, Medical,
Environmental, and Test Facilities.
The top priority of LEM is a commitment
to the quality of our products and
This, together with a combination of
design, manufacturing, test, and
customer service competencies, ensures
the long-term success and satisfaction of
our customers.
Isolated Current and Voltage Transducers
Characteristics - Applications - Calculations
Optimal solutions with 6 different
technologies of LEM transducers
Determining parameters for transducer
Which parameters need to be considered ?
Understanding the LEM documentation
Additional selection criteria
Current transducers – selection check list
Voltage transducers – selection check list
Power transducers – selection check list
Type of output
Hall effect technologies
Open loop Hall effect current transducers
Construction and principle of operation
Advantages and limitations
Nominal and extreme currents
Output signals
Measurement accuracy
Magnetic offset considerations
Demagnetization to eliminate magnetic offset
Bandwidth and core losses
Core losses
Core loss rule-of-thumb
A core loss example
Addressing core losses
Response time, delay time, and di/dt behavior
Typical applications
Calculation of the measurement accuracy
Closed loop Hall effect current transducers
Construction and principle of operation
Advantages and limitations
Nominal and extreme currents
Output signal – Measurement resistance
Measurement accuracy
Observations regarding magnetic offset
Bandwidth and core losses
Response time and di/dt behavior
Typical applications
Transducer parameter examples
Calculation of the measurement accuracy
Unipolar power supply
Eta Technology Hall effect current transducers
Construction and principle of operation
Advantages and limitations
Nominal and extreme currents
Output signal
Measurement accuracy
Dynamic behavior
Typical applications
Closed loop Hall effect voltage transducers
Construction and principle of operation
Voltage transducer with internal resistor
Voltage transducer without internal resistor
Transducer Output
Typical applications
Other Hall effect voltage transducers
Fluxgate technologies
Working principle of Fluxgate technologies
„Standard“ Fluxgate – working principle
Sensing head – a current response to a voltage step
Detecting the sensing head inductance variation
Current transformer effects
Existing types of Fluxgate transduers
General performance of Fluxgate technologies
„C-type“ Fluxgate transducers
Construction and principle of operation
„CT-type“ current transducers
„CD-type“ differential current transducers
„CV-type“ voltage transducers
„C-type“ transducers - typical applications
Calculation of the measurement accuracy &
noise rejection
„IT-type“ Fluxgate transducers
Construction and principle of operation
„IT-type“ transducers – advantages & limitations
„IT-type“ transducers - typical applications
Calculation of the measurement accuracy
Air-core technologies
Basic working principle and sensitivity
LEM~flex - the flexible AC current transducer
Construction and principle of operation
Characteristics and features
Typical applications
Calculation of the measurement accuracy
PRiMETM Transducers
Construction and Principle of Operation
Characteristics and Features
Advantages and Limitations of PRiME Technology
Other types of voltage transducer
technolo gies
OptiLEM voltage transducers
„AV type“ voltage transducers
Construction and Principle of Operation
Characteristics and Features
Typical applications
Calculations & Properties
Current probes
Power supply polarity inversion
Capacitive dv/dt noise
Magnetic disturbances
Typical misapplication of a transducer parameter
LEM ASIC based transducers
LEM – the leader in electrical parameter
Glossary A-Z
Optimal Solutions with 6 Different Technologies of LEM Transducers
• Fluxgate transducers, which include „IT-types“, „C-types“,
„standard types“, and „low frequency types“
Optimal solutions with 6 different technologies
of LEM transducers
• Air-core transducers, which include LEM~flex
(Rogowski) and PRiME™ transducers
Since 1972 LEM has been able to respond to customer
demands, creating a wide range of galvanically isolated
current and voltage transducers that have become
standards in the measurement field. Our customers are
offered a wide range of LEM transducers to meet their
needs, based on the application requirements and transducer performance. This document is intended to provide
our customers with a technical background to allow them to
determine the best transducer to meet their needs.
• Various voltage transducers, which include Hall effect,
Fluxgate, the AV and OptiLEM™ products
The key attributes of these technologies are summarized in
Table 1.
Finally, while most applications will find their best solution
with a standard transducer selected from one of these
technologies, LEM is fully capable of developing customized
solutions to meet customer specific requirements. A review
of this application note will allow the customer to better
understand the critical aspects of transducer design,
determine the closest standard device for the application,
and better communicate the specific requirements of a
custom device to LEM.
We will begin by splitting the LEM transducer portfolio into
four main categories covering six different technologies,
discussed in detail later in this document:
• Hall effect transducers, which include closed loop, open
loop, and Eta™ transducers
Table 1: Overview of the various LEM transducer technologies with their corresponding main characteristics
Hall effect technologies ①
Fluxgate technologies ①
Air-core technologies ②
‘Eta’ types
types (CTS)
0 to 15 kA
25 to 150 A
0 to 15 kA
0 to 600 A
0 to 150 A
0 to 400 A
0 to 500 A
0 to 10 kA
0 to 10 kA
0 to 100 kHz
0 to 500 kHz
0 to 100 Hz
0 to 200 kHz
0 to 200 kHz 0 to 100 kHz 0 to 25 kHz
10 Hz to 100 kHz
Typ. response td
time (@ 90 %)
< 1 µs
< 1 µs
< 3-7 µs
< 1 µs
< 0.4 µs
< 5 ms
< 1 µs
10 to 50 µs
2 to 50 µs
Typ. accuracy X
at 25 °C, in %
of IPN
±0.5 %
±1.5 % (DC)
±0.5 % (AC)
±1.5 %
±0.0002 %
±0.1 %
±0.1 %
±0.2 %
±1.0 % ③
±0.5 % ③
±0.1 %
±0.5 % (DC)
±0.1 % (AC)
±0.5 %
±0.0001 %
±0.05 %
±0.1 %
±0.1 %
±0.2 %
±0.2 %
Low power Best resolution High resolution Resolution
Low power Small size Best accuracy High accuracy
Low voltage Low cost
Best speed Low frequency
Low cost
Light weight Light weight
AC only
AC only
① The Hall effect and Fluxgate attributes shown are based on a „solid core“ (non-opening) implementation. Split core (opening) versions are also
available with reduced specifications.
② LEM~flex is a „split core“ device, implying that it can be opened to place around the conductor(s) to be measured. PRiMETM can also be
designed for split-core operation which does not affect the performances.
③ The accuracy of air-core technologies is given in percent of reading (above 10% of nominal current) rather than percent of nominal current.
Measuring range
Electronic Isolated
AV 100
Optically isolated
0 to 9.5 kV
0 to 7 kV
50 V to 2 kV
100 V to 6 kV
Several kHz
0 to 2/10/800 kHz ④
0 to 13 kHz
0 to 13 kHz
Typical response
time (@ 90%)
10 to 100 µs
< 30 µs
< 30 µs
Typical accuracy at
25°C, in % of VPN
±1 %
±0.2 %
±0.7 %
±0.9 %
< 0.5 %
±0.05 %
±0.1 %
±0.1 %
Standard performance
High accuracy
Best speed
Low speed
Small size
Limited voltage
Low speed
Excellent EMI immunity
High common mode rejection
High isolation capability
Low speed
④ Design dependent
Hall effect technologies Fluxgate technologies
Closed loop
C types
Voltage Transducers and Basic Working Principle
Determining parameters for transducer
The wide variety of available LEM transducers is the direct
result of our know-how and many years of experience,
enabling us to address the many variations of customer
requirements within the greatly diversified field of power
electronics. The application requirements create selection
criteria, guiding the user to an appropriate product.
Which parameters need to be considered?
Understanding the LEM documentation
The first step for transducer selection is a detailed understanding of the application, including parameters such as
continuous RMS and repetitive peak measurement level,
maximum possible peak or fault level, rate of change
(e.g. di/dt and dv/dt) to be measured, allowable response
time, etc. One must also consider external influences that
will impact the application, such as temperature, shock,
vibration, and external fields as well as any necessary
compliance standards (EN, IEC, UL, CSA, etc.) that need to
be met.
The selection of a transducer is the result of a technical and
economic trade-off, considering the transducer as well as
the associated sub-systems. All aspects of an application
must therefore be taken into account during transducer
selection and system design, with particular attention to the
Using this information, refer to the general catalog of LEM
transducers to locate the product line(s) that have the key
characteristics to meet the requirements from the full range
of available products. Then select the specific product(s)
from those lines that meet the measurement requirements.
• electrical requirements, including power supply
requirements, peak measurement, response time, di/dt,
dv/dt, etc.
The individual datasheet (see the LEM website: http://
www.lem.com) of the selected transducer(s) will then
provide more details to determine if the transducer(s) match
the requirements. The checklist provided in the following
tables (§2.4 and §2.5) may help define critical concerns and
assist in the transducer selection process.
• mechanical requirements, including aperture size, overall
dimensions, mass, materials, mounting, etc.
• thermal conditions, including current profile versus time,
maximum RMS measurement, thermal resistances,
cooling, etc.
• environmental conditions, including vibration,
temperature, proximity of other conductors or magnetic
fields, etc.
Additional selection criteria
As previously discussed, some applications have a higher
level of complexity and combine several potentially critical
elements such as:
• electromagnetic interference
The transducer development process includes a full regimen
of tests that comprise the characterization report. These tests
follow the scientific method, varying individual parameters to
characterize the transducers response to each.
During production a quality plan indicates the tests to be
carried out, on each product or batch of products, to verify
compliance with specifications. Unless otherwise specified,
performance is tested under nominal conditions of current,
voltage, temperature, etc. in the production and laboratory
In the actual application, several factors can act simultaneously and potentially provide unexpected results. It is
essential to assess the transducer in those conditions to
verify acceptable performance. Based on LEM experiences,
this assessment is typically not difficult if the working
conditions are known and defined.
• significant common mode voltage transients (dv/dt)
• mechanical disturbances (vibration, shock, etc.)
• special isolation or partial discharge requirements
• compliance with specific standards, etc.
Obviously the best scenario is to perform tests in the specific
application environment. If this is not reasonable or feasible,
provide LEM with a diagram of your installation and a
detailed description of the transducer operating conditions
(e.g. description of the environmental conditions, graph of
the waveform to be measured, nearby potentially disturbing
elements such as inductors, current carrying conductors, the
presence of magnetic materials or the desired location of
other transducers).
The operating temperature range is based on the materials
and construction of the selected transducer. The minimum
temperatures are typically -40, -25, or -10 °C while the
maximums are +50, +70, +85, or +105 °C.
Determining Parameters for Transducer Selection
Current transducers – selection check list
Electrical parameters
Type of current to be measured
Selection criteria
- DC, AC or complex waveform current
- Selection of the adequate technology (Table 1)
Range of current to be measured
- Nominal current to be measured
- Peak current to be measured
- Transient current overloads to be measured
- Maximum peak value and duration not measured
Required output signal
- Current or voltage output
- Output value at nominal or peak current
- For current output transducers, selection of the necessary or desired output load
Measurement accuracy
- Required accuracy at 25 °C, taking into account the DC offset and non-linearity
of the output signal
- Global accuracy within the operating temperature range, adding the accuracy at
25 °C, the offset drift and, if applicable, the gain drift.
Available power supply
- Power supply voltage, including tolerances
- Maximum allowable current consumption
- Primary working voltage
- Applicable standard for isolation (design or test)
- Compliance to the relevant standards
- Single or reinforced insulation
- Dielectric withstand voltage (e.g. 4 kVRMS, 60 Hz, 1 min.)
- Pollution degree (e.g. class 2)
- Over voltage category (e.g. 0V cat. 1)
- Impulse withstand voltage (e.g. 8 kV with 1.2/50 µs)
- Partial discharge extinction level and electric charge
Dynamic operating parameters
Frequency range
Selection criteria
- Frequency range to be measured
- Fundamental operating frequency
- Harmonic content (e.g. drive’s switching frequency)
- Current Harmonics not to be measured but likely to create transducer losses
(e.g. drive’s ringing current)
- Match between the di/dt to be measured and the transducer’s response and rise
times (Figure 8)
- Maximum possible di/dt overloads, to be withstood by the transducer, but not to
be measured
- Maximum transducer recovery time after a di/dt overload
- Maximum error allowed during dv/dt disturbance
- Maximum transducer settling time after a dv/dt disturbance
Determining Parameters for Transducer Selection
Environmental parameters
Selection criteria
- Minimum and maximum working temperatures where the transducer
performance is applicable
- Extreme storage temperatures
Vibration & shock
- Standards and levels to be considered (when applicable)
Presence of external fields
- External current identical to the one being measured
- Other currents of similar or greater amplitude
- Fields from transformers or inductors
- Fields from magnetized materials in the area
Mechanical Interfaces
Primary electrical connection
Selection criteria
- Printed wiring board (PWB/PCB) pins
- Through-hole aperture size and shape
- Busbar dimensions
- Other connections (e.g. screw terminals)
Secondary electrical connection
- Printed wiring board pins
- Connector
- Faston tabs
- Screw lugs
- Other connections (e.g. screw terminals)
External dimensions
- Maximum specified dimensions
- Aperture / primary location
- Connector / secondary location
- Required creepage / clearance distances
Package fastening
- Printed circuit mount
- Panel mount
- Aperture (cable or busbar) mount
- DIN rail mount
Determining Parameters for Transducer Selection
Voltage transducers – selection check list
LEM produces a wide variety of galvanically isolated voltage
transducers capable of measuring up to 9.5 kV. These are
based on four different technologies, with different
performances, as outlined in Table 1.
The selection criteria for a current transducer are also
applicable to voltage transducers, especially when the
Electrical parameters
voltage measurement is based on a ‘current measurement
principle’, as described later in § 3.4. The main difference
lies in the primary parameters which affect the link between
the voltage to be measured and the current, IMES, detected by
the transducer (Figure 21), namely the effect of the series
resistance R1 and the parameters linked to the transducer
primary impedance. This leads to the following additional
selection criteria:
Selection criteria
Measurement accuracy
- The transducer’s primary resistance and series resistance
R1 (built-in or external), taking into account the effect of
the manufacturing tolerances and temperature variations
Power budget
- The total power lost from the primary circuit due to power
dissipation in the primary measuring circuit
- The selected series resistor, R1, shall be capable of
dissipating many times the nominal power
Dynamic operating parameters
Bandwidth / response time
Selection criteria
- Dependent on the L / R time constant of the primary circuit,
the series combination of primary inductance (LP), the
primary resistance (RP), and the series resistance (R1)
Power transducers – selection check list
The combination of current and voltage transducer in a
single device creates an instantaneous power transducer.
The selection criteria for this device are identical to those
previously listed for current and voltage transducers. There
will be a limitation on bandwidth and response time to
ensure proper multiplication of the signals.
Type of output
The output of a LEM transducer typically represents the
instantaneous primary signal, providing both output scaling
and isolation. Transducers may also provide average, peak,
sine-wave RMS, or true RMS output(s).
The secondary signal can be a current, voltage, current loop
(4-20mA), or digital output. This signal will have an offset
level, the output with no primary signal, and a range, the
output swing with a change in primary signal. These can be
matched to a specific interface, such as the A/D converter of
a DSP or microprocessor (Figure 1).
Devices can be ratiometric, implying that the offset level and/
or output range are dependent on the power supply or a
reference input voltage. This is ideal when driving an analog
to digital converter (ADC) or other reference based
electronic processing circuitry, allowing the output to track
the external reference, minimizing drift between the
transducer and this reference. Alternatively, the internal
Figure 1: Output curve of the LTS and HTS product families
reference of the transducer may be provided to act as a
reference source for other electronic processing circuitry,
potentially eliminating a discrete reference source.
Loop powered devices are also available. These are two
wire devices that obtain their power from a 4-20mA current
loop and provide a variable current load to represent the
output signal. These operate with a voltage drop across the
transducer to power the internal circuitry, which must be
taken into account when supplying the loop.
Hall Effect Technologies
Conversion of a current output to a voltage is easily
accomplished by the user with the addition of a measurement resistor, RM, in series with the output. This allows the
user to easily scale a single device to the desired output
level. There is a range of acceptable RM values dependent
on the operating parameters of the application with the
maximum value limited by available voltage and a minimum
value required to limit the internal power dissipation. The
transducer datasheet provides a range of values based on
nominal parameters.
Hall effect technologies
Three of the LEM technologies (open loop, closed loop and
Eta) are based on the Hall effect, discovered in 1879 by
American physicist Edwin Herbert Hall at Johns Hopkins
University in Baltimore. The Hall effect is created by Lorentz
forces, FL = q • (V x B), which act on charges moving through
a magnetic field.
Open loop Hall effect current transducers
Open loop transducers use the simplest implementation of
the Hall effect. They provide the smallest, lightest, and most
cost effective current measurement solution while also
having very low power consumption.
3.1.1 Construction and principle of operation
A current carrying conductor creates a magnetic field. This
field is concentrated by a magnetic core. The core has a gap
cut through it and a hall generator is used to sense the
magnetic flux density in the gap. The control current, IC, and
differential amplification are supplied by electronics (Fig. 3)
built into the transducer.
Figure 3: Conversion of the primary current into an output voltage
Figure 2: Representation of the electrical parameters of the Hall
A thin sheet of conducting material is traversed lengthwise
by a control current, IC (Fig. 2). The mobile charge carriers of
this current are affected as the external magnetic flux, B,
generates a Lorentz force, FL, perpendicular to the direction
of current flow. The resulting deflection of current causes
more charge carriers to be located at one edge of the sheet,
creating a potential difference referred to as the Hall voltage,
VH. For the arrangement described above, with the magnetic field, current, and sheet edges mutually perpendicular,
we obtain:
Within the linear region of the hysteresis loop of the material
used for the magnetic circuit (Fig. 4), the magnetic flux
density, B, is proportional to the primary current, IP, and the
Hall voltage, VH, is proportional to the flux density. Therefore
the output of the Hall generator is proportional to the primary
current, plus the Hall offset voltage, VOH.
VH = K / d • IC • B + VOH
where ‘K’ is the Hall constant of the conducting material, ‘d‘
is the thickness of the sheet, and ‘VOH’ is the offset voltage of
the Hall generator in the absence of an external field. Such
an arrangement is referred to as a Hall generator and the
product ‘K / d • IC’ is generally referred to as the Hall
generator sensitivity.
The sensitivity and offset voltage of Hall generators are
temperature dependent. However, these effects can be
greatly compensated by the biasing and amplification
electronics driving and sensing the Hall generator.
Figure 4: Magnetization curve
The measurement signal is then compensated to remove the
offset component and address temperature effects and
amplified to supply the user with the desired output.
In the case of low current measurement (< 50 A) multiple
turns are recommended or implemented internally to
achieve 50 Ampere-turns nominal providing reasonable flux
density levels for measurement.
Hall Effect Technologies
3.1.2 Advantages and limitations
• gain error (current source, hall generator, core gap)
• linearity (core material, hall generator, electronics)
Open loop transducers measure DC, AC and complex current
waveforms while providing galvanic isolation. As mentioned
earlier, the advantages include low cost, small size, lightweight, and low power consumption and are especially
advantageous when measuring high (> 300 A) currents. As
with most magnetic based measurement techniques,
insertion losses are very low. Primary current overloads can
be easily handled although it may result in some
magnetization of the core creating an offset shift, called
remanence or magnetic offset (§ 3.1.6).
Compared to other technologies the limitations of open loop
transducers are moderate bandwidth and response time, a
larger gain drift with temperature, and a limitation on the
current frequency product (power bandwidth).
In many applications the advantages outweigh the limitations
and an open loop solution is advised.
3.1.3 Nominal and extreme currents
LEM open loop transducers are made for nominal currents,
IPN, from several amperes to 10 kA, with a peak current rating
up to 30 kA. This wide range of products addresses virtually
all of the industrial requirements.
The maximum current an open loop transducer can measure
is dependent on the design and material used for the magnetic circuit and on the design of the processing electronics.
In general, LEM open loop transducers are designed such
that the maximum measurable current is 200 % to 300 % of
the nominal RMS current rating.
Even so, open loop transducers can withstand current
overloads significantly beyond the maximum measurable
value, for example 10 times the nominal current. However, as
described earlier, this can create a magnetic offset resulting
in an additional measurement error, to be removed by
applying a dedicated demagnetization procedure.
3.1.4 Output signals
• output noise floor (hall generator, electronics)
• bandwidth limitation (attenuation, phase shift,
current frequency)
Temperature changes also create drift in:
• DC offset
• gain
LEM transducers are factory calibrated at nominal temperature and nominal current, leading to the „accuracy at 25 °C“
given in the LEM datasheets. The datasheet also provides
the temperature drift specifications.
The location of the primary conductor through the aperture
as well as the positioning of the return conductor can affect
dynamic performance of the transducer. LEM generally
recommends an optimal routing / position for the primary
and return conductors. In addition, high frequency
disturbances can affect the transducer output due to
capacitive coupling, so the routing and layout of the transducer output must be considered (e.g. twisted and shielded
cables, appropriate routing of the output PWB tracks).
3.1.6 Magnetic offset considerations
Depending on the type of transducer and the magnetic
material used, the residual flux (BR or remanence) of the
magnetic core induces an additional measurement offset
referred to as ‘magnetic offset’. The value depends on the
previous core magnetization and is at a maximum after the
magnetic circuit has been saturated. This might occur after a
high overload current.
As an example, measurements carried out on the HAL and
HTA types of open loop transducers give the following
results: after a cycle of current varying from 0 to 300 % of IPN
and then back to zero, the magnetic offset is 2.5 mV for HAL
transducers and 3 mV for HTA transducers, less than 0.1 %
of the nominal output, VSN.
The output of an open loop transducer is generally a voltage
directly proportional to the measured current. This voltage is
typically equal to 0V without primary current and 4V at the
nominal current, IPN. Variations are possible, including
different offset, and/or nominal values or a current output.
In the case of a higher current overload (e.g. 1000 % of IPN),
a larger magnetic offset error may occur. Recovering from
this condition requires demagnetization, either by
appropriate reversal of the primary current or a dedicated
degauss cycle. This process will return the transducer to the
initial, pre-overload, performance.
3.1.5 Measurement accuracy Demagnetization to eliminate magnetic offset
The typical open loop transducer has an overall accuracy of a
few percent. There are a number of error terms that combine
to create this error, at nominal temperature (25°C) and across
the temperature range.
The elimination of magnetic offset requires demagnetization.
A degauss cycle requires driving the core through the entire
B-H loop with a low frequency AC source, then gradually
decreasing the excitation returning the B-H operating point
to the origin (Fig. 5). As a minimum, provide 5 cycles at full
amplitude and then decrease the excitation smoothly no
faster than 4 % per cycle, requiring 30 cycles or 500 ms at
60 Hz. For closed-loop devices, additional care must be
taken to be sure the compensation coil does not negate the
demagnetization effort (see § 3.2.6).
The accuracy is limited by the combination of:
• DC offset at zero current (hall generator, electronics)
• DC magnetic offset (remanent magnetization of core
Hall Effect Technologies
Demagnetization current
> 5 cycles
Figure 5: Degauss cycle current
Alternatively, a partial demagnetization of the core is possible
by providing an appropriate signal in the opposite polarity of
the magnetization. The difficulty is determining the exact
amplitude and duration to obtain a satisfactory result. With a
well-defined application it may be feasible to determine the
required value empirically and apply this correction as
3.1.7 Bandwidth and core losses
The bandwidth limitation of open loop transducers is mainly
due to two factors:
• limitations of the processing electronics
• magnetic core heating due to core losses, a combination of
eddy current and hysteresis losses Core losses
The magnetic material and core design as well as the current
amplitude versus frequency spectra define the level of core
• eddy current losses are proportional to the square of three
different parameters: the peak flux density in the core, the
frequency of induction and the lamination sheet thickness
of the core
• hysteresis losses are proportional to frequency, core
volume and the square of peak flux density
For LEM transducers, this leads to the following conclusions:
• core losses become significant at high frequencies and it
is essential to limit the current amplitude at these
frequencies to acceptable levels (dependent on ambient
and maximum transducer temperatures); this implies not
only limiting the maximum frequency of the fundamental
current, but also harmonic content, since even a low
amplitude signal may create unacceptable losses at high
frequencies. Core loss rule-of-thumbs
Iron losses calculation are complex and, as a „rule-of-thumb“
judgment, it is possible to consider that the iron losses are
minimized if the product "N • I • f" is kept as small as
possible, where:
N = number of internal or external primary turns
I = primary current or amplitude of a current harmonic
f = frequency of the primary current or current harmonic
As a result, when one of the three factors is increased (i.e.
the current), the iron losses are increased unless at least
one of the two other factor is decreased (i.e. the frequency of
the measured current and/or the number of primary turns).
While this formula implies that the core losses will increase
with an increase of any of these parameters, it is not
intended to say that acceptable core losses are realized if
the product of the three parameters is kept constant. For
example, it is wrong to say that one can operate at twice the
frequency if the Ampere-turns are cut in half.
At a given frequency, it is nevertheless correct to assume
that keeping constant the "N • I" product implies similar iron
losses, even the probable change on the primary conductor
magnetic coupling may affect the iron losses value.
To conclude, trouble-free operation of a current transducer
requires limiting the temperature rise to avoid overheating
the internal components. Parameters affecting temperature
rise go beyond core losses and include the primary busbar
resistive losses, the losses of the electronics and the various
thermal resistances. In particular, to keep losses constant
requires to decrease the transducer primary current while
the working frequency increases. A core loss example
Considering the LEM open loop HY 10-P transducer, where
the primary is integrated into the transducer and the number
of primary turns cannot be changed, tests show the following
maximum working conditions:
• these losses are directly proportional to the square of the
flux density, which is directly related to the primary ampereturns, implying core losses are theoretically proportional to
the square of primary ampere-turns if no magnetic
saturation occurs. When increasing sensitivity by using
multiple primary turns, core losses are increased by the
square of the turns
Hall Effect Technologies
At an ambient temperature of 25 °C
10 A
13 kHz
33 kHz
340 kHz
At an ambient temperature of 70 °C
10 A
6 kHz
12 kHz
90 kHz
Obviously these values are far from constant, but they can be
considered as limits for acceptable operation. Even so, this
transducer cannot operate above 50 kHz as limited by the
maximum bandwidth (3dB) of the electronics. Addressing core losses
Often the adverse effects of core losses are not considered or
cannot be predicted accurately during the initial design
stages. Therefore many designers find themselves in a
difficult situation when a design, or a specific application of
that design, causes overheating of the transducer due to core
losses. There are solutions to this problem, but a careful
analysis of the tradeoff between reducing core losses and
maintaining acceptable response time of the transducer is
Although the insertion loss of a transducer is extremely low
this impedance is, in fact, a combination of the resistance and
inductance of the primary. Placing a series resistor-capacitor
in parallel with the primary (Fig. 6) diverts the high frequency
components of the current around the primary, significantly
reducing core losses. This also removes these frequencies
from the measurement path increasing the response time. An
example is shown in Fig. 7 where the VOUT response of the
HX 15-NP transducer is given, with and without parallel
resistor R (C = 0).
Figure 7: HX 15-NP transducer response
response, also called di/dt following (the transducers ability
to follow a fast change in primary current).
LEM defines the response time tr as the delay between the
primary current reaching 90 % of its final value and the
transducer reaching 90 % of its final amplitude (Fig. 8). The
primary current shall appoximately behave as a current step,
with an amplitude close the nominal current value.
This is actually the combination of three effects: the input rise
time (from 10 % input to 90 % input, also called di/dt), the
90 % of IN
IP: primary reference
time tr
10 % of IN
IS: secondary current
of the LEM
Rise time
Reaction time tra
t [µs]
Figure 8: Definition of the transient response parameters
3.1.8 Response time, delay time, and di/dt behavior
Three different criteria are used to characterize the dynamic
behavior of a transducer: bandwidth, response time, and step
reaction time tra (from 10 % input to 10 % output), and the
output rise time (from 10 % output to 90 % output, also called
slew rate).
Open loop transducer response is dependent on the transducer design and the magnetic coupling of the signal to be
measured, as well as those not to be measured. The latter
places some responsibility on the user to investigate the
coupling effects and determine the appropriate placement of
the primary and other conductors to optimize the response of
the transducer in the application. Careful consideration of the
cabling in and around the transducer usually resolves
response time and di/dt performance problems.
Fig. 9 shows the response of a HAL 600-S transducer, where
a slight difference between the input and output currents can
be noticed at the corners. In this case, the response time is
less than 3 µs with a di/dt of 50 A/µs.
Figure 6: Schematic of high frequency bypass
Hall Effect Technologies
600 A
-50 A/µs
Figure 9: Dynamic behavior of an HAL 600-S transducer at 600 A
3.1.9 Typical applications
Open loop current transducers are used in numerous
industrial applications as the key element of a regulation loop
(e.g. current, torque, force, speed, position) or simply to drive
a current display.
two values are independent because the accuracy (40 mV)
is confirmed with an AC signal while the offset (10 mV) is a
DC measurement. Therefore, when measuring a 200A DC
current at 25 °C the output could be in error by as much as
50 mV, which is 1.25 % of the 4 V output.
Operating at a different temperature causes both offset and
gain drift. The maximum offset drift is specified as 1 mV/K
and the maximum gain drift is 0.05 %/K. When we operate
the transducer at 85 °C there can be an additional 1 mV/
K • (85 – 25) °C = 60 mV of offset voltage and 0.05%/K •
4 V • (85 – 25) °C = 120 mV of gain drift. The total error from
all of these effects is 230 mV, or 5.75 % of the nominal 4 V
Closed loop Hall effect current transducers
Typical applications include:
Compared to the open loop transducer just discussed, Hall
effect closed loop transducers (also called Hall effect
‘compensated’ or ‘zero flux’ transducers) have a
compensation circuit that dramatically improves
• frequency inverters and 3-phase drives, for the control of
the output phase and DC bus currents
3.2.1 Construction and principle of operation
• power factor correction converters, for monitoring of the
mains current(s)
• electric welding equipment, for the control of the welding
• uninterruptible power supply (UPS) or other battery
operated equipment, for the control of charge and discharge
• electric vehicles, for motor drives and battery current control
While open loop current transducers amplify the Hall
generator voltage to provide an output voltage, closed loop
transducers use the Hall generator voltage to create a
compensation current (Fig. 10) in a secondary coil to create
a total flux, as measured by the Hall generator, equal to
zero. In other words, the secondary current, IS, creates a flux
equal in amplitude, but opposite in direction, to the flux
created by the primary current.
• electric traction systems, trackside circuit breaker and
rectifier protection, rolling stock traction converters and
• energy management systems, switching power supplies,
electrolysis equipment, and other applications
3.1.10 Calculation of the measurement accuracy
As indicated previously, the accuracy indicated in the
datasheets applies at the nominal current at an ambient
temperature of 25 °C. The total error at any specific current
includes the effects of offset, gain, non-linearity, temperature
effects and possibly remanence. The LEM datasheet provides
the worst-case value of each of these factors individually.
The theoretical maximum total error corresponds to the
combination of the individual worst-case errors, but in practice
this will never occur.
Figure 10: Operating principle of the closed loop transducer
Example: Current transducer HAL 200-S (see datasheet)
In this example it is assumed the power supplies are accurate
and stabilized and magnetic offset is negligible. A current of
200A is measured at an ambient temperature of 85 °C.
Operating the Hall generator in a zero flux condition
eliminates the drift of gain with temperature. An additional
advantage to this configuration is that the secondary winding
will act as a current transformer at higher frequencies,
significantly extending the bandwidth and reducing the
response time of the transducer.
The datasheet indicates the output voltage is 4 V at the
200 A nominal current. The worst-case accuracy at IPN, 25 °C
and with ±15 V supplies is 1 %, or 40 mV. In addition there is
a maximum offset voltage at Ip = 0 and 25 °C of 10 mV. These
When the magnetic flux is fully compensated (zero), the
magnetic potential (ampere-turns) of the two coils are
identical. Hence:
NP • IP = NS • IS which can also be written as IS = IP • NP / NS
Hall Effect Technologies
Consequently, the secondary current, IS, is the exact image
of the primary current, IP, being measured. Inserting a
„measurement resistor“, RM, in series with the secondary coil
(Fig. 10) creates an output voltage that is an exact image of
the measured current.
To give an order of magnitude, the typical number of
secondary turns is NS = 1000…5000 and the secondary
current is usually between IS = 25…300 mA, although it
could be as high as 2 A. For higher output currents an
output power stage is needed to produce the transducer
output current.
At low frequencies the transducer operates using the Hall
generator. At higher frequencies the secondary coil operates
as a current transformer, providing a secondary output
current again defined by the turns ratio and converted to a
voltage by the measuring resistor. These effects are
illustrated in Fig. 11.
Frequency limit
of the electronics
transformer area
Figure 11: Bandwidth of the „Hall generator“ and „current
The unique design of closed loop transducers provides an
excellent bandwidth, typically from DC to 200 kHz. The
challenge is to ensure a flat frequency response across the
entire range, especially where the two response curves cross,
to provide excellent dynamic response and accuracy for all
possible signals.
Finally, while closed loop transducers work theoretically at
zero flux, various magnetic imperfections (leakage flux, non
perfect coupling) imply a residual flux into the core which
results in iron losses at high frequencies. Consequently, the
heating phenomena described in § 3.1.7 for open-loop
transducer also applied in this case, even if much less
3.2.2 Advantages and limitations
Closed loop transducers are capable of measuring DC, AC
and complex current waveforms while ensuring galvanic
isolation. The advantages of this design include very good
accuracy and linearity, low gain drift, wide bandwidth, and
fast response time. Another advantage is the output current
signal that is easily scalable and well suited to high noise
environments; nevertheless, closed loop transducers are
available in voltage output configurations. Again, as with most
magnetic based measurement techniques, insertion losses
are very low.
The main limitations of the closed loop technology are the
high current consumption from the secondary supply (which
must provide the compensation as well as bias current), the
larger dimensions (more noticeable on high current
transducers), a more expensive construction compared with
the simpler open loop designs and a limited output voltage
due to the internal voltage drops across the output stage and
secondary coil resistance.
Again, depending on the application requirements, the
advantages often outweigh the limitations and the accuracy
and response of a closed loop solution is desirable over other
3.2.3 Nominal and extreme currents
LEM closed loop current transducers are available in models
with nominal currents from 2 A to more than 20 kA. Specific
designs even allow current measurements up to 500 kA.
With closed loop technology the maximum measurable peak
current is typically 150 to 300 % of the nominal current rating.
For a given closed loop transducer, the maximum peak
current which can be measured can be defined in three
different ways:
• from DC to mid-frequencies (in the closed loop
compensation mode, § 3.2.1), the maximum measurable
current is limited by the ability of the electronics to drive
compensation current, IS, in the secondary coil; this limit is
based on the available supply voltage, internal voltage
drops, and current through the total series resistance and
exceeding this limit at low frequency will result in „electronic
• also, each transducer is designed for a specific measurable
current range and exceeding the transducer ratings will
result in non nominal magnetic effects (excessive fringing)
that does not allow the electronics to properly compensate
the loop resulting in „magnetic saturation“
• for transient currents at higher frequencies the transducer
operates as a current transformer and the current can reach
higher values, limited by magnetic (pulse duration in
ampere seconds) and thermal (core losses related to
ampere-hertz) constraints; the transducer user should
consult LEM when considering working in the transformer
mode, beyond the limits set in LEM datasheets.
In some specific LEM transducer types (e.g. the LTS family),
primary currents exceeding the peak measuring range can
create abnormal, though non-destructive, results due to the
unique electronic configurations.
In case of current overload, a too long overload duration (e.g.
> 1 ms) may in some case start to overheat the snubber which
protects the transducer against short time overloads.
3.2.4 Output signal – Measurement resistance
The majority of closed loop transducers have a current output
that can be easily converted to a voltage for measurement by
adding a measurement resistor in series with the output.
Refer to § 2.7 for more details.
Hall Effect Technologies
3.2.5 Measurement accuracy
Due to the closed loop working principle, operating at nearly
zero flux (some flux remains due to system loop gain and
magnetic leakage phenomena), LEM closed loop current
transducers have excellent linearity and minimal gain drift
over a wide measuring and temperature range, with total
accuracy typically remaining below 1 %.
At ambient temperature, the accuracy is given by the
combination of:
• if one or both of the secondary supply voltages are missing,
disabling the electronic compensation process, there will no
longer be zero flux compensation
• when an external conductor creates localized core
saturation, not totally detected by the Hall generator and
compensated by the electronics, the total flux in different
areas of the core will be non-zero
If any of these conditions occur the result could be magnetic
offset, resulting in an additional measurement error. This can
be corrected with demagnetization (§
• output offset at zero primary current (IP = 0)
• non-linearity of the Hall generator, electronics, and
• gain tolerance (tolerance of the number of secondary coil
• tolerance of the measuring resistor, RM (internal or external)
And temperature changes imply:
• offset drift (or with respect to the reference voltage, if
• drift of the measuring resistor value, RM
While those factors may be simple to assess for DC current,
AC signals and complex waveforms may have their total
accuracy affected by transducer bandwidth limitations
(Fig. 11), possibly introducing frequency attenuations and
phase shifts.
To make the best use of the transducer, the mounting
conditions must be such that it optimizes the primary to
secondary magnetic coupling, specifically for AC signals
where the transducer works as a current transformer. The
designer must consider both the primary conductor (wire or
busbar being measured) and other conductors in close
proximity, such as the return conductor or the conductors of
other phases.
Additionally, the routing of the transducer output wires, or
paths of the PWB tracks, at the transducer output should limit
high frequency disturbances created by external conductors.
The output wiring should have minimal loop area, to minimize
di/dt effects, and long runs parallel to power wires must be
avoided, to limit capacitive coupling and minimize dv/dt
3.2.6 Observations regarding magnetic offset
In standard working conditions, a closed loop transducer is
always working near zero flux, either when the low frequency
Hall based closed loop is effective or when the high
frequency current transformer is working (§ 3.2.1). However,
this does not imply that closed loop transducers are not at risk
of having a permanent magnetic offset. As discussed for open
loop transducers:
• if a low or medium frequency primary current exceeds the
measuring range (based on supply voltage, transducer
parameters, and measuring resistor value) the electronics
can no longer drive sufficient secondary coil current to
maintain the zero flux condition
With compensated devices care must be taken to ensure the
compensation does not negate the demagnetization effort.
Ideally the output can be disconnected to open the
compensation loop. If this is not possible, disabling the power
supplies accomplishes the same goal if a low frequency is
used for excitation, to avoid the current transformer effect.
3.2.7 Bandwidth and core losses
Closed loop transducers demonstrate excellent bandwidth
characteristics. Typically the bandwidth is from DC to 200 kHz,
while a LEM patented design (the LB family) achieves a
bandwidth better than DC to 300 kHz.
Nominal current can not be considered over the full frequency
range. To keep constant the transducer losses, current value
shall be decreased while working frequency increases.
While the current transformer effect of closed loop transducers
provides excellent high frequency performance they are still
subject to core losses due to hysteresis and eddy current
losses. As with open loop transducers (§ 3.1.7), care must be
used when attempting long term measurement of high
currents at high frequencies.
3.2.8 Response time and di/dt behavior
The response time of a transducer characterizes how it will
respond to a step current with a controlled rate of change,
called di/dt following. It is defined by several parameters such
as the delay time, rise time and reaction time (§ 3.1.8, Fig. 8).
Closed loop transducers show fast reaction times, typically
better than 1 µs.
The correct following of di/dt depends on the intrinsic
construction of each product and, as mentioned in § 3.2.5, the
mounting conditions of the transducer within the circuit to be
Dependent on the closed loop transducer model, it is possible
to measure a di/dt of 50 to 400 A/µs or more. This makes them
well suited for the short-circuit protection of semiconductors in
power equipment.
3.2.9 Typical applications
Closed loop transducers are particularly well suited for
industrial applications that require high accuracy, wide
bandwidth and fast response time. They are often used as the
key element of a regulation loop for the control of current,
Hall Effect Technologies
torque, force, speed and/or position as well as for the
protection of semiconductor devices.
Applications are identical to those for open loop transducers,
except higher performance results can be expected:
• frequency inverters and 3-phase drives, for the control of
the output phase and DC bus currents as well as
protection of the power semiconductors from fault
conditions such as output short-circuits
• converters for servo-motors frequently used in robotics, for
high performance speed and position control
• special wide bandwidth power supplies for special
equipment, such as radar
Other applications include energy management systems,
switching power supplies, electrolysis equipment, lasers,
rectifiers for electrolysis and, finally, many applications for
laboratories or test and control benches.
In the specific case of the LTS family, with a through-hole
and several U-shaped primary conductors, series-parallel
connections combined with wire sensing offers a multitude
of possibilities, such as the measurement of differential
currents or performing phase current mathematic calculation
(Iu – Iv or Iu + Iv + Iw) with a single device.
3.2.10 Transducer parameter examples
The following examples are intended to help the user
estimate the limits of the operating values for a closed loop
current transducer based on operating parameters and the
measurement resistor. Two approaches can be used to
determine the appropriate measuring resistance; based on
the range provided on the datasheet or based on acceptable
operating values outside those listed on the datasheet. In
some cases it is also possible to measure beyond the
maximum current given on the datasheet and two examples
are provided for this case.
Example 1: LA 55-P operating within nominal datasheet
parameters (see datasheet)
a) What maximum measuring voltage can be obtained with
the following parameters ?
IP = 70 A, TA = 70 °C, VC = ±15 V
The number of secondary turns, 1000, determines the
secondary current, IS = IP • NP / NS = 70 mA
The LEM datasheet indicates RMmax = 90 Ω for these
conditions leading to a maximum measuring voltage of:
VM = RM • IS = 90 Ω • 70 mA = 6.3 V
b) Using the following parameters, which measurement
resistance must be selected to achieve a measuring
voltage of 3.3 V at the nominal primary current?
IPN = 50 A, TA = 85 °C, VC = ±12 V, and ISN = IPN • NP/
NS = 50 mA
We have RM = VM/IS = 3.3 V / 50 mA = 66 Ω and, for the
given parameters, the datasheet recommends a
measuring resistance between RMmin = 60 Ω and
RMmax = 95 Ω; therefore a 66 Ω resistor can be used.
c) For the same parameters, is it possible to have a 6 V
measuring voltage?
RM = VM/IS = 6 V/50 mA = 120 Ω
The 50 A current will not be measured with this resistance
since it exceeds the specified RMmax value of 95 Ω. The
maximum measuring voltage is obtained with the RMmax
resistance and is equal to 50 mA • 95 Ω = 4.75 V.
If a 120 Ω resistor is used, the measurement will only be
correct for primary currents less than 50 A (actually up to
43.9 A and 5.25 V). In this case the calculation of the
maximum current value is not straightforward and is
described in the following paragraphs.
Example 2: LA 55-P operating outside of nominal
datasheet parameters
When operating at lower measurement currents it is possible
to get more output voltage by selecting a larger measurement resistor than specified on the datasheet. Alternatively,
when operating at power supply voltages other than those
specified on the datasheet the measurement range may be
a) What is the maximum voltage available at the amplifier
Fig. 12 shows the schematic diagram of the output of a
typical closed loop current transducer. The output
voltage is limited by the voltage drops across each of the
circuit elements: the maximum output stage driving
voltage (VA), the transistor drop (VCE), the secondary coil
resistance (RS), and the measuring resistance (RM).
Knowing the maximum possible value of the voltage at
the output of the amplifier stage (VA) will allow the
determination of the measurable current range. The
worst-case situation will be considered, with the
maximum temperature and the minimum supply voltage,
but other situations can be evaluated with this same
Using datasheet values for peak current (70 A), coil
resistance (80 Ω @ 70 °C), and maximum measurement
resistance (90 Ω @ 70 °C with ±15 V supplies) we have:
VA = IS • (RS + RM) = 0.07 A • (80 Ω + 90 Ω) = 11.9 V
b) What is the maximum output voltage with IP = 60 A,
TA = 70 °C, and VC = ±15 V ± 5 %, IS = 60 mA?
Using the results from a) above, the maximum level for
VA under similar circumstances with a min 14.25 V
supply is 11.9 V. We can determine the maximum value
of VM and then RM.
VM = VA - ∆VC - RS • IS = 11.9 V - 80 Ω • 60 mA = 7.1 V
RMmax = VM/IS = 7.1 V/60 mA = 118 Ω
Hall Effect Technologies
b) The datasheet allows an RMmin value equal to zero.
VCmin = 14,25 V
VCE(sat) = ?
RS = 80 Ω
RM = 90 Ω
Figure 12: Diagram for calculation of the available voltage, VA, at
the output stage
VCmin = 14,25 V
VCE(sat) = 2,35 V
RS = 80 Ω
RM = ?
Figure 13: Equivalent diagram to calculate the measuring
resistance, RM
Example 3: Measuring current higher than the maximum
value given in the datasheet
LEM datasheets indicate the transducer operating conditions,
with a measuring range that is generally limited to 150 to
200 % of the nominal current (IPN). However, it is possible to
measure currents exceeding this range if the following two
parameters are considered:
• the measurement resistance must not be smaller than the
RMmin value given in the datasheet; this minimum value is
required to limit the power dissipated in the transducer
output stage; depending on the selected transducer RMmin
can be very small, in some cases equal to zero
• the primary conductor temperature must not exceed the
value specified in the datasheet (e.g. 90 °C), to keep the
transducer within acceptable operating temperature levels
The following examples demonstrate the calculation of the
maximum measurable current. Various scenarios are
examined to address different types of LEM transducers.
a) The datasheet requires an RMmin value greater than zero.
For example, the following parameters are defined for the
LA 55-P transducer: VC = 15 V ± 5 %, TA = 70 °C, RS =
80 Ω, RMmin = 50 Ω, VA = 11.9 V (as calculated in example
2a). The maximum value of the measurable secondary
current is:
IS = VA / (RS + RMmin) = 11.9 V / (80 + 50)Ω = 91.5 mA
The corresponding maximum measurable primary current
is 91.5 A.
For example, the following parameters are defined for an
LA 305-S transducer: VC = 15 V ± 5 %, TA = 70 °C, IP = 300
A, RS = 35 Ω (at 70 °C), RMmin = 0 Ω, NP:NS = 1:2500. The
minimum measurement resistance RM can be zero and it is
up to the user to select the most suitable value. The
smaller the value the larger the maximum measurable
current, but the smaller the available output voltage signal.
The available voltage at the amplifier output (VA) is
determined as follows:
RMmax = 75 Ω at 300 A for VC = 15 V, IS = 300 A/2500 = 120 mA
VA = (RS + RMmax) • IS = (35 + 75)Ω • 0.12 A = 13.2 V
The maximum value of the measurable secondary current
depends on the selected measurement resistance.
For example, with RM = 5 Ω, we have:
IS = (VA-∆VC)/(RS + RM) = (13.2 V) / (35 + 5)Ω = 330 mA
The maximum measurable primary current is consequently equal to 330 mA • 2500 = 825 A, corresponding to
2.75 • IPN. The voltage measured on the measurement
resistance is VM = 330 mA • 5 Ω = 1.65 V.
With RM = 2 Ω, we have IS = 357 mA, a maximum
measurable primary current of 3 • IPN (892 A) with a smaller
output voltage of 0.71 V.
If a zero ohm measurement resistor is considered, the
transducer can provide IS = 377 mA, which represents
3.14 • IPN (943 A). This arrangement provides no output
voltage, so a virtual ground referenced current to voltage
converter will be required to obtain a usable output signal.
It is up to the user to decide exactly what is needed.
Under these higher current conditions it is especially
important to keep the temperature of the primary conductor
below the maximum allowable temperature to avoid
permanent damage.
Example 4: Measuring transient current higher than the
maximum value given in the datasheet
In some cases higher currents can be measured using the
current transformer effect, provided the frequency and/or the
di/dt are high enough. This can be useful for short circuit
detection, as this is typically a high di/dt event, but care must
be exercised when attempting to use this capability for
standard or continuous measurement. See § 3.2.3 for more
It is important to note that some transducers (e.g. LTS) have
an internal measurement resistor value that cannot be
adjusted and, in these circumstances, the measuring range
cannot be adjusted.
Hall Effect Technologies
Example 5: Periodic transient current overload
IPt1 = 100 A
DT = 0.07
The following measurement sequence is considered,
illustrated for the LTS 25-NP transducer:
IPt1 = 75 A
DT = 0.105
• The transducer is measuring the nominal current of 25 A
IPt1 = 50 A
DT = 0.26
• During the time t1 a current overload occurs (IPt1), exceeding
the 25 A nominal value
• During the time t2 the current returns to the initial value (IPt2)
of 25 A, allowing the transducer to cool after the overload
t2 (s)
IPt1 = 34 A
DT = 1
• In the worst-case, the sequence is continuously repeated
Fig. 14 shows DC current levels, but these can also represent
the RMS value of the primary current over time. The following
examples use DC values, but total RMS (using AC and DC
values) has an equivalent effect.
8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40
t1 (s)
Fig. 16 Overload diagram of the LTS 25-NP for a rest current of
25 Arms, linear zone
Duty cycle = DT = t1 / (t1 + t2)
t2 = (1 – DT) • t1 / DT
Figure 14: Periodic overload with a standby current of 25 A at 85 °C
The maximum environmental temperature of 85 °C is
considered and, in this specific example of an LTS 25-NP
transducer, the three configurable primary terminals must be
connected in parallel to allow the long-term measurement of
25 A.
What is the acceptable limit for t1 and t2 based on the
magnitude of IPt1?
For given overload duration (t1) and amplitude (IPt1), Fig. 15
and Fig. 16 give the minimum time (t2) required for the
transducer to cool when the continuous current, IPt2, is the
nominal 25 A value with a maximum ambient temperature of
85 °C. Fig. 16 is a close-up view of the linear zone shown in
the lower left corner of Fig. 15 for short overload duration. In
this case the curves are linearized, allowing to define a
simple duty cycle relation:
IPt1 = 75 [A]
t2 (min.)
IPt1 = 100 [A]
IPt1 = 50 [A]
Considered as
linear zone
IPt1 = 34 [A]
For a current overload of 50ARMS applied during t1 = 14 s, the
recovery time shall be at least t2 = 40 s. This value can either
be taken from Fig. 15 or Fig. 16, or calculated according to the
duty cycle relation, considering a 26 % duty cycle in this case:
t2 = (1 - 0.26) • 14 / 0.26 = 40 s.
At 85 °C, 34A can be measured continuously by the LTS 25NP transducer. In this condition the various parts of the
transducer (the ASIC, coil, and primary pins) remain at or
below their maximum critical temperatures.
Unfortunately the failure rate of a transducer increases
significantly as the temperature increases. Therefore these
overload conditions will reduce the transducer reliability. This
overload capability is considered a key element of the safety
factor inherent in all LEM designs and, while temporary
excursions are anticipated, long term operation at levels
beyond those specified in the datasheet must not be allowed.
Important! The maximum operating temperature defined in
the transducer datasheet must always be observed. If not,
abnormal and possibly destructive behavior may result.
Always consult LEM when actual operating or storage
conditions fall outside of those specified on the datasheet.
To conclude, Fig. 17 and Fig. 18 give the maximum overload
conditions for a rest current of IPt2 = 0A.
3.2.11 Calculation of the measurement accuracy
100 110
130 140 150
t1 (s)
Figure 15: Overload diagram of the LTS 25-NP with a standby
current of 25 A
For closed loop transducers, examples of the maximum error
calculations are provided below. It is important to note that
there are four regions of operation that exist:
• the region at or near zero primary input, which is
characterized by the offset specification
Hall Effect Technologies
IP =
100 A
IP = 50 A
t2 [min]
±0.65 %
±1.20 %
Worst-case error (+85 °C)
±1.85 %
IP = 75 A
The worst-case error when measuring a 50 A current is
consequently equal to ±1.85 % • 50 A = ±0.93 A.
IP = 40 A
When a 40 A current is measured the output current is 40 mA
and the errors are:
IP = 34 A
t1 [sec]
Figure 17: Overload diagram of the LTS 25-NP for a standby
current of 0A
IP = 75 A
IP = 50 A
IP = 40 A
±0.81 %
±1.50 %
Worst-case error (+85 °C)
±2.31 %
The errors when measuring lower currents are larger as a
percentage of reading due to the offset currents becoming
significantly larger with respect to the output signal current.
The initial offset current, IO, of ±0.2 mA and the residual
current, IOM, (magnetic offset) of ±0.3 mA after a current
overload, are also given in the data sheet. It shall be read as
follows: When measuring a 0 A current the output can be
±0.2 mA, or ±0.4 % of IPN = ±0.2 A. In addition, after
experiencing an accidental 300 % overload (150 A) there
can be an additional ±0.3 mA of offset for a total of ±0.5 mA,
or 1.0 % of IPN = ±0.5 A.
IP =
100 A
Accuracy at 25 °C
±0.65 % of IPN
Offset drift with temperature ±0.6 mA / 40 mA
The worst-case error when measuring a 40 A current is
consequently equal to ±2.31 % • 40 A = ±0.93 A.
t2 [sec]
Accuracy at 25 °C
±0.65 % of IPN
Offset drift with temperature ±0.6 mA / 50 mA
IP = 34 A
Current transducer LTS 25-NP (see datasheet)
t1 [sec]
Fig. 18 Overload diagram of the LTS 25-NP for a rest current of
0 A, linear zone
For the LTS 25-NP transducer, the accuracy at 25 °C is given
as ±0.7 % of IPN. This breaks down as follows:
• the region at or near the nominal primary level, which is
characterized by the accuracy specification
Number of secondary turns ±0.1 % of IPN
±0.1 % of IPN
Tolerance on RIM
±0.5 % of IPN
• the region between these two, characterized by the linearity
Worst-case error
• and the region near the limit of the measuring range, where
saturation effects may begin
The internal measurement resistance, RIM, has a value of
50 Ω and a thermal drift specified as 50 ppm/K. Considering a
-10…85 °C temperature range, the maximum variation is
±0.15 Ω and the corresponding error due to the RM drift is
equal to 50 ppm/K • (85 - 25)°C = ±0.3 % of IPN.
Maximum error takes all factors into account using the worstcase parameters. Because of the statistical distribution of each
parameter it is highly unlikely that all will be at worst-case
values in a single device. Nonetheless, this is the basis of
worst-case design analysis.
Current transducer LA 55-P (see datasheet)
In this example, a DC current is measured with the LA 55-P
current transducer with a ±15 V supply.
The datasheet provides the following information: IPN = 50 A,
ISN = 50 mA, accuracy (@ 25 °C and ±15 V) is 0.65 % of IPN.
The accuracy is breakdown in several parameters, including
linearity < 0.15 %, offset error IO = 0.2 mA, and offset drift
IOT = 0.6 mA for 25…85 °C.
When a 50 A current is measured (one primary turn) the
output current is 50 mA and the errors are:
±0.7 % of IPN
In addition, the voltage output of the LTS 25-NP transducer
uses a 2.5V ‘reference voltage’ corresponding to a zero
primary current (Fig. 1). The offset on this reference voltage
(given in the datasheet) is ±25 mV at 25 °C. Considering a
-10…85 °C temperature range, the worst-case offset value
goes up an additional 100 ppm/K • (85 - 25)°C = 0.6 % to a
total of ±40 mV at 85 °C, corresponding to 1 % initial offset
plus 0.6 % drift.
Because a primary nominal current (IPN) generates an output
voltage of 625 mV at 25 °C, the error introduced by the offset
on the reference voltage is:
• error due to the standard offset ±25 / 625 = ±4.0 % of IPN
• maximum error due to the drift
±15 / 625 = ±2.4 % of IPN
Hall Effect Technologies
The worst-case measurement error throughout the
temperature range is: 0.7 + 0.3 + 2.4 = ±3.4 % of IPN
The error due to the standard offset (4%) is not taken into
account because it represents a constant deviation which
can easily be eliminated by an adequate electronic design.
3.2.12 Unipolar power supply
The vast majority of LEM closed loop transducers are specified for use with bipolar supply voltages (e.g. ±15 V). Even
so, most transducers can also be operated from a single,
unipolar, supply for the measurement of unidirectional
currents. In these cases the following must be taken into
• the supply voltage should be equal to the sum of the
positive and negative voltages indicated in the datasheet
(e.g. a ±15 V product should be powered with +30 V)
• selection of the measurement resistance and the
maximum current must not imply excessive power
dissipation in the transducer output stage; assessment of
the worst-case condition is not straightforward
3.3.1 Construction and principle of operation
Hall effect Eta transducers are similar in construction to
closed loop transducers (Figure 20), with the same magnetic
circuit geometry, a Hall generator and secondary winding.
Differences lie in the details of the magnetic core and
processing electronics designs, leading to the Eta specific
features. A Hall effect Eta transducer is a mix of open loop
and current transformer technologies providing the following
• it works as an open loop transducer at low frequencies (up
to 2…10 kHz depending on the specific transducer
design), with the Hall generator providing a signal proportional to the primary current to be measured (§ 3.1.1)
• it works as a current transformer at higher frequencies,
where the output current is proportional to the AC primary
current (§ 3.2.1)
Both the Hall effect and transformer signals are
electronically added to form a common output signal.
• the output stage is designed for use with a bipolar power
supply and diodes must be inserted in series with the
output (Fig. 19) to allow a minimum output bias voltage
without creating a measurement offset
The LEM portfolio also includes standard transducers
dedicated to unipolar operation and use of these is advised
as the electronic design and specifications are based
directly on expected operating conditions.
Applications that only require a second supply voltage for
the transducers, with the associated cost, complexity, space
and reliability concerns of this additional circuitry, would
justify the use of a unipolar transducer.
Eta technology is better suited than others when the
following performance is expected:
Figure 19: Addition of series diode(s) when operating with a
unipolar power supply
Eta Technology Hall effect current transducers
Eta is the name of the Greek symbol ‘η‘, representing
efficiency. This product family has been given this name
because of its very low secondary power requirements while
also providing exceptional performance. In terms of
performance, Eta falls between open loop and closed loop
technologies. Because the construction is similar to closed
loop, this technology offers no price advantage.
Figure 20: Hall effect Eta principle
• wide bandwidth – DC to 100 kHz or more
• low power consumption
• use of a low voltage secondary supply (e.g. +5 V)
Power consumption is minimal because the power supply is
not required to drive the secondary coil with compensation
current, which also makes them suitable for use with a low
voltage secondary supply, as they do not require the voltage
‘headroom’ of a closed loop device (§ 3.2.1).
Indeed, it is very difficult to design closed loop transducers
that will operate from low voltage secondary supplies (less
than or equal to 5 V) with primary currents exceeding 25 A.
This design issue is due to the fact that there is very limited
voltage ‘headroom’ to drive a suitable secondary coil and
measuring resistance. It is this combination of requirements
that are best address with Eta technology.
Hall Effect Technologies
3.3.2 Advantages and limitations
3.3.5 Measurement accuracy
Eta transducers are capable of measuring DC, AC and
complex current waveforms while ensuring galvanic isolation and low insertion loss. Their significant advantages
are the low power consumption and suitability for small
secondary supply voltages, such as a unipolar 5 V supply,
as with open loop transducers along with the high
bandwidth and fast response time of a current transformer.
The accuracy of an Eta transducer is dependent on the
working frequency:
These characteristics also lead to the limitations of an open
loop transducer at low frequencies; offset and gain drift with
temperature and moderate accuracy. With higher
frequencies (> 2…10 kHz) the current transformer effect
provides very good accuracy and negligible temperature
drift. In addition, the flux canceling nature of a current
transformer reduces the concern of core losses with high
frequency currents.
Refer to the related open loop Hall effect (§ 3.1.5 and
§ 3.1.10) and closed loop (§ 3.2.5 and § 3.2.10) sections for
more details on the factors affecting the accuracy and to
review calculation examples.
With Eta technology the major inconveniences are the size
of the magnetic circuit, with a large core sized for low
frequencies as with open loop transducers, and the need for
a large secondary coil for detecting high frequencies, as
compared to closed loop transducers. This typically results
in a more expensive construction than is required for a
similarly rated closed loop transducer.
3.3.3 Nominal and extreme currents
The LEM Eta transducer range is made for nominal currents
IPN from 25 to 150 A. This rather narrow range is not limited
by technical issues, but rather by the market itself: for
currents less than or equal to 25 A it is possible to work with
the higher performance closed loop technology, while for
currents greater than 150 A secondary voltage supplies with
a greater amplitude (e.g. ±15 V) are generally available,
again allowing the use of closed loop transducers. The
reduced secondary power consumption of the Eta
technology is often not a sufficient asset to promote Eta
technology beyond this current range, although there are
always exceptions.
• for low frequencies (< 2…10 kHz), the overall accuracy is a
few percent, as with open loop designs
• for higher frequencies, the overall accuracy is typically
below one percent
3.3.6 Dynamic behavior
The bandwidth, response time and di/dt behavior of Eta transducers is very close to those of the closed loop technology
(§ 3.2.7 and § 3.2.8), although slight performance reductions
may come from the use of a less efficient magnetic circuit
(material, design) for high frequency operations.
Measurements carried out on Eta transducers show a typical
bandwidth range of DC to 100 kHz or more.
The response to a current step demonstrates the ability to
correctly reproduce a transient. It is defined by several
parameters such as the delay time, rise time, reaction time
(§ 3.1.8) with a particular waveform. Eta transducers show
fast reaction time, better than 1 µs, similar to closed loop
The correct following of di/dt depends on the intrinsic
construction of each product and, as mentioned previously,
the mounting conditions of the transducer in the circuit to be
measured. Depending on the selected Eta transducer model,
it is possible to measure di/dt from approximately 50 A/µs to
400 A/µs or more. This feature makes them well suited for the
protection of semiconductor devices.
3.3.7 Typical applications
For a given Eta transducer, the maximum current which can
be measured depends on the limitations set by the open
loop (§ 3.1.3) or current transformer (§ 3.2.3) behavior, at
low and high frequencies respectively. This leads to a
maximum measurable current of typically 150 to 200 % of
the nominal current at low frequencies, and a capability
going significantly beyond that at higher frequencies.
Eta current transducers are used in numerous industrial
applications, generally as an important element of a
regulation loop (for current, torque, force, speed, position
feedback), but also for current monitoring and display.
Examples of applications are similar to the ones of open loop
(§ 3.1.9) and closed loop (§ 3.2.9) transducers.
The risk of magnetic offset after an unexpected current
overload is similar to that of open loop and closed loop
transducers (§ 3.1.6 and § 3.2.6), again at the expected
frequency of disturbance.
3.3.4 Output signal
By design an Eta transducer has a voltage output, although
the internal design could be modified or voltage signal postprocessed to provide other output types. Refer to § 2.7 for
more details.
Closed loop Hall effect voltage transducers
Previous sections address the measurement of current with
galvanic isolation, but it is also possible to measure a primary
voltage with galvanic isolation using the same LEM
technology. These voltage transducers are based on the
more sensitive and accurate current measurement
technologies, such as closed loop Hall effect designs.
The main difference from a current transducer is the addition
of an internal primary winding with a large number of turns,
Hall Effect Technologies
allowing the transducer to create the necessary ampere•turns
to measure the small primary current.
3.4.1 Construction and principle of operation
The operating principle of closed loop Hall effect voltage
transducers is to measure a small current that is directly
proportional to the voltage of interest. Dividing the voltage to
be measured by a large resistance, R1, creates a small
current, Imes, that can be measured by an ‘optimized’ transducer (Fig. 21), enabling it to accurately measure the small Imes
current, while also having controlled insertion impedance to
maintain suitable accuracy and measurement bandwidth.
The R1 resistance is added in series with the transducer
primary coil to obtain the optimal Imes current value for the
nominal voltage level. This resistance is often split into two
resistors, placed on both sides of the primary coil, to improve
common mode rejection.
R1 / 2
R1 / 2
Figure 22: Equivalent diagram for the R1 primary series resistance
3.4.3 Voltage transducer without internal resistor
Figure 21: A basic principle for voltage measurement
Two situations exist with the LEM voltage transducer
• the series resistance, R1, is built into the LEM voltage
transducer providing optimal performance when used at or
near the product ratings, as with the LEM LV 100-100
• the series resistance, R1, is external, allowing the
customer to tune the transducer working voltage range
and/or response time, as with the LEM LV 100 and LV 200
The equivalent electric diagram of a voltage transducer
primary part is show on Fig. 22, including the serial
resistance R1, the primary coil resistance RP and the primary
coil inductance LP. The latter generally creates a negligeable
insertion reactance (ωLP), in most cases neglected.
Example 1: LV 100 closed loop voltage transducer with
external R1 resistor (see datasheet)
a) What external resistance, R1, is required to measure a
nominal voltage of VPN = 230 VAC?
The LV 100 datasheet provides the following information:
nominal current, IPN = 10 mA, measuring range, IP = 20 mA,
primary coil resistance, RP = 1900 Ω at TA = 70 °C. We
R1 = VPN / IPN - RP = (230 / 0.01) - 1900 = 21.1 kΩ
We select the nearest standard resistance value of 21.0 kΩ
The total resistance of the primary circuit is consequently
equal to 22.9 kΩ at 70 °C.
The nominal power PN dissipated in R1 is:
PN = IPN2 • R1 =
+ RP
) •R
= 0.012 • 21000 = 2.1 W
In order to avoid excessive thermal drift of the resistor, and
to improve reliability, the user would preferably select a
resistor with a power rating of 300 to 400 % of the
calculated nominal power. In this example, a 21.0 kΩ, 8 W
resistor should be considered.
b) With this resistor, can we measure a maximum voltage
of 500 V ?
3.4.2 Voltage transducer with internal resistor
The measurement of a voltage higher than the nominal
value is possible during transient operation, depending on
the following two conditions:
For the „LV 100-[voltage]“ transducers (e.g. LV 100-100) the
series resistance, R1, is built into the transducer and cannot
be changed by the user. This design has a 10 W input power
at nominal voltage. The internal series resistor allows factory
calibration of the transducer at the specified nominal
voltage, implying better accuracy.
• the RMS value of the primary voltage shall be such that
it keeps the transducer current below nominal value
(10 mA for the LV 100); in our example, if this rule is not
fulfilled at 500 V with the selected 21.0 kΩ resistance, a
higher resistance value must be considered
The measuring range is limited to 150 % of the nominal
value, except for transient measurements where the RMS
transducer input power remains below the specified 10 W.
• the primary current at the higher voltage should normally
be lower than the transducer specified ‘measuring
range’, 20 mA for the LV 100; we have: IP = (VP/RPtot) =
500/22900 = 21.8 mA and are therefore beyond the
Hall Effect Technologies
specified measuring range; a check on the transducer
secondary side is required to see if this transient
measurement is nevertheless possible
For the latter point, as discussed in § 3.2.10 – example 2,
it is sometimes possible to measure a current beyond that
specified in the transducer datasheet, more precisely
when the measurement resistor is not exceeding a
maximum RMmax value. In this example, we have:
• the conversion ratio of the transducer is 10000 / 2000,
giving a secondary current of IS = 5 • 21.8 = 109 mA
• the voltage available at the amplifier output is (Fig. 12)
VA = (RS + RMmax) • IS = (60 + 150)Ω • 50 mA = 10.5 V
For the resistance R1, typical values for the temperature
drift and tolerance are 50ppm/°C and ± 0.5% respectively.
The minimum value becomes:
R1 = 21’000 • (1 - 50 • 50 • 10-6) • (1 – 0.005) = 20’942 Ω
The primary resistance RP is made of copper and exhibits
a resistance change with temperature as follows:
RP20 (1 + α • ∆T) where
RPF : resistance at Final temperature (e.g. +70 °C)
RP20 : resistance at +20 °C
α : copper temperature coefficient = 0.004 K-1
∆T : change in temperature (Final – 20 °C), positive
when above +20 °C
• the maximum measurement value is
RMmax = (VA / IS) -RS = (10.5 / 0.109) – 60 = 36.3Ω
Note that the temperature of the RP copper is likely to be
higher than the maximum ambient temperature due to
internal transducer heatings.
In conclusion, measuring a 500 V transient is possible if a
measurement resistance lower than 36.1 Ω is used.
In our case:
c) Measurement accuracy: influence of the serial resistance
R1 and primary resistance RP
The measurement principle discussed in § 3.4.1 is based
on the measurement of a current Imes (Fig. 21) proportional
to the voltage to be measured. The two main factors
affecting the measurement accuracy are:
• The accuracy of the measurement of the collected
current Imes
• Unexpected changes of the ratio between the collected
Imes current and the voltage to be measured.
Thus, changes of the resistances R1 and RP will affect the
measurement accuracy, since it will change the value of
the Imes current.
The example below calculates the impact on accuracy of
a change in resistances, due to both resistance tolerances
and temperature effects. A LV 100 voltage transducer is
considered, with a +20°C to +70°C worst case working
Reference case
The reference value of the current to be measured, Imes, is
defined as the value obtained at +70°C when both
resistors R1 and RP have their nominal value. These
values are R1 = 21.0 kΩ (as calculated in point a), at
+70°C) and RP = 1.9 kΩ (data-sheet, measured value).
The collected Imes current is then equal to 230 V / (21 +
1.9) kΩ = 10 mA
Maximum error
The value of the resistances are smaller at +20 °C when
compared to +70 °C. The worst case shall additionally
consider the tolerance on the resistances' value when at
their minimum.
RP20 = 1’900 / (1 + 0.004 • 50) = 1’538 Ω
Collected current become equal to Imes = 230 V / (20’942 +
1’538) = 10.21 mA, corresponding to a +2.1 % error
compared to the reference value.
The measurement accuracy of the Imes current has still to
be added to this 2.1 % error.
d) What is the accuracy of the transducer, converting the
primary current into an isolated output signal ?
In this case, we assume that the resistor R1 has been
selected in order to have a 10 mA primary current IPN.
According to the LV 100 datasheet, the accuracy of the
current measurement at 25 °C is ±0.7 % of IPN. The
thermal drift of the offset current is ±0.3 mA max. With a
conversion ratio of 10000:2000, the input current of
10 mA generates an output current of 50 mA. The values
of the errors are:
Accuracy at 25 °C
±0.7 % of IPN
Temperature offset drift ±0.3 mA/50 mA
±0.7 %
±0.6 %
Maximum error of the current measurement: ±1.3 %
e) What is the total measurement error?
The effects listed in c) and d) above provide the total
measurement error of the output current of the transducer.
Typical applications include a measuring resistance, RM,
to convert this output into a voltage for measurement.
Assuming typical parameters for RM, we have an intial
tolerance of ±0.5 % and a 50 ppm/K temperature drift,
giving a 0.225 % variation for a 20 °C to 70 °C
temperature range. The total RM error becomes 0.5 +
0.225 = 0.725 %.
For the LV 100 transducer, the total measurement error at
the 230 V nominal, considering a temperature range of
20 to 70°C, is the combination of the effects in c) and d)
and RM above:
total measurement error =
2.1 % + 1.30 % + 0.725 % = 4.125 %
Hall Effect Technologies
Example 2: Design for continuous measurement of
1000 V nominal
Select the value of the external primary resistance R1 to
continuously measure a voltage of VPN = 1000 V nominal
and determine the corresponding measurement accuracy.
a) Calculation of the primary resistance
Again using the LV 100 datasheet values: nominal
current, IPN = 10 mA, measuring range, IP = 20 mA,
primary coil resistance, RP = 1900 Ω at 70 °C or RP =
1615 Ω at 25 °C. We have: R1 = (VPN/IPN) -RP = (1000/
0.01) - 1615 = 98 385 Ω.
The nominal power, P1N, dissipated in R1 is:
P1N = IPN2 • R1 = 0.012 • 98385 = 9.8 W and, as already
discussed, it is suggested to use a resistor with 300 to
400 % of this power rating: a 40 W resistor.
The total resistance, RPtot, of the primary circuit is:
RPtot = RP + R1 = 100.3 kΩ
b) Voltage measurement accuracy: effects of the series
resistance, R1, and coil resistance, RP
Following the same methodology as in example 1c
above, we have a maximum R1 value of:
R1max = 98 385 Ω • 1.005 • 1.00225 = 99 099 Ω
Conclusions based on these two examples
The use of the LV 100 transducer for the measurement of a
230 V or a 1000 V nominal voltage leads to a worst-case
measurement accuracy of 3.84 % or 2.97 %, respectively.
The measurement accuracy is significantly better for larger
voltages because the variation of the primary coil resistance
is less significant in comparison to the primary total
resistance (R1 + RP) due to the larger series resistor value.
To obtain better accuracies when measuring low voltages
one must select transducers with a larger conversion ratio.
This will, however, require higher power dissipation in the
primary circuit and thus increase the power taken from the
source being measured. A second benefit of considering a
larger conversion ratio (smaller number of primary turns) is
the resulting larger bandwidth (reduced inductance value L
in R / L time constant limitation).
Finally, if needed, the calibration of the output signal of a
voltage transducer can be made by either tuning the
external resistor value, R1, or, as with current transducers, by
adjusting the measuring resistor, RM.
3.4.4 Transducer Output
The output of a closed loop voltage transducer is identical to
that of a closed loop current transducer. Therefore, the
methodology previously described in § 3.2.4 and § 2.7 for
the selection of the transducer measuring resistor and output
voltage also applies to these devices.
and a minimum value of:
R1min = 97.6 kΩ • 0.995 • 0.99875 = 97 771 Ω
3.4.5 Typical applications
The maximum value of RP at 70 °C is 1.9 kΩ while the
minimum value, at 20 °C, is equal to:
RP70 °C
RPmin =
1583 Ω
(1 + 0.004 • 50)
Closed loop Hall effect voltage transducers are used in
many industrial applications to detect, monitor and regulate
voltages. A typical application is the monitoring of input,
output and DC filter voltages of frequency inverters, where
accuracy and isolation are of primary importance.
The primary resistance calculations are the same so the
value of the total resistance, RTOT, is between
97 771 + 1583 = 99 354 Ω and
99 099 + 1900 = 100 999 Ω
The range of primary current collected at nominal voltage
1000 V / 100 999 Ω = 9.90 mA to
1000 V / 993 54 Ω = 10.07 mA
This can be interpreted as a value of 9.985 mA ± 0.085
mA, corresponding to a ± 0.85 % tolerance.
c) The current measurement accuracy is identical to the
calculation in example 1d before, namely 1.3 %.
d) For the LV 100 transducer, the total measurement error
for a 1000 V nominal voltage over the temperature range
20 to 70 °C, making the same RM assumptions as in
example 1e is 0.85 % + 1.3 % + 0.725 % = 2.875 %.
Other Hall effect voltage transducers
Actually, most current transducers technologies can be
modified into a voltage transducer by ensuring the small
measurement current (proportional to primary voltage) is
applied to a large number of primary turns to create the
Ampere-turns necessary for measurement. This is commonly
made with Fluxgate technologies (§4.4.4) but less typical
with open loop and Eta Hall based transducers.
Fluxgate Technologies
Fluxgate technologies
The Fluxgate technologies discussed in this chapter cover
several types of isolated current and voltage transducers
based on the same basic measurement principle: the
magnetic field created by the primary current to be
measured is detected by a specific sensing element. The
latter is driven through its B-H loop by a dedicated electronic
and the resulting magnetic effects are used for primary
current detection. There are a wide variety of methods for
concentrating the field, driving the magnetic core, and
sensing the field intensity, but in all cases the underlying
working principle is the same.
Working principle of Fluxgate technologies
To start, the ‘standard’ Fluxgate design will be investigated
as this has a construction similar to the closed loop Hall
effect current transducer already described (§ 3.2.1). After
this the Fluxgate principle should be understood and other
topologies can be covered, highlighting their specific
characteristics and performances.
4.1.1 „Standard“ Fluxgate – working principle
An isolated Fluxgate current transducer (Fig. 23) can be
designed in the same manner as a Hall effect based closed
loop transducer (§ 3.2.1), using the same magnetic circuit
including a gap and secondary winding. The secondary
winding is driven to provide zero flux in the gap as sensed
by the Fluxgate element, rather than a Hall generator.
Magnetic core
Fluxgate Sensing head
L = f(Bext; Isi)
0.5 mm
Figure 24: Fluxgate magnetic sensing head (saturable inductor)
As with any inductor, the value of inductance of the „saturable
inductor“ depends on the magnetic permeability of the core.
When the flux density is high, the core is saturated, its
permeability low, and the inductance high. At low field
density, the inductance is high.
The „saturable inductor“ is purposely designed so that any
change in the external field, BEXT (Fig. 24), affects its saturtion
level, changing its core’s permeability and consequently its
inductance. Thus, the presence of an external field change
the inductance value of the field sensing element. This
change can be very pronounced if the saturable inductor is
adequately designed. The second factor affecting this
inductance is the current, ISI, injected into the coil of the
saturable inductor (Fig. 24). This current produces a flux,
channeled into the magnetic core, resulting in an additional
magnetic field components, BSI. The saturable inductor is
generally designed in a way that BEXT and BSI have the same
order of magnitude, both affecting the inductance value.
With the „standard Fluxgate“ design (Fig. 25), the primary
current creates an airgap flux, ΦP (corresponding to BEXT),
added to the flux ΦSI created by the saturable inductor current
ISI. The addition of these two flux creates an over saturation of
the saturable inductor’s core and its inductance drops. When
the polarity of the ISI current is reversed, the flux ΦP and ΦSI
are substracted, giving a lower total flux when ΦSI is smaller
than Φp, a zero total flux when ΦP and ΦSI are equals, or a
reverse total flux when ΦSI is larger. The latter case lead to a
strong core saturation when ΦSI is much larger than ΦP. The
design of the saturable inductor shall provide high inductance
near zero total flux and low inductance under saturation.
Isi (positive)
Figure 23: Structure of the „standard Fluxgate“
The main difference between the closed loop Hall
technology and the Fluxgate is on the way the airgap field is
detected: by a Hall cell in the first case, and by a so-called
„saturable inductor“ (Fig. 24) in the second. This implies a
drastic change of the transducer electronics, to drive the
sensing element and to process the resulting signal.
The Fluxgate sensing element is intentionally a „saturable
inductor“ (Fig. 24) made of a small, thin magnetic core with a
coil wound around it. It is generally made of discrete pieces
of material (lamination sheet & copper wires), but various
designs can be considered, including advanced concepts
based on MEMS technologies (not yet commercially viable).
Φ si
Figure 25: Standard Fluxgate - gap flux distribution – additive flux
In conclusion, changes in the magnetic saturation of the
Fluxgate sensing head leads to inductance variations to be
detected by the processing electronics (Fig. 26). The closed
loop principle is then used, where variations in inductance
Fluxgate Technologies
Small inductance
current iSI (t)
created by the primary current, IP, can be detected and
compensated using the closed loop principle, feeding a
current into the transducer’s secondary coil, IS, to return the
gap field to zero and thus inductance back to a reference
case. The relation between primary and secondary current is
then simply given by the primary to secondary turns ratio
(§ 3.2.1). The next paragraphs details the methodology
followed to detect inductance variations.
L = L (IP = 0)?
lS = IP /NS
Figure 28: Current response to a voltage step – constant inductance
Figure 29 again shows the current response to a voltage
step, but in this case with a saturable inductor, where the ISI
current increase lead to an increase or decrease of the
inductance value, depending if the field BSI (created by the
same ISI current) is added or substracted from the external
BEXT airgap field. When those fields are for example added,
an ISI current increase leads to an increase of the saturation
level, implying a decrease of the inductance value and thus
a faster current variation.
Adjusts IS
Figure 26: Principle of the „standard Fluxgate“
4.1.2 Sensing head – a current response to a voltage
The electric diagram of the saturable inductor is given in
Fig. 27. The resistance represents the copper resistance of
the inductor coil, constant at a given temperature, while the
inductance is variable, depending on both the external field
applied on the saturable inductor and the injected i(t) current
current iSI (t)
Since the Fluxgate working principle is based on the
detection of an inductance change, it is necessary to first
have a good understanding of the electric behavior of the
saturable inductor. Thus, before analyzing the way
inductance changes are detected (§ 4.1.3), lets first see what
is the current response of the saturable inductor when a
voltage step is applied.
Figure 29: Current response to a voltage step – saturable inductance
In practice, Fluxgate sensing heads are designed so that the
current response to a voltage step is very „sharp“, as shown
in Figure 30 (BEXT = 0), where the current behaves in three
different stages: (1a) For small current values, the current
variation is slow, the saturable inductor being designed with
a very high inductance when non-saturated; (2a) When the
current exceeds a pre-defined level, its variation becomes
very fast due to a sudden drop of the inductance value (use
of magnetic materials with a sharp magnetic characteristic);
(3a) The current reaches the asymptotic level defined by the
excitation voltage and coil resistance.
Figure 27: Electric diagram of the sensing head inductor
If the inductance of the sensing head were constant, the
current response to a voltage step would be as shown in
Fig. 28, given for three different inductance values: small,
medium and large. The initial slope is defined by di/dt = V / L
for t=0, giving lower slopes, or slower current responses,
with higher inductances, while the final value is always the
same and is defined by i = V / R.
This sharp profile is desired because it acts as a „closed“ or
„open“ gate for the current, restraining or allowing the
current flow depending on the saturation level of the core.
This behavior, of the core flux controlling the current „gate“,
is the origin of the name Fluxgate.
current iSI (t)
Fluxgate Technologies
BEXT = 0
BEXT = 0
Assuming the loop is not closed, and compensating secondary
current is not available, Fig. 32 shows the current behavior
when a primary current (or external field BEXT) is present, as
described in Fig. 30. The current almost immediately goes to a
non-zero positive value, then stays relatively stable until the
core saturates in the other direction. Again, the voltage polarity
changes prior to reaching the asymptotic current level.
Compared to the Figure 31 current, the main differences are
on the peaks amplitude (comparing the peaks above and
below the DC value of the signal) and the appearance
of a DC current component.
Figure 30: Current response to a voltage step – Fluxgate sensing
head (BEXT set at a parameter)
In the case where the external field BEXT is non-zero, the
current response appears as in Fig. 30, where the current
behaves in four different ways: (0b) For small ISI current
values, the external field BEXT is pre-dominant and drives the
core into saturation, leading to a low inductance and a fast
current change; (1b) As the inductor current ISI reaches a
level where it creates a BSI field equal and opposite to the
BEXT field (total core flux equal to zero), the saturation level is
low, the inductance high and the resulting current variation
slow; (2b) As the inductor ISI current continues to grow, the
BSI field becomes pre-dominant and drives again the
inductor core into opposite saturation, resulting in an
inductance drop and a fast current variation; (3) the current
reaches the asymptotic level defined by the excitation
voltage and coil resistance.
4.1.3 Detecting the sensing head inductance variation
As mentioned before (Fig. 26), the Fluxgate working
principle is based on the detection of an inductance change.
This change can be detected in different ways, the most
common being described here.
Looking at Fig. 31, the saturable inductor is driven with a
square wave voltage, u(t), leading to the sensing head
current, i(t), when no primary current IP (or no external field
BEXT) is applied. This current shape is directly related to the
behavior shown in Figure 30, with the applied voltage
changing polarity prior to reaching the asymptotic current
Figure 32: Voltage steps and current response (IP = 0)
The polarity of the applied voltage can be triggered to
change by different parameters (e.g. when the current
reaches pre-defined trigger levels) or by having a fixed
Different techniques are then used to sense the change of
inductance (Fig. 26), leading to various complexities and
performances. The most common are (1) to measure the DC
current component of Fig. 32; (2) to perform a spectral
analysis of the Fig. 32 current and measure the amplitude of
a selected current harmonic, the one which is the most
sensitive to current shapes changes (generally the second
harmonic order), or (3) to measure the duty-cycle of the
Fig. 32 voltage. Once the changes of these signals are
detected, the closed loop principle of Fig. 26 is used to
compensate the flux in the gap.
4.1.4 Current transformer effects
As with a closed loop Hall effect transducer (§ 3.2.1), the
secondary coil of Fig. 26 acts as a current transformer to
measure high frequency currents. Depending on the type of
Fluxgate considered, this is not always possible and, in
those cases, the result is a reduced bandwidth and limited
response time.
Figure 31: Voltage steps and current response (IP = 0)
Fluxgate Technologies
• A large dynamic range, allowing measurement of both
small and large currents with the same transducer
Existing types of Fluxgate transducers
The main types of Fluxgate transducers are shown in
Figure 33 and briefly described here:
• Very high resolution, provided by the low offset. See
comment on the noise hereunder.
(1) The „standard“ Fluxgate (§ 4.1), similar to a closed loop
Hall effect design
• Large temperature range, the low offset drift make
Fluxgate technologies suitable for broader operating
temperature ranges (still limited by the transducer
materials and component limits)
(2) The „C-type“ Fluxgate (§ 4.4), where the performance is
significantly improved by:
• Making the field sensing element with the entire
toroidal core, without a gap
• Ensuring high frequency performance by using a
separate core for the transformer effect
(3) The „IT-type“ Fluxgate (§ 4.5), where the performance is
improved a step further by:
• Duplicating the field sensing element, using two
toroidal cores with opposing excitation coils
• Improving the design of the high frequency current
transformer and processing electronics
(4) the „Low frequency“ Fluxgate, using only the low
frequency part of the „C-type“ Fluxgate transducer, not
considering the current transformer, to have a cost
effective and efficient transducer for low frequencies
• High bandwidth – fast response time, provided by the
current transformer effect (when relevant) which is further
enhanced on C-type and IT-type Fluxgates.
The technology has in general the following limitations:
• Limited bandwidth for the simpler designs
• A large noise level at the excitation frequency, present at
the output and possibly coupled into the primary
• Voltage noise injection into the primary lines (acceptable
for the vast majority of applications)
• Relatively high secondary power consumption, similar to
that of closed loop Hall effect transducers
• More efficient designs are more complex and thus more
expensive to produce
• The design of Fluxgate transducers is relatively complex
and makes for more difficult customization
More details are given in the following sections outlining the
details of specific Fluxgate designs.
Figure 33: Main types of Fluxgate transducers - field sensing
element in dark color
General performance of Fluxgate technologies
In general, Fluxgate technologies offer the following
• Low offset & offset drift, because the magnetic core is
cycled throughout its B-H loop suppressing any magnetic
offset in the Fluxgate core (not avoiding offset or offset
drift of the processing electronics or, for the „standard“
Fluxgate, the magnetic offset created by the main toroid).
„C-type“ Fluxgate transducers
„C-type“ closed loop Fluxgate transducers are a significant
part of the LEM current and voltage product portfolio. This
technology was developed in co-operation with the
University of Auckland - New Zealand (Prof. Dan Otto) and
provides very high performance in terms of accuracy,
temperature drift, bandwidth and response time. This high
performance is the result of a patented design used for the
compensation of Ampere-turns.
4.4.1 Construction and principle of operation
• Excellent accuracy due to the quasi absence of offset.
Compared Hall based technologies, this advantage is
more noticeable for small currents measurements, where
the relative effect of the offset is more significant
The basic working principle and performance of a „standard“
Fluxgate has been discussed in § 4.2 and § 4.1. An understanding of this device is the foundation for a complete
description of „C-type“ operation, provided here.
• Excellent over-current recovery, again because any
permanent magnetization of the field sensing element is
reset with subsequent B-H cycles (and again, not
affecting the main toroid of the „standard“ Fluxgate)
A „C-type“ Fluxgate transducer (Fig. 34) is made with two
identical magnetic cores (T1 and T2), each wound with an
equal number of secondary turns, NS. The primary winding,
NP, is common to both cores. The two secondary windings
are connected in series and the shared center point is also
connected to the electronics.
• Much higher sensitivity than other technologies, allowing
the measurement of very low Ampere-turns (determined
by design and dependent on the required magnitudes of
One core, T1, uses the Fluxgate principle to measure the
lower frequencies (T1 being the so called Fluxgate sensing
head), while the other, T2, acts as a current transformer for
Fluxgate Technologies
higher frequencies. Both secondary windings of T1 and T2
also acts as the zero flux compensation winding, ensuring the
transducer works in a closed loop mode.
This design has the ability to supply a single coil (secondary of
T1) with the „Fluxgate“ current (Fig. 32 and Iµ in Fig. 34) as well
as the secondary current (IS in Fig. 34) required for flux
compensation. The processing electronics then removes the
Fluxgate current in a „filter“ (ref. 2 in Fig. 34) to prevent noise
at the transducer output. The output current (point C of
Figure 34) is then equal to IS = IP • NP/NS. Finally, for a wide
operating temperature range, the electronics are designed to
automatically compensate the electronic offsets and voltage
drops, eliminating the need for adjustments.
„C-type“ transducers internally control the loading of the
secondary current and therefore have a voltage output.
• Very high isolation levels and excellent resistance to
partial discharge (e.g. CT 5-T/SP3 transducers have
50 kVrms isolation with a partial discharge extinction level
of 14.5 kV with < 20 pC)
The limitation of the CT transducer is the injection of a
rectangular voltage ripple on the primary line. This is caused
by the Fluxgate excitation voltage, typically at a frequency of
500 Hz, and is a function of the primary to secondary turns
ratio. This primary voltage ripple induces a current noise,
with an amplitude dependent on the primary circuit
impedance; the lower the impedance the higher the current
noise (see calculation examples in § 4.4.6b). While this
current noise is generally not a problem, it is measured by
the transducer and will be seen as an additional
measurement error.
IS +Iµ
Control Loop
„CD-type“ differential current transducers
A LEM „CD“ transducer is designed to measure differential
currents, the difference between two primary currents
flowing in opposite directions. This application takes
advantage of the very high sensitivity of this topology,
allowing measurement of a differential current that is only
0.1 % of the main current flowing in each primary conductor.
For example, a differential current of 1 A can be measured
with a main current of 1000 A, or 0.1 A with a main current of
100A. The measurement accuracy is about 5 to 10 % within
the specified operating temperature range.
The main advantages are:
• Very good differential resolution (able to measure low
differential currents with high main currents)
Figure 34: Block diagram of the C-type Fluxgate transducers
4.4.2 „CT-type“ current transducers
Please note that „CT“ is a LEM product designation and is not
used as an abbreviation for current transformer.
The LEM „CT“ current transducers measure currents up to a
maximum of 150 A. They typically have an accuracy of 0.1 %
and a remarkable 0 to 500 kHz bandwidth.
The main advantages are:
• Possibility to have an external adjustment by the user of
the level of the differential current to be measured (a
special design provides external terminals in the housing,
where an adjustment resistor is connected)
• Special designs allow the adjustment of the time constant
of the measured differential current, which is convenient
when the output signal of the transducer is used as a
trigger in a safety system
• Options to define several levels of differential currents to
be measured (the transducer is then designed with a
separate individual output for each current level)
• Protection against primary current overloads
• Excellent accuracy across the entire operating temperature
The main limitations are:
• Wide bandwidth
• Limited frequency bandwidth, typically DC to 2kHz
• Extremely short response time
• A differential current measurement should theoretically be
independent of the main current value, but the main
current must never exceed the transducer rated value to
avoid permanent local saturation affects, due to both
internal and external magnetic effects, which will reduce
the accuracy of the transducer
• Excellent immunity to surrounding magnetic fields
• High overload current capability (e.g. a CT 1-S transducer,
with a 1 A nominal current, can withstand a overload above
15 kA for 150 ms)
• Output short circuit protection
Fluxgate Technologies
4.4.4 „CV-type“ voltage transducers
The „CV“ family of transducers measure voltages up to 7 kV.
The typical accuracy is 0.2 % and 1 % for the CV3 and CV4
series respectively. The bandwidth is from DC to a maximum
frequency ranged between 10 kHz and 800 kHz, depending
on the selected transducer reference. CV voltage transducers
generally include the primary resistor as this value is tuned
for optimum performance. Because of the high sensitivity of
the „C-type“ design the required primary ampere-turns is
small, leading to a lower primary inductance and resistance,
improving accuracy, bandwidth, and response time.
Their main advantages are:
• Excellent accuracy over a broad operating temperature
• Measurement of the heating current in the cathode of a
KLYSTRON accelerator.
CD differential current transducers
• Measurement and detection of earth leakage currents.
• Replacement of the classic differential relays, with a better
accuracy and the detection of much smaller currents.
• Measurement of differential currents, as a safety function
in electric traction equipment.
CV voltage transducers
• Measurement of AC voltages in high power industrial
• Voltage measurement in electric traction converters (DC
and AC).
• Low primary power consumption
• Wide frequency bandwidth and fast response time
• Voltage measurement between phases of power cycloconverters.
• Excellent fast dv/dt measurement capability
• Calibration benches for power converters and motors.
• Very good immunity against surrounding magnetic fields
• Voltage measurement in photovoltaic plants (precise
measurement of the maximum power point).
• Very good immunity against common mode voltage
The limitation of most of the CV transducers is the dielectric
withstand voltage of 6 kVRMS, with a partial discharge
extinction level of 2 kVRMS with < 10 pC. The CV4 transducer
has been designed with an extended isolation capability.
4.4.5 „C-type“ transducers - typical applications
„C-type“ transducers are used in industrial applications
requiring very high accuracy, for example calibration units,
diagnosis systems, test platforms and laboratory equipment.
It is also appropriate when the application needs an absolute
robustness of performance with temperature changes.
CT current transducers
• Current measurement in transmitters.
• Measuring the magnetizing current and DC current in
power transformers, avoiding unexpected magnetic
saturation (industrial equipment and electric traction).
• Current measurement in induction heating systems.
• Measurement of charge and discharge currents for battery
• Laboratory measurement instruments: isolated voltage
measurement, power measurement for inverters, as an
interface with a power analyzer, etc.
4.4.6 Calculation of the measurement accuracy & noise
The following examples show the high performance of the
„C-type“ transducers.
Example 1: Accuracy of the CT 100-S current transducer
(see datasheet)
In this example a 100 A DC current is measured and,
according to the transducer datasheet, the output voltage
will be 5 V. Using this part within its operating temperature
range of 25 to 70 °C the indicated accuracy is ±0.15 %,
including an initial offset of maximum ±0.4 mV, and the
temperature drift of the offset voltage can be ±0.6 mV.
Offset drift with temperature
±0.15 % of IPN
±0.6 mV/5 V
Worst-case error
±0.15 %
±0.012 %
±0.162 %
This worst-case error is expressed as a percentage of the
nominal value
• Calibration benches for power converters and motors.
• Current measurement in the electric energy distribution
simulators and substations.
Example 2: Ripple rejection and the CT 5-T current
transducer (see datasheet)
• Current measurement in photovoltaic plants (precise
measurement of the maximum power point).
This example demonstrates the ripple rejection in the
primary circuit of a CT 5-T transducer. We have:
• Laboratory instruments: isolated current measurement for
use with an oscilloscope or a digital multi-meter; power
measurement for inverters, as an interface with a power
analyzer, etc…
• The internal square-wave generator (Fig. 34 - ref. 1)
provides a voltage of USW = ± 6.8 V
• Number of primary turns is NP = 10
Fluxgate Technologies
• Number of secondary turns is NS = 1000
4.5.1 Construction and principle of operation
• I / V converter resistance (Fig. 34 - ref. 4) is RC = 100 Ω
If the primary circuit impedance is ZP = 100 Ω, the induced
primary current becomes:
IPind = VSW • NP / (NS • ZP) = 6.8 • 10 / (1000 • 100) = 0.68 mA
The voltage induced in the secondary by this parasitic
current is:
VSind = IPind • (NP/NS) • RC = 0.68 mA • (10/1000) • 100 = 0.68 mV
For a primary current giving a 5 V output signal, the
corresponding measurement error is 0.68 mV/5 V = 0.014 %.
The „IT-type“ transducer consists of a current measuring
head made of three magnetic cores, C1, C2, and C3, with
primary (wP1) and secondary (wS1 to wS4) windings as shown
in Fig. 35. Closed loop compensation is obtained by a
secondary current, IC, fed into one of the secondary coils
(wS2). The latter coil, magnetic coupled to the three magnetic
cores, is connected in series with the measuring resistor to
obtain an output voltage of typically 1 V with secondary
currents of 200 or 400 mA.
Low frequency
If, on the other hand, the primary impedance is small, for
example ZP = 1 Ω the measurement error becomes much
more significant, 1.4 % in this case.
High frequency
Example 3: CV voltage transducer – Accuracy
+25 °C
Typical accuracy of the primary
TA max
+70 °C
IP & wP1
Square wave
Zero flux
Output voltage
0.05 %
0.05 %
0.10 %
Variation with temperature
(typical coefficient 20 ppm/°C)
Figure 35: Principle of the IT transducer
For the upper frequency range, the secondary current results
from a transformer effect in two secondary coils (w 1 and w 2).
For lower frequencies, including DC, the design functions as
a closed loop Fluxgate transducer (§ 4.1), with the winding
wS3 and wS4 being the Fluxgate sensing coils. These features
are described in details hereunder.
Typical resistance accuracy
of the converter
0.05 %
0.05 %
0.10 %
Variation with temperature
(typical coefficient 20 ppm/°C)
Secondary offset voltage:
- nominal is 5 mV
- Maximum with temperature
is 10 mV
Maximum total error Transformer effect – high frequencies
0.10 %
0.20 %
0.20 %
0.5 %
Example 4: Accuracy of the CV 1-1500 voltage
transducer (see datasheet)
The CV 1-1500 voltage transducer is designed to measure a
1500V peak voltage. According to the transducer datasheet,
the output voltage will then be 10 V peak. Using this part
within its operating temperature range of 25 to 70 °C the
indicated accuracy is ±3. 0% including an initial offset of max
±100 mV. This worst-case error is expressed as a
percentage of the peak value.
„IT-type“ Fluxgate transducers
LEM „IT-type“ closed loop Fluxgate transducers allow AC
and DC current measurement with very high accuracy,
linearity and stability, while eliminating the injection of noise
into the primary.
The current transformer is made with a primary (wP1) and two
secondary coils (wS1 and wS2), wound around the same
magnetic core (C1). The secondary coils are connected to a
compensation amplifier in order to improve the overall
performances. This system works as follow:
• The coils wP1 and wS2 work as a classical current
transformer, where a current IC is induced into the
secondary coil, proportional to the primary current to be
measured, leading theoretically to a zero total flux into the
magnetic core.
• In practice, a residual flux remains, reflecting the imperfect
coupling between the primary (wP1) and secondary (wS2)
coils as well as the transformer burden. For a current
transformer, this residual flux is an image of the
measurement imperfections.
• The coil wS1, connected on a high impedance, collects a
non zero voltage when an AC residual flux occurs: a signal
is then sent into the correction amplifier, adjusting the
secondary current IS until the total flux is equal to zero.
The resulting current transformer performances are
Fluxgate Technologies
I µ+
iµ+ (t)
Second harmonic detector
iµ- (t)
Low frequency
Figure 37: Iµ+ and Iµ- currents
IT transducer working principle
Figure 36: Block diagram of the symmetrical zero flux detector Fluxgate detector – low frequencies
The Fluxgate sensor (Fig. 35) is made of two magnetically
symmetric parts (two saturable inductors), each of them
made with one magnetic core (C2 or C3), one primary
winding (wP1), one compensation winding (ws2) and one
secondary winding (wS3 or wS4).
Close-loop not active
At low frequency, assuming in first instance that the zero flux
compensation created by wS2 is inactive, the flux ΦP created
by the primary current is identical in both C2 and C3 cores
(Fig. 35), with the same magnetic polarity.
The secondary windings are connected to a square-wave
generator that supplies the Iµ currents (Fig. 36), bringing the
core C2 and C3 into saturation. The resulting Iµ currents
have a shape as shown on Fig. 31 and Fig. 32.
Due to the magnetic construction and electric connections of
wS3 or wS4 windings, the fluxes created by the Iµ currents into
the C2 and C3 cores have an opposite magnetic polarity
(±Φµ). The total flux in the C2 and C3 core are consequently
different, since at a given time the flux Φµ and ΦP are
additive in C3 and subtractive in C2 (Fig. 35). The level of
saturation in the two cores being different, the shape of the
Iµ+ and Iµ- currents will be at any time different, including
differences on the DC value and when the current ‘peaks’
(or saturation) occur. Assuming a frequency of the Iµ currents
significantly larger than the primary current frequency, the Iµ+
and Iµ- currents have typically the shape shown on Fig. 37.
In term of noise, the fact that the switched currents (Iµ+ and Iµ-)
create fluxes with an opposite polarity (± Φµ) explains why
the conducted noise created by the IT transducer is very
In normal working conditions, the close-loop coil wS2 is
active and the principle can be further described as follow :
When there is no primary current, the system is magnetically
symmetric and the Iµ currents are identical, with a shape
looking like that shown in Fig. 31.
An increase of the primary current has the following
consequences (Fig. 37):
• It creates a DC current component on the Iµ+ and Iµcurrents, positive or negative respectively.
• The „AC“ behavior of the two Iµ currents different, with
peaks with large and small amplitudes not occurring at
the same time.
The IT transducer only monitors the differences in the Iµ
currents shapes, reflecting a change in the primary current.
When a change is noticed, the compensating Is current in the
secondary coil wS2 is adjusted in order to be back to the
initial magnetic stage. This is identical to the general closed
loop principle described before.
The shape change is detected by a „second harmonic
detector“ (Fig. 36), described hereunder, which detects
changes in the harmonic content of the Iµ current waveform.
The second harmonic order is selected because it is the
most sensitive.
Due to the targeted very high resolution and accuracy,
preference is given to this harmonic detection principle
rather than a monitoring of the changes in the Iµ current’s
DC offset. Indeed, while the latter principle is regularly
considered for less efficient Fluxgate designs, it suffers from
offset errors introduced by the processing electronic.
A key element of this detector is the center-tapped
transformer, which naturally sort out the second harmonics
of its Iµ primary currents. The transformer primary coil is
made in two parts, connected in a way that the Iµ+ and Iµcurrents create flux with an opposite polarity:
Fluxgate Technologies
• When the two Iµ currents are identical (IP = 0), the flux
cancellation is such that there is no signal induced into
the secondary of the center-tapped transformer
• When the two Iµ currents are different (IP different from
zero), only their AC differences create a signal. In this
case, where the Iµ currents have the Fig. 37 shapes, all
the Iµ currents uneven harmonics (1, 3, 5…) create a zero
total flux, while the even harmonics (2, 4…) create an AC
flux and thus a signal into the secondary winding of the
center-tapped transformer.
Finally, among the harmonic content of the transformer
output signal, only the second harmonic is detected by the
dedicated processing electronics (Fig. 36), using an analog
switch driven at twice the excitation frequency, ensuring
„synchronous rectification“ of the second harmonic signal.
The output of the synchronous rectifier is then filtered using
a low-pass filter, before controlling the amplifier that drives
flux compensation coil, wS2 (Fig. 35).
This WS2 coil is consequently compensating both the low
frequencies, using the Fluxgate principle, and the high
frequencies, through current transformer effects.
4.5.2 „IT-type“ transducers – advantages & limitations
The main advantages are:
• Very high accuracy and stability
4.5.4 Calculation of the measurement accuracy
Set an IT 600-S transducer to measure nominal current and
check accuracy at 100 % and 10 % of this level.
Example 1: Accuracy of the IT 600-S at 600A (see
When measuring a 600 A DC current, an IT 600-S can only
provide a 1 V output. With a current transfer ratio of 1500:1,
the secondary current is 400 mA and therefore the
measurement resistance must be 2.5 Ω. We will assume the
worst-case rated operating temperature of 50 °C for the
analysis. This amounts to a 25 °C deviation which creates a
0.1 µA/°C • 25 °C = 2.5 µA offset drift. The error is given in
both mA and ppm of reading.
At 25 °C ambient
DC offset on secondary current
at IP = 0
< 4 mA
Non-linearity: <1 ppm of full scale < 0.4 mA
1 ppm
Measuring ratio stability of
< 2ppm of reading
< 0.8 mA
2 ppm
Total error at 25 °C
< 5.2 mA
13 ppm
Drift with temperature (from 25 °C to 50 °C)
DC offset drift: 0.1 mA/K
2.5 mA
• Excellent linearity (< 1 ppm)
Measuring ratio stability of
0.3 ppm of reading/K
3 mA
• Very good temperature stability (< 0.3 ppm/K)
Maximum global error at 50 °C
< 10.7 mA
• Very low initial offset and drift with temperature
• Large bandwidth (DC to 100 kHz)
10 ppm
6.25 ppm
7.5 ppm
26.75 ppm
Example 2: Accuracy of the IT 600-S at 60 A
• Very low distortion for AC current measurement
• Very low noise on the output signal
The limitations are:
• Operating temperature presently limited to laboratory or
clean environment usage (basically 10 °C to 50 °C)
With the same configuration, capable of measuring 600 A,
the IT 600-S will only give 40 mA and 0.1 V at 60 A. The
error is given in both mA and ppm of reading.
At 25 °C ambient
• High power supply current consumption
DC offset on secondary current
at IP = 0
• Relatively large dimensions
Non-linearity : <1 ppm of full scale < 0.4 mA
4.5.3 „IT-type“ transducers - typical applications
Measuring ratio stability of
< 2ppm of reading
< 0.08 mA
• Feedback measurement in precision current regulated
power supplies.
Total error at 25°C
< 4.48 mA
• Precision current control in gradient amplifiers for medical
Drift with temperature (from 25 °C to 50 °C)
• Current measurement for power analysis.
• Calibration device for test benches.
• Battery charging equipment requiring high-resolution
< 4 mA
100 ppm
10 ppm
DC offset drift: 0.1 mA/K
2.5 mA
62.5 ppm
Measuring ratio stability of
0.3 ppm of reading/K
0.3 mA
7.5 ppm
Maximum global error at 50°C
< 7.28 mA
182 ppm
• Laboratory/metrology equipment requiring high accuracy
Air-Core Technologies
Air-core technologies
The performance of current and voltage transducers are often
limited by the characteristics of the magnetic core material
itself (e.g. remanence, hysteresis, non-linearity, losses,
saturation) so the design of an air-core, or coreless,
transducer is often considered.
In this case the following issues have to be taken into
• the measurement of DC current requires the use of a field
sensing element; due to the absence of a field focused
area (e.g. gap), a highly sensitive field sensing device
must be considered (e.g. GMR, Hall cell) ideally in an array
around the conductor.
• when available, a magnetic circuit can be used as a shield
to external magnetic field disturbances (e.g. earth’s field,
external conductors); with air-core technologies the
sensitivity to external disturbances must be managed in a
different way, for example an array of field sensors instead
of a single sensor or, when coils are considered, special
design execution such as the Rogowski method of routing
the return wire; the ability to accurately measure the
desired current while also rejecting external fields is a
significant challenge for air-core technologies.
the waveform of the measured current requires the
integration of the induced voltage. Therefore, the current
transducer includes an integration function in the processing
In the LEM~flex and PRiME™ transducer datasheets the
value of sensitivity (S12) is provided, linking the amplitude of
a sinusoidal current to the amplitude of the transducer output
voltage at a specific frequency. The same sensitivity parameter can also be used to link the RMS values of a sine
wave primary current and the corresponding sine wave
output voltage, namely:
EPEAK = S12 • f • IPEAK and ERMS = S12 • f • IRMS
To give an order of magnitude, the typical sensitivity is:
• LEM~flex probe
S12 = 2.0 [µVs/A]
• PRiME™
S12 = 1.0 [µVs/A]
LEM~flex - the flexible AC current transducer
LEM~flex is a transducer based on the Rogowski coil
principle (Fig. 38), providing a flexible air-core sensor that
can be opened, allowing it to be installed on a primary
conductor without interrupting the circuit.
Two efficient air-core transducer technologies are presented
in this chapter, both related to AC measurements: the
LEM~flex and PRiME™(1) technologies.
(1) patented and licensed to LEM by Suparule Ltd.
Basic working principle and sensitivity
LEM~flex and PRiME™ technologies both work on the same
basic principle; a pick-up coil is magnetically coupled with
the flux created by the current to be measured. A voltage is
induced on the pick-up coil proportional to the derivative of
flux and thus proportional to the derivative of the current to be
measured. Because the derivative of DC is zero these
technologies are only useful for the measurement of AC or
pulsed currents.
The instantaneous voltage induced in the pick-up coil is
EOUT(t) = L12 • di(t)/dt
For a sinusoidal current, we have:
EOUT (t) = L12 • IPEAK • 2 • π • f • cos(2 • π • f • t)
= EPEAK • cos (2 • π • f • t)
As shown in this example, where a sinusoidal current i(t)
creates a phase delayed (cosine) voltage EOUT(t), reproducing
Figure 38: Rogowski AC current measurement
5.2.1 Construction and principle of operation
where i(t) is the primary current [A] and L12 is the mutual
inductance [H] between the primary and pick-up coil.
i(t) = IPEAK • sin(2 • π • f • t)
The key element of the LEM~flex transducer is the flexible
measuring head (Figure 39), which is a coil uniformly wound
around a flexible former of insulating material. The end of
the coil wire is returned to the same end as the start by
returning it co-axially through the coil former (Fig. 38). This
construction technique minimizes the sensitivity to external
field disturbances.
To make a measurement, it is not necessary to have the
LEM~flex transducer formed as a circle or to have the
primary conductor(s) centered within the perimeter. In
practice, the flexible measuring head is wrapped around the
conductor(s) carrying the current to be measured and the
two ends are brought together and mechanically connected
by a coupling latch (Fig. 40) to form a closed path.
Air-Core Technologies
sectional area. Note that changing the length of a Rogowski
coil does not affect the sensitivity.
5.2.2 Characteristics and features
The LEM~flex family of transducers has been designed to
conveniently measure single and 3-phase AC currents, as
well as pulsed DC currents. Standard ranges include 30/300/
3 kARMS and 60/600/6 kARMS, but scaling is easily designed for
other currents. Theoretically the size of the measuring head
and/or measurement range is unlimited.
The standard LEM~flex current transducer provides a
sensitivity of 50 or 100 mV/A at 50 Hz and at the output of the
processing electronic. The analog output voltage, galvanically
isolated from the measured current, is 0 to 3 VRMS, or to be
precise 4.25 VPEAK.
Fig. 39 LEM-flex transducers
Due to the flexibility of the measuring heads, it is possible to
position them around one or more irregularly shaped or
difficult to access conductors or busbars. The transducer can
be quickly and easily installed as well as removed. Installation
and measurement is performed without mechanical or
electrical interruption of the current carrying conductor, while
also ensuring galvanic isolation. LEM~flex transducers are
also very lightweight.
In terms of bandwidth, the performance of the LEM~flex
transducer is similar in concept to a band-pass circuit,
exhibiting both high and low cut-off frequencies. As the
integrator gain can be very high, shielding and appropriate
filtering prior to the integrator must be used to minimize the
influences from low frequency influences. For high frequency
performance, the upper cut-off frequency is determined by the
coil inductance and capacitance. It should also be noted that
the integrator includes compensation circuits that limit the
thermal drift. The typical bandwidth targeted by the LEM~flex
portfolio is 10 to 100 kHz, with possible extension to 1 MHz.
As the LEM~flex measuring head is fundamentally an air-core
coil, there is no magnetic hysteresis, no saturation
phenomena or non-linearity, as is present with magnetic
cores. The main factors that affect the accuracy of this
technology are:
Phase Degree
The sensitivity (§ 5.1) of a LEM~flex transducer can be
detailed as follows:
S12 = 8 • 10-7 • π2 • N • A
where N is the density of the coil turns made around the
flexible insulating cylinder [turns/m] and A is the crosssectional area of the coil [m2].
Phase Degree ( ° )
Fig. 40 Typical LEM-flex use, surrounding conductor(s) and latched.
Relative Amplitude
8 10
Frequency (Hz)
Figure 41: Integrator circuit & bandwidth
To design a Rogowski coil with a high sensitivity requires
either a large number of turns per meter or a large cross35
Air-Core Technologies
• the manufacturing tolerances of the wound coil (consistent
turns/m and m2)
• the fact this is a mechanically open structure, leading to a
slight gap in the coil perimeter where the two ends are
brought together
• the error introduced by the processing electronics, such as
the integrator circuit phase angle error and relative
amplitude as shown in Fig. 41.
These factors lead to slight errors based on the position of the
current conductor inside the LEM~flex and sensitivity to fields
from conductors just outside the perimeter of the LEM~flex.
These errors are typically below 1 %, with a maximum of 2 %,
and can be avoided entirely by proper positioning of the
5.2.3 Typical applications
LEM~flex is a lightweight measuring head combined with
remote electronics (distance between head and electronics
can be as great as 4 meters, or 12 feet). This, along with all of
the previous described attributes, lead to a device suitable for
use in a wide range of applications.
• Measuring currents in busbar sets, in particular in induction
heating equipment.
• Frequency converters, variable speed drives and
• Control of power semiconductors.
• Analysis of the current distribution in mains networks.
• Analysis of harmonics, power measurements,
measurement of the peak load in the mains, and in UPS.
Linearity error
±0.2 % of reading
Accuracy at 25 °C ±1 % of range
Accuracy drift
±0.08 %/K • 25°C • 280
Position error
±2.0 % of reading
±0.56 A
±3 A
±5.6 A
±5.6 A
Worst-case error
±14.76 A ⇔
±5.3 % of reading
Care must be exercised in properly identifying errors based
on reading and those based on range.
PRiMETM Transducers
PRiMETM has been developed to measure AC and pulsed DC
currents with minimal effect from external sources, achieved
by the application of a novel compensation technique.
PRiMETM is an air-core technology based on planar magnetic
sensors, constructed using no ferromagnetic material. It is
similar to the LEM~flex technique, but uses discrete coils at
regular intervals around a fixed perimeter rather than a
continuous uniform coil. A specific coils arrangement gives
the robustness to external field perturbations.
5.3.1 Construction and Principle of Operation
The transducer head comprises two parts, a number of
sensor printed wiring boards (PWBs), and a base-PWB (Fig.
42). The sensor PWB consists of two separate coils
constructed on a multi-layer PWB (Fig. 43). Several sensors
are mounted onto the base-PWB, at right angles to it, and
connected in series to form two concentric inner and outer
loops (Fig. 44). A larger number of sensor PWBs provides
higher accuracy, lower sensitivity to conductor position and
improved external field rejection performance.
• Switched mode power supplies.
• Low or medium voltage distribution installations.
• Power electronics installations.
• Sensing devices for watt meters and network analyzers
installed by electric power distribution companies.
• Electrical maintenance, repair and machine installation and
start-up applications.
To make measurements, the current carrying conductor
needs to be positioned within the aperture of the transducer
head, within both loops (Fig. 42). This creates an AC
magnetic flux, which is coupled into the sensors and induces
a voltage proportional to the rate of change of the current (di/
dt) in both the inner and outer loops (Fig. 44). As a result, a
• Connection to most measuring instruments including
multimeters, oscilloscopes, recording devices, data loggers,
Conductor to be
Sensor PWB
5.2.4 Calculation of the measurement accuracy
An example of a total error calculation is given for the
LEM~flex RR 3020, (see datasheet) having three
selectionable working ranges of 30/300/3000 A nominal
current. Considering a 280 ARMS current giving a 2.8 VRMS
output signal (10 mV/A), what is the measurement accuracy at
an ambient temperature of 50 °C? According to the
datasheet, the error values are:
Base PWB
Figure 42: PRiMETM current transducer
Air-Core Technologies
measured. The integrator circuit (Fig. 45) is kept as close
to the transducer head as possible to reduce stray inductive
pickup that would give rise to errors.
Finally, the comments made previously (§ 5.1) regarding the
instantaneous voltage collected by the coils of an air-core
technology are again applicable.
5.3.2 Characteristics and Features
Figure 43: PRiMETM sensor multi-layers printed wiring board
Figure 44: PRiMETM – two coils concentric loops
constant DC current will not produce a voltage and
therefore cannot be measured. However, as with LEM~flex,
it will respond to pulsed currents.
The output signal VTOTAL (Fig. 45), used for the current
measurement is:
For an external current source, the essence of the
technology is that the ratio of the induced voltage in each
loop of sensor PWBs (VOUTER / VINNER) is almost constant,
irrespective of the magnitude or position of this external
source. As a result, the external field effects are cancelled if
the correct proportion of the voltage induced in the outer
loop (VOUTER) is substracted from that in the inner loop
(VINNER). Making λ = VOUTER / VINNER will force VTOTAL = 0 or, in
other words, an external current source will not produce any
signal at the output.
To measure the current from the conductor placed within the
aperture, an optimized transducer design leads to a typical
VOUTER / VINNER ratio of λ / 2, again irrespective of the
conductor position. As a result, the usable signal for the
measurement is close to VTOTAL = VINNER / 2, meaning that
approximately half of the signal collected by the inner
concentric chain of sensing PWB coils is used for the
In short, this novel arrangement provides one-half of the total
possible signal for measurement while effectively canceling
the unwanted influence of signals from external current
As with the LEM~flex, the collected signal is proportional to
the derivative of the current and it is necessary to integrate
the voltage induced in the transducer head to obtain both
amplitude and phase information for the current being
Since PRiMETM utilizes an air-core coil as the sensing
element, there is no magnetic hysteresis, saturation, or nonlinearity, as is present in a current transformer with a
magnetic core.
There is no theoretical maximum limit to the measuring
range, but the typical dynamic range is 1000:1,
corresponding to the ratio of the maximum to minimum
measurable current with a given transducer. The accuracy is
typically specified as a percentage of reading, above 10 %
of the nominal rated current, leading to a highly accurate
solution when the current is only a fraction of the nominal
current. The accuracy is generally better than 0.8 % of
reading and the gain variation due to temperature is low,
typically 0.01 %/K.
The output voltage is directly proportional to the measured
current and provides accurate phase information. The level
of sensitivity depends on the required measuring range and
supply voltage.
The aperture sizes for existing transducers range from
20 mm to 160 mm in diameter, in both ring and split format.
There is no theoretical limit to the size of the aperture. With
split versions, installation and measurement can be
performed without mechanical or electrical interruption of the
current carrying conductor, while also ensuring galvanic
The bandwidth of PRiMETM has both a high frequency and a
low frequency cut off. The high frequency limit is dependent
on the resonant frequency of the sensors while the low
frequency limit is a function of the integrator design.
Products are designed for a given bandwidth, typically 5 to
100 kHz, but an upper limit in the MHz range appears to be
Figure 45: PRiMETM – processing of the coil signals
Air-Core Technologies
Attenuation (dB)
Phase shift (°)
Figure 46: PRiMETM – typical bandwidth 5 to 100 kHz
5.3.3 Advantages and Limitations of PRiMETM Technology
• Capable of measuring AC and pulsed DC currents.
• Wide current measuring range, capable of withstanding
high overload.
• Accuracy given in percent of reading; high accuracy over a
wide measuring range.
• Large bandwidth, not including DC.
• Lightweight in comparison with current transformers or
• On-board electronics that can potentially be merged with
the users electronics.
• Provides an isolated output signal (e.g. 4-20 mA, 0-10 V)
usable with PLCs without conditioning.
• Requires a power supply, but has low current consumption
This makes them suitable for portable applications and power
quality monitoring where weight and battery life are a
concern. This performance also make them suitable
replacements for current transformers.
Other Types of Voltage Transducers Technologies
and transmitted to the secondary side through an optical
link. The same is done with the required data
synchronization signals.
Other types of voltage transducer technologies
Two additional voltage measurement technologies have been
developed by LEM to achieve differentiated measurement
performance compared to Hall (§ 3.4) or Fluxgate (§ 4.4.4)
based voltage transducers. The first technology, OptiLEM,
uses fiber optics for the transmission of the voltage
measurement information, providing a high level of dielectric
isolation. The second technology is the AV-type of voltage
transducer based on electronic isolation circuitry that also
provides galvanic isolation, but at the component level rather
than using discrete fiber optics.
On the secondary side the data stream is converted back
into an analog signal and converted into an output current
for high noise immunity and easy scaling. A very critical
function of the transducer is to provide a low voltage supply
for the components on the primary side, requiring a very
high level of voltage isolation and very low capacitive
coupling from primary to secondary. A sine wave signal is
used to minimize noise levels.
OptiLEM voltage transducers
The OptiLEM voltage transducer has been developed to
provide an optimal solution for higher isolation voltages. The
result is an interesting product containing numerous patented
The following key attributes are reached with the first voltage
transducer model:
• isolated voltage measurements up to 12 kVRMS
• 100 V to 6 kV measuring range
• overall accuracy better than 1.5 %
• bandwidth from DC to 12 kHz
• low stray capacitance between primary and secondary
(less than 10 pF)
Figure 48: OptiLEM voltage transducer
• low partial discharge extinction level of 5 kV with < 10 pC
The working principle of the OptiLEM voltage transducers is
shown in Fig. 47. The primary voltage is directly applied to the
transducer primary connections, ±U. An internal resistor
divider network and differential amplifier measure the primary
voltage signal. This output is converted to a serial data string
12 bits A/D
Figure 47: OptiLEM working principle
Other Types of Voltage Transducers Technologies
voltage value. Any kind of signal can be measured: DC, AC,
or complex waveforms. The output current is always a true
image of the primary voltage.
„AV type“ voltage transducers
6.2.1 Construction and Principle of Operation
The working principle of the AV-type voltage transducers is
shown in Fig. 49. The primary voltage (VPN) is applied
directly to the transducer primary connections (±HT). An
internal resistor network and amplifier perform the voltage
measurement and drive an isolation amplifier. The latter
ensures an accurate and isolated transfer of the measured
information from the primary to the secondary side of the
transducer, based on a capacitive isolation principle.
Another important function of the transducer is to provide a
low voltage supply for the components on the primary side,
which requires the use of an efficient, well isolated, voltage
transformer capable of withstanding the specified primary to
secondary insulation levels.
At the output, the transmitted signal is recovered and then
conditioned to supply a current at the transducer output that
is an exact representation of the primary voltage. While the
AV-type transducer is a new LEM product, it utilizes only
well-known electronic components and techniques to
ensure quality and reliability.
6.2.2 Characteristics and Features
The measurement bandwidth ranges from DC to 13 kHz.
The delay time is less than 13 µs while the response time is
less than 33 µs. Compared to magnetically isolated voltage
transducers, the frequency bandwidth is not linked to the
model chosen, as there is no appreciable primary
inductance to create an L / R rolloff.
The linearity errors are within 0.1 % while the overall
accuracy is 1.7 % of VPN between –40 and 85 °C.
The main advantage of the AV technology is the small size
and lightweight, with a common compact design for each
model from 50 V to 1.5 k VRMS nominal. Compared to Hall
based voltage transducers, the large heat sink often used for
power dissipation of the integrated primary resistors is not
needed. This is a significant space and weight reduction. As
a comparison, the AV 100 family has approximately one-half
the volume (3 • 10-4m3) of the LV 100 family. The limitations
are a lower accuracy and limited insulation capability
compared to some of the other available voltage
The AV 100 family delivers an output current of 50 mA at VPN.
The AV-type is a galvanically isolated voltage transducer
made for measurements from 50 V to 1.5 kVRMS. The peak
voltage measurement capability is 150 % of the nominal
Power Supply +VC
Figure 49: AV type voltage transducers
Other Types of Voltage Transducers Technologies
6.2.3 Typical applications
+ HT
Presently, the main market for the AV 100 is in traction
inverters, but they can also be used in or adapted for any
industrial application.
+ HT
• Auxiliary and main converters (input voltage, DC link,
output phase voltages).
- HT
+ Vc
• Chopper and power factor correction circuits.
• Battery chargers.
• Sub-Stations and others.
- HT
Figure 50: Electric connection – AV 100 with positive unipolar
power supply
6.2.4 Calculations & Properties
Calculation of the measuring resistor RM
The AV 100 family provides a current output. This current
can be transformed into a voltage with the addition of a
measuring resistor, RM, at the output which cannot exceed a
defined RMmax value, based on the available voltage at the
transducer supply and the maximum voltage measured. For
the AV 100 family, RMmax can be easily calculated using the
following formula:
Negative unipolar power supply
In this case, the zener diode must be inserted as shown in
Figure 51 and the zener voltage, VZ, must be greater than
5.1 volts. The maximum measuring resistance becomes:
RMmax = [(abs(VCmin) - 2.0 - VZ) • VPN / (VPmax • ISN) - 31] • 0.9
with abs (VCmin) corresponding to the absolute value of the
VCmin voltage.
+ HT
R Mmax
= ((VCmin - 5.1) • VPN / (VPmax • ISN) - 31) • 0.9 [Ω]
= Minimum value of the secondary power supply [V]
= Internal voltage drop of the electronics [V]
= Nominal measurable primary voltage [V]
V Pmax
= Maximum measurable primary voltage [V]
= Secondary output current at VPN [A]
= Secondary internal resistance [Ω]
= Safety factor [-]
+ HT
0 V
- HT
- Vc
- HT
Figure 51: Electric connection – AV 100 with negative unipolar
power supply
The RMmax values indicated in the datasheets are the values
calculated using VPmax.
Unipolar Power Supply
For unipolar voltage measurements the AV 100 family can
operate from a unipolar secondary supply. A positive supply
allows positive voltage measurements while a negative
supply allows negative voltage measurements.
Positive unipolar power supply
In this case, a zener diode must be inserted as shown in
Figure 50 and the zener voltage, VZ, must be greater than
2.0 V. The maximum measuring resistance becomes:
RM max = [(VCmin - 5.1 - VZ) • VPN / (VPmax • ISN) - 31] • 0.9
Current Probes
Current probes
A comprehensive range of LEM clamp-on current probes
(Figure 52) allows current measurements from 5mA to 6kA.
The technologies used for the probes have been presented
before, including Hall effect, Fluxgate and air-core
technologies. Probe performance depends on the selected
sensing technology with bandwidths from DC up to 50MHz
and accuracies as low as 0.1% - 1%.
Compared to current transducers, additional issues need to
be considered when specifying a probe, linked to the way an
end-user handles a probe. For example, the effect of the
position of the conductors in the head, the manner in which
the jaw opens, the need for the head to fit into tight areas,
and safety concerns, such as „tactile barrier“ (indicating a
safe working distance from the operator’s hand to the live
conductor) and a reinforced or double-isolated output cable
with safety connectors.
In term of applications, clamp-on current probes open many
opportunities: in maintenance, repair-shops, and for the
installation and commissioning of industrial machines and
equipment. Applications include automobile diagnostics in
the factory and in garages, electroplating plants, telecommunication and computer equipment, drives controlled by
frequency inverters, industrial controllers and electrical
vehicles. In Hi-Fi amplifiers, the different loudspeaker
currents can be easily measured. Earth-leakage measurements in single or 3-phase AC networks are also possible,
by inserting two or three conductors in a single probe head.
Finally, current probes are also frequently used in process
control applications, for measuring the status of 4 to 20mA
current loops.
Figure 52: Clamp-on current probes
Here are additional concerns when using the types of
measurement devices described in this document.
Power supply polarity inversion
A LEM transducer may be damaged by an inversion of the
power supply voltage or connection of supply voltages to the
output or common pins. If this is a concern, LEM advises the
user to insert a diode on each power supply line, both
positive and negative, or to look for a specific LEM transducer that incorporates these protection diodes.
Capacitive dv/dt noise
Any electrical component with galvanic isolation has
capacitive coupling between the isolated potentials.
Applications with fast switching speeds, and consequently
fast voltage changes (dv/dt), across this capacitance
experience some coupling of the primary transient to the
secondary side creating undesirable interferences.
For example, a voltage change of 10 kV/µs in combination
with a 10 pF coupling capacitance generates a parasitic
current of i = C • dv/dt = 100mA. This represents an error of
two times the nominal output current for a transducer with a
50 mA nominal output.
This issue typically occurs with power converters where a
power component, such as a MOSFET (Metal Oxide
Semiconductor Field Effect Transistor) or IGBT (Insulated
Gate Bipolar Transistor), switch the voltage at frequencies in
the 10 kHz to 1 MHz range, generating dv/dt values in the
5 to 50 kV/µs range.
Figure 53: dv/dt disturbance and transducer signal
The following points can be highlighted:
• when long cables are used for the secondary connections
of the transducer, it is advisable to use a shielded cable
connecting the shield to a driven common at the
measurement end of the cable
• when possible, it is recommended to desynchronize the
measurement from the dv/dt occurrence, never performing
a measurement during the dv/dt disturbance and settling
• it is possible to attenuate the dv/dt disturbances using a
low pass filter; while this is detrimental to the transducer
bandwidth, in many cases it is a reasonable and desirable
- CH2 is the output of an LAS 50-TP representing a primary
current of 16 A/div;
• during printed wiring board (PWB) layout it is important to
keep secondary traces and primary traces, wires, or bus
bars separated and avoid long parallel runs to minimize
capacitive coupling; the best scenario has the secondary
traces running perpendicular to the primary providing
immediate separation; it is also advantageous to include a
screening trace or plane, connected to a star ground point,
between the primary and secondary to shunt the
disturbance to the screen rather than the secondary
- CH3 is the output of an LAH 50-P representing a primary
current of 4 A/div.
Figure 53 is an example of a primary dv/dt and the resulting
output noise:
- the time scale is 200 ns/div;
- CH1 is the primary voltage of 250 V/div, a 1 kV excursion
creating a 6 kV/µs transient (800 V in 133 ns);
For these two transducers, the settling time is close to 800 ns
and the peak disturbance is 50% and 7% of the nominal
current, for the LAS 50-TP and LAH 50-P respectively.
dv/dt disturbances can be minimized at two different levels:
• at the transducer level the design is optimized to limit the
primary-secondary capacitive coupling and to minimize
the settling time after a dv/dt disturbance
• at the user level care must be taken in the application of
the transducer into the system
For the latter, good EMI control practices must be observed.
Magnetic disturbances
Because the majority of transducers use magnetic coupling,
it is important to be concerned with external magnetic fields
likely to disturb a measurement. Potential sources include
transformers, inductors, wires, busbars, as well as other
transducers. Typical power electronics equipment has
multiple conductors, oftentimes in close proximity to each
other. A transducer may be disturbed by the magnetic fields
from adjacent conductors, with the disturbance being the
largest with short distances and high currents. A key
parameter in this case is the relative position between the
field sensing element (e.g. Hall or Fluxgate cell) and the
conductor creating the disturbance.
To minimize the disturbance please try:
• to increase the distance between the transducer and the
external conductor as much as possible
• to modify the transducer and/or conductor layout to
optimize the conductor position with regard to the field
sensing element of the transducer
• to twist, layer or parallel, if possible, the source and return
conductors to minimize the external field
• to divide the external conductors into equal parts and
place them symmetrically on each side of the transducer,
canceling or minimizing the external magnetic influence
• to magnetically shield the transducer
Typically, tests are performed by LEM to record external field
influences. The results of these tests and further advise are
available on request.
Typical misapplication of a transducer parameter
The incorrect use of a transducer may impact the performance or reliability of the design. Here are a few classical
cases of misapplication, although an analysis at a specific
operating condition may lead to these results.
Measurement resistance
Risk of overheating (current output) or
- RM < RMmin
measurement range reduction (voltage
- RM > RMmax
Measurement range reduction and
electronic saturation (current output)
- IN > I P
- IN < IPN
Magnetic saturation and permanent offset
Reduced accuracy
Secondary supply voltage
- VC < VCmin
Reduced accuracy and/or limited output
signal range
- VC > VCmax
Risk of overheating or permanent damage
Temperature range
- Excessive measurement error
- Mechanical damage to components
LEM ASIC based transducers
One of LEM’s answers to the market need for size and cost
reduction, along with accuracy and reliability improvement,
has been the move toward Application Specific Integrated
Circuits (ASICs) which replace all the discrete circuitry in a
transducer with an „all in one“ chip solution.
While the impact of an ASIC in terms of size reduction is
obvious, the challenge is to make the best use of ASIC
capabilities to improve accuracy. LEM uses state-of-the-art
techniques to minimize offset and other parameters affecting
the transducer accuracy. The transducer environment is also
taken into account, in particular the presence of very high di/
dt and dv/dt, which could impact measurement quality if not
appropriately considered.
LEM today employs ASIC technology in a number of
products including Hall effect based open loop, closed loop
and Eta transducers as well as various Fluxgate
technologies. This puts LEM in the position to produce a
wide range of ASIC based transducers and to better address
customer specific requirements, providing a continuous
improvement path into the future.
LEM – the Leader in Electrical Parameter Measurement
LEM – the leader in electrical parameter
With over 30 years of research, design, development, and
production, LEM is your source for electrical parameter
measurement components. LEM commitment to quality and
its customers has made it the leader in its market for over
20 years in a row. We work towards continuous
improvement of our products and processes.
Let us show you our wide array of products, developed
specifically for customers like you. If you can’t find the
product you are looking for, we have R&D resources in
every area of the world to address your needs.
Please take this opportunity to contact the LEM regional
office nearest you to learn more about how LEM can help
you solve your problems and improve your products, making
you more competitive.
It is the objective of all LEM employees to adhere to the LEM
Quality Policy:
The key to success for our company is the customer’s
satisfaction regarding respect for their demands as well as
their expectations.
In a continuous effort to fulfill the LEM Mission Statement:
To Help optimize the Utilization of World Energy and Natural
Resources, LEM contributes to the Mastery of Electricity by
providing innovative and cost effective Components, Instruments, Systems and Services.
Glossary A-Z
Glossary A-Z
The ability of a coil to generate a magnetic field. This is given by the
product of the coil current and the number of coil turns.
Bandwidth, small signal
The range of low level sine-wave frequencies that can be
reproduced with a specified reduction in signal amplitude, typically 1 or -3dB. Typically, the bandwidth of most transducers given in the
data sheet are assuming a current derating with frequency
Bandwidth, power
The range of nominal amplitude sine-wave frequencies that can be
reproduced, typically limited by internal heating.
Closed loop current transducer
A current transducer in which the magnetic field created by the
primary (measured) current is cancelled by a magnetic field with an
opposite direction, created by a secondary (output) current. The
secondary current is a true image of the primary current scaled
based on the primary to secondary turns ratio.(§3.2; §3.5)
C-type transducer (§ 4.4)
Highly accurate Fluxgate transducers operating on the closed loop
principle. Very good frequency behavior. Sensitivity to temperature
changes is minimized. C-type transducer portfolio includes CT
(current: § 4.4.2), CD (differential: § 4.4.3) and CV (voltage: § 4.4.4)
Current transformer
A transformer used to measure AC current while also providing
isolation. Current is scaled based on primary to secondary turns
Degauss cycle
Applying a decaying AC field to return a magnetic material to a zero
remanence (BR) condition (§ See also Demagnetization,
Magnetic offset and Remanence.
Returning a magnetic material to a zero remanence (BR) condition
(§ See also Degauss cycle, Magnetic offset and
di/dt following
The ability of a transducer to accurately reproduce a step in
primary current with a controlled rate of change (di/dt).
Differential current transducer
A transducer used to measure the difference between two opposing
currents. These are often used to measure leakage current.
Electro-Magnetic Compatibility, implying conformance with defined
Eta technology is a combination of open loop and current
transformer technologies. In general, Eta transducers have open
loop performance at low frequencies and current transformer
performance at high frequencies (§ 3.3).
Fluxgate technology implies driving a magnetic core material
through its complete B-H loop. The field from a primary current
causes a shift in this behavior that is measurable. See also C-type,
IT-type, Low frequency Fluxgate, Standard Fluxgate and § 4.
Global error
The worst-case error considering all possible factors. This is not
necessarily the addition of all error factors as some factors are
multiplicative while others cannot be simultaneously in their worstcase conditions. This is an absolute limit used in worst-case design
scenarios. In practice errors should be considered statistically, with
the probability and distribution of deviations leading to a much
smaller actual total error.
Hall effect / Hall generator
The Hall effect is the force applied to charge carriers in a magnetic
field causing an imbalance in voltage.
The device referred as Hall generators or Hall cells are designed to
take advantage of the Hall effect by providing a usable output for
measurement of magnetic fields.
The hystereses curve of a magnetic material described his overall
magnetic performances, advantages and drawbacks included
(Figure 4).
Because a Hall generator measures the magnetic field, transducer
Hysteresis has typically an impact on the transducer accuracy
(magnetic offset), gain (possible saturation effects in open loop),
behavior after a current overload (eventual need of demagnetization), heating at high frequency (hysteresis losses), influence of
external magnetic field (local saturation).
IT-type transducer
One of the most accurate transducer technologies available, based
on a closed loop Fluxgate principle (§ 4.5). See also C-type,
Fluxgate, Low frequency Fluxgate, Standard Fluxgate and § 4.
Low-frequency Fluxgate
A low cost product from the Fluxgate transducer portfolio with a
limited bandwidth. See also C-type, IT-type, Fluxgate, Standard
Fluxgate and § 4.
Magnetic offset
The output offset resulting from the remanence of the magnetic
material being used. See also Degauss cycle, Demagnetization
and Remanence.
Measurement resistor
When a device provides an output current a resistor is typically
used to convert the output current into a voltage for measurement.
This „measurement resistor“ is also commonly referred to as a
„burden“ or „load“ resistor.
Measurement voltage
The voltage created across a „measurement resistor“ is referred to
as the „measurement voltage“.
Nominal (current or voltage)
The maximum rated continuous RMS value of input signal.
Sometimes called „continuous“ or „rated“ current or voltage.
Offset drift
The drift of offset current or voltage versus temperature at the
transducer output, specified in mA/K or mV/K.
Offset error / voltage
Output signal of the transducer when subject to no primary
excitation (e.g. IP = 0). It is mostly due to electrical parameters but
also depends on magnetic influences.
Glossary A-Z
Open loop current transducers (§ 3.1)
Current transducers where the output signal is an image of the
magnetic field created by the primary current to be measured. It
generally uses a magnetic circuit which focuses the field on a field
sensing element, e.g. made of a Hall generator. The output signal is
a direct amplification of the Hall signal, giving a true image of the
measured current if the system is designed for a linear behavior.
The input, driving or measured side of a circuit and the components
related to that side of the circuit. Galvanic isolation is provided
between the primary and the secondary side. For example, the
primary winding carries the primary current. See also Secondary.
PWB (printed wiring board)
Often referred to as a „PCB“ or printed circuit, or PWB depending
on the geographic location.
The output (offset and/or transfer function) is directly proportional to
the supply or a reference voltage. See also Offset and Transfer
Reaction time
The delay between the measured signal reaching 10% of final value
and the output signal reaching 10% of final value. See also
Response time, and Rise time (Figure 8).
Recovery time
The time required to ‘recover’ and resume normal operation after an
event that interrupts operation, such as saturation. Not to be
confused with Settling time.
The output, driven or measurement side of an isolated circuit and
the components related to that side of the circuit. Isolation is
provided between primary and secondary side components. For
example, the secondary supply provides secondary compensation
current. See also Primary.
Settling time
The amount of time required to ‘settle’ to within a defined tolerance
after a disturbance, such as dv/dt. Not to be confused with
Recovery time.
Standard Fluxgate
A LEM definition, Standard Fluxgate transducers operate on the
closed loop principle and provide the most „standard“ performance
in term of accuracy or bandwidth. Non-standard Fluxgate
transducers include the more efficient IT or C-types, or the less
dynamic „low frequency“ Fluxgates. See also C-type, IT-type,
Fluxgate, Low frequency Fluxgate and § 4.
Transfer function
The ratio of output signal to input signal. For example, if a 100A
input creates a 1V output the transfer function is 1V / 100A or 1mV /
100mA. The output of a ratiometric device is dependent on the
supply voltage, so VS will be part of the transfer function.
Turns ratio
The ratio of primary turns to secondary turns in a transformer. With
closed loop current transducers, the turns ratio typically assumes a
single primary turn. For example, 1:1000 implies 1000 secondary
turns and a secondary current of 1mA with a single primary turn
carrying 1A.
After traversing a hysteresis loop the output will not return to zero
when the input is at zero. The amount remaining at the output is the
remanence and, in essence, creates an additional output offset.
See also Magnetic offset and Hysteresis.
It is minimized by design in LEM transducers and is often resulting
from a non-expected primary current overload. The magnetic offset
defines the offset on the transducer output signal, positive or
negative depending on the direction of the current overload.
Response time
The time difference between the driving signal reaching 90% of its
final value and the measured signal reaching 90% of its final value.
This is a combination of delay time, driving signal rise time, and
measured signal rise time. See also Reaction time and Rise time
(Figure 8).
Rise time
The time difference between a signal reaching 10% and 90% of its
final value. See also Delay time, Reaction time, and Response time
difference (Figure 8).
Saturable inductor
An inductor designed to operate into its saturation region. The
inductance value varies from a high value at low currents (based on
the permeability of the core) to a low value at high currents (the
core permeability becomes unity when saturated). (Figure 25).
LEM International Sales Representatives
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South Africa
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Australia and New Zealand
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122/49, 27th Cross
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Youngwoo Ind. Co.
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Beijing LEM Electronics Co. Ltd
No. 1 Standard Factory
Building B
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2-1-2 Nakamachi
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Acei Systems SDN BHD
No. 3, SB Jaya 7
Taman Industri SB Jaya
47000 Sungai Buloh
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Tope Co., Ltd.
P.O. Box 101-356
3F, No. 344, Fu Shing Road
ROC-10483 Taipei
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Beving Elektronik A.B.
Jägerhorns väg 8
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LECTRON Co., Ltd..
9F, No 171, SEC. 2
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Taipei Hsien 221
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Fax +886 2 8692 6098
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BAC/E, 05.04
LEM Components
8, Chemin des Aulx, CH-1228 Plan-les-Ouates
Tel. +41/22/7 06 11 11, Fax +41/22/7 94 94 78
e-mail: [email protected]; www.lem.com
Publication CH 24101 E/US (05.04 • 15/8 • CDH)
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