Polar Instruments T1500A Operator's Manual

Polar Instruments Ltd.
Garenne Park
Rue de la Cache
St. Sampson
Guernsey
Channel Islands
GY2 4AF
England
Fax: +44 (0)1481 52476
MAN 151-9510
T1500A
OPERATOR
MANUAL
T1500A FAULT LOCATOR OPERATOR MANUAL
WARRANTY
For a period of one year from its date of purchase new and undamaged from Polar
Instruments Ltd, POLAR INSTRUMENTS LTD or its authorized distributors will, without
charge, repair or replace at its option, this product if found to be defective in materials
or workmanship, and if returned to POLAR INSTRUMENTS LTD or its authorized
distributors transport prepaid. This warranty is expressly conditioned upon the product
having been used only in normal usage and service in accordance with instructions of
POLAR INSTRUMENTS LTD and not having been altered in any way or subject to
misuse, negligence or damage, and not having been repaired or attempted to be
repaired by any other than POLAR INSTRUMENTS LTD or its authorized distributors.
EXCEPT FOR THE FOREGOING EXPRESS WARRANTY OF REPAIR OR
REPLACEMENT POLAR INSTRUMENTS LTD MAKES NO WARRANTY OF ANY
KIND, INCLUDING BUT NOT LIMITED TO, ANY EXPRESS OR IMPLIED WARRANTY
OF MERCHANTABILITY OR FITNESS FOR ANY PARTICULAR PURPOSE, AND
POLAR INSTRUMENTS LTD SHALL NOT BE LIABLE FOR ANY DAMAGES,
WHETHER DIRECT OR NOT OR OTHERWISE, BEYOND REPAIR OR REPLACING
THIS PRODUCT.
Copyright Polar Instruments Ltd. 1995
T1500A OPERATOR MANUAL
DECLARATIONS
ELECTROMAGNETIC COMPATIBILITY
European Community Directive Conformance Statement
This product is in conformity with the protection requirements of EC Council Directive
89/336/EEC on the approximation of the laws of the Member States relating to
electromagnetic compatibility.
A declaration of conformity with the requirements of the Directive has been signed by
POLAR INSTRUMENTS (UK) LTD
11 College Place
London Road
Southampton
England
SO1 2FE
This product satisfies EN50081-1:92 and EN 50082-1:92
ii
SAFETY
SAFETY
WARNING
The LIVE and NEUTRAL lines on this unit are BOTH fused.
This unit contains no user-serviceable parts. When the unit is connected to its supply,
the opening of covers or removal of panels is likely to expose dangerous voltages. To
maintain operator safety, do not operate the unit unless the enclosure is complete and
securely assembled.
GROUNDING
This unit must be earthed (grounded); do not operate the instrument with the safety
earth disconnected. Ensure the instrument is connected to an outlet with an effective
protective conductor terminal (earth). Do not negate this protective action by using an
extension cord without a protective conductor.
Note: This instrument is fitted with 3-wire grounding type plug designed to fit only into a
grounding type power outlet. If a special local plug must be fitted to the power cord
ensure this operation is performed by a skilled electronics technician and that the
protective ground connection is maintained. The plug that is cut off from the power cord
must be safely disposed of.
Power cord color codes are as follows:
Europe
brown
blue
green/yellow
live
neutral
earth (ground)
United States
black
white
green
live
neutral
ground
iii
T1500A OPERATOR MANUAL
POWER SUPPLY
Check that the indicated line voltage setting corresponds with the local mains power
supply. See the rear panel for line voltage settings.
To change the line voltage settings refer the instrument to a skilled electronics
technician. Instructions for changing the line voltage settings are contained in the
T1500A Service Manual published by Polar Instruments.
T1500A OPERATION
This manual contains instructions and warnings which must be observed
by the user to ensure safe operation. Operating this instrument in ways
other than detailed in this manual may impair the protection provided by
the instrument and may result in the instrument becoming unsafe. Retain
these instructions for later use.
The T1500A is designed for use indoors in an electrical workshop environment at a
stable work station comprising a bench or similar work surface.
Use only the accessories (e.g. test probes and clips) provided by Polar Instruments.
The T1500A must be maintained and repaired by a skilled electronics technician in
accordance with the manufacturer’s instructions.
If it is likely that the protection has been impaired the instrument must be made
inoperative, secured against unintended operation and referred to qualified service
personnel. Protection may be impaired if, for example, the instrument:
•
•
•
•
•
Shows signs of physical damage
Fails to operate normally when the operating instructions are followed
Has been stored for prolonged periods under unfavourable conditions
Has been subjected to excessive transport stresses
Has been exposed to rain or water or been subject to liquid spills
CAUTION
Electrical Isolation
Always disconnect the board under test from the local mains supply (including ground)
before using this instrument.
SPECIFICATIONS
iv
SPECIFICATIONS
ASA Test Ranges
Junction
Logic
Low
Med
High
Hi-Cap (1Hz fixed)
Low-Cap (2kHz fixed)
Open Circuit Voltage
1V
10V
10V
20V
40V
10V
10V
ASA Test Frequencies
Low
Medium
High
Short Circuit Current
500µA
5mA
150mA
1mA
1mA
150mA
20µA
95Hz
500Hz
2kHz
Internal CRT
Display
Power Requirements
230V ± 10%, 115V ± 10% or 100V ± 10% at 50/60Hz, 40VA.
Physical characteristics (excluding accessories)
Dimensions
300 mm (11.8 in.) wide
110 mm (4.4 in.) high
260 mm (10.3 in.) deep
Weight
1.5 kg (3.3 lb.)
ENVIRONMENTAL OPERATING CONDITIONS
The instrument is designed for indoor use only under the following environmental
conditions:
Altitude
Temperature
Relative humidity
Mains borne transients
Pollution Degree
Up to 2000m
+5°C to +40°C ambient
RH 80% maximum at 31°C — derate
linearly to 50% at 40°C
As defined by Installation Category II
(Overvoltage Category II) in IEC664
2 (IEC664)
v
T1500A OPERATOR MANUAL
ACCESSORIES
Standard Accessories
Probe set
Test clip set
Operator manual
MMP159
ACC110 (Red)
ACC111 (Black)
MAN151
SYMBOLS
vi
CAUTION
To prevent damage to this product and to ensure its safe use
observe the specifications given in this manual when connecting to
terminals marked with this symbol.
COM
This terminal is internally connected to earth (ground).
SPECIFICATIONS
GUIDE TO THE MANUAL
INTRODUCTION
An overview of the Polar T1500A fault locator
and its applications.
GENERAL DESCRIPTION
A description of the principles of operation and
front and rear panels.
INSTALLATION AND SET UP
Connecting the T1500A to a power supply.
ASA DEVICE TESTING
Information about the different ways in which
the ASA facilities of the T1500A can be used
and how to test devices.
ASA SEMICONDUCTOR TESTING
Describes methods of testing signal diodes,
zener diodes, LEDs and transistors in circuit.
SIMPLE MAINTENANCE
Details of maintenance and cleaning
procedures.
vii
T1500A OPERATOR MANUAL
CONTENTS
DECLARATIONS........................................................................................................ii
ELECTROMAGNETIC COMPATIBILITY...................................................................... ii
European Community Directive Conformance Statement .................................... ii
SAFETY .....................................................................................................................iii
WARNING ................................................................................................................... iii
GROUNDING .............................................................................................................. iii
POWER SUPPLY........................................................................................................ iv
T1500A OPERATION .................................................................................................. iv
CAUTION..................................................................................................................... iv
Electrical Isolation ......................................................................................... iv
SPECIFICATIONS .....................................................................................................iv
ENVIRONMENTAL OPERATING CONDITIONS .........................................................v
ACCESSORIES........................................................................................................... vi
Standard Accessories.......................................................................................... vi
SYMBOLS ................................................................................................................... vi
SECTION 1 – INTRODUCTION.............................................................................. 1-1
THE T1500A FAULT LOCATOR .............................................................................. 1-1
1-1 Introduction to the T1500A ......................................................................... 1-1
1-2 Areas of application.................................................................................... 1-1
1-3 Analog Signature Analysis.......................................................................... 1-1
SECTION 2 – GENERAL DESCRIPTION .............................................................. 2-1
PRINCIPLES OF OPERATION ................................................................................ 2-1
2-1 T1500A controls, connectors and probes................................................... 2-1
The T1500A front panel ............................................................................. 2-1
The T1500A rear panel .............................................................................. 2-1
2-2 Producing signatures.................................................................................. 2-1
2-3 Four-quadrant signatures ........................................................................... 2-2
2-4 The T1500A equivalent circuit.................................................................... 2-3
SECTION 3 – INSTALLATION AND SET-UP ........................................................ 3-1
PREPARATION FOR USE ....................................................................................... 3-1
3-1 Unpacking .................................................................................................. 3-1
3-2 Connecting the T1500A to a power supply................................................. 3-1
3-3 Setting up the T1500A................................................................................ 3-2
3-4 Connecting the test cables ......................................................................... 3-3
Single Channel Applications ...................................................................... 3-3
Dual Channel (Comparison) Applications .................................................. 3-3
viii
CONTENTS
SECTION 4 – ASA DEVICE TESTING................................................................... 4-1
TESTING COMPONENTS ....................................................................................... 4-1
4-1 ASA techniques.......................................................................................... 4-2
Troubleshooting without documentation .................................................... 4-2
Typical faults .............................................................................................. 4-3
4-2 Testing resistors ......................................................................................... 4-4
4-3 Testing capacitors ...................................................................................... 4-6
Choosing capacitor testing ranges............................................................. 4-7
Leakage current in capacitors. ................................................................... 4-8
Capacitors in circuit.................................................................................... 4-9
4-4 Inductors .................................................................................................. 4-10
Signatures of inductors ............................................................................ 4-10
SECTION 5 – ASA SEMICONDUCTOR TESTING ................................................ 5-1
USING ASA TO TEST SEMICONDUCTORS........................................................... 5-1
5-1 Diodes, LEDs and Zeners .......................................................................... 5-1
5-2 Testing transistors in circuit........................................................................ 5-3
Identifying Transistor Terminals ................................................................. 5-3
5-3 Field Effect Transistors (FETs)................................................................... 5-5
Junction FETs (JFETs) .............................................................................. 5-5
Metal-Oxide Semiconductor FETs (MOSFETs) ......................................... 5-5
5-4 Integrated Circuits ...................................................................................... 5-5
5-5 Testing Devices in Circuit ......................................................................... 5-10
Circuit Example ........................................................................................ 5-10
5-6 Testing Bus-connected devices ............................................................... 5-13
SECTION 6 – SIMPLE MAINTENANCE AND CLEANING .................................... 6-1
Fault diagnosis ................................................................................................. 6-1
Cleaning ........................................................................................................... 6-1
Technical Support ............................................................................................ 6-1
ix
SECTION 1 – INTRODUCTION
THE T1500A FAULT LOCATOR
1-1 Introduction to the T1500A
The T1500A Fault Locator provides a fast and efficient means of testing
components, either in isolation or in circuit.
All testing is done with power disconnected from the circuit, so there is no risk to the
user and components under test cannot be damaged.
Two channels, A and B, enable the characteristics of a reference and a faulty
component or circuit to be compared. Faults in complex circuits can thus be
diagnosed without a high degree of technical skill or detailed knowledge of the circuit
functions — this facility will be found particularly useful when documentation is not
available.
1-2 Areas of application
The T1500A is an ideal instrument for a wide range of applications:
•
Field Service
–
fault finding.
•
Manufacturing
–
goods inwards testing and troubleshooting.
•
Repair
–
rapid diagnostics.
•
Education
–
examination of device characteristics.
1-3 Analog Signature Analysis
The T1500A Fault Locator tests components by applying a current-limited alternating
(AC) drive voltage across the component and monitoring the resultant current flow to
display impedance signatures. Signatures for different types of components have
distinctly different shapes that can be easily distinguished.
The inclusion of two channels in the T1500A allows the user to compare the
behaviour of two devices simultaneously.
1-1
SECTION 2 – GENERAL DESCRIPTION
PRINCIPLES OF OPERATION
2-1 T1500A controls, connectors and probes
The T1500A front panel
The front panel comprises:
Display Intensity — use to adjust display brightness
Trace Rotation — use to control trace orientation.
ASA range and frequency switches — use the voltage range and frequency switches to
select appropriate drive voltages and frequencies during signature analysis.
Channel A and B probes — probe across devices (between Channel A and COM or
Channel B and COM) to observe device signatures.
COM (common return) connector — connect the COM test clip to the ground of the
board (or boards) under test.
Channel A position controls — use the Channel A position controls to move the
channel A trace vertically and horizontally with respect to the Channel B trace.
The T1500A rear panel
The IEC power connector and power ON/OFF switch are mounted on the rear of the
instrument.
2-2 Producing signatures
The T1500A applies an alternating voltage to a component or circuit and displays the
resulting current versus voltage display on a CRT screen.
The voltage applied across the component is displayed horizontally, the current through
the component is displayed vertically, so the resulting V/I graph represents the
resistance, or impedance, of the component. This is defined as the Impedance
Signature of the device.
The T1500A generates voltages of both polarities, i.e. applies both positive and
negative voltages to a component, causing current to flow in both directions. The result
is a four quadrant graph (see Figure 2-1).
2-1
T1500A OPERATOR MANUAL
+I
2nd Quadrant
1st Quadrant
Vertical (Current) Axis
-V
+V
Horizontal (Voltage) Axis
3rd Quadrant
4th Quadrant
-I
Figure 2-1 The T1500A CRT Display
2-3 Four-quadrant signatures
Impedance signatures are graphs of current against voltage, plotted on a scale which
has its origin at the centre of the CRT display screen.
As the voltage applied to the component is driven positive and negative, positive
voltages and currents are displayed in the upper right quadrant on the CRT, negative
voltages and currents are displayed in the lower left quadrant.
The graticule marked on the CRT face is graduated so approximate current and
voltage measurements can be made.
It is usually not necessary, however, to calculate actual current values in a component
to verify its correct operation. When fault finding on an electrical circuit, the technician
is frequently looking for components that have failed completely. Often, a brief glance
at the signature of a suspect device will be sufficient to show whether it is good or
defective.
2-2
GENERAL DESCRIPTION
2-4 The T1500A equivalent circuit
The T1500A can be represented by a voltage source, VS, in series with an internal (or
source) impedance, ZS, and the component under test by a simple impedance, ZL
(Figure 2-2).
A
Zs
Vs
VL
ZL
Figure 2-2 T1500A equivalent circuit
The T1500A contains circuits which measure the voltage across, and the current
through, the component to be tested and output a display on the CRT.
The voltage, VL, across the impedance, ZL, will depend on the impedance of the
component under test and controls the horizontal deflection of the display.
Current through the component under test will cause a voltage to be developed across
the T1500A internal impedance, ZS. This voltage controls the amount of vertical
deflection on the display.
A high value impedance in the component under test (that is, high compared with the
value of ZS) will result in low circuit current (and therefore a low voltage across ZS) with
most of the T1500A source voltage, VS, appearing across ZL, producing a signature
with a shallow slope.
A low value impedance will result in high circuit current and most of the source voltage
developed across ZS; the result is a steeply sloping signature.
2-3
T1500A OPERATOR MANUAL
Figure 2-3 shows the signatures of two resistors, measured on the same voltage range,
superimposed. Component B has a greater resistance then component A.
A
B
Figure 2-3 T1500A display showing two impedance signatures
2-4
SECTION 3 – INSTALLATION AND SET-UP
PREPARATION FOR USE
3-1 Unpacking
The instrument is shipped in a sturdy transit pack. Open the pack carefully and remove
the instrument and its accessories.
If the instrument is damaged in any way contact the local distributor or supplier.
Retain the pack for possible future use.
The T1500A pack should contain:
T1500A
Power cord
Pair of probes
Pair of test clips
Operator manual
Note: If the instrument has been shipped or stored in a cold environment, allow the
instrument to reach the temperature of its new location before applying power.
3-2 Connecting the T1500A to a power supply
Refer to the voltage label on the rear panel of the instrument and make sure that
the marked rating is suitable for the local mains power supply.
If the rating on the label is not suitable for the local power supply refer the instrument to
a skilled electronics technician. Instructions for changing the line voltage settings are
contained in the T1500A Service Manual.
Note: If a special local plug must be fitted to the power cord ensure this operation is
performed by a skilled electronics technician and that the protective ground connection
is maintained. The plug that is cut off from the power cord must be safely disposed of.
3-1
T1500A OPERATOR MANUAL
Power cord color codes are as follows:
Europe
brown
blue
green/yellow
live
neutral
earth (ground)
United States
black
white
green
live
neutral
ground
3-3 Setting up the T1500A
1. Connect the power cord to the T1500A and to the power supply and
switch on (the power switch is located at the rear of the instrument).
The screen should display the Channel A and B outputs (two horizontal
traces) about the screen centre.
Caution: The CRT phosphor could be burnt if the intensity is set too high
and the same display is left on the screen for several hours.
Rotate the A POSITION horizontal and vertical controls and ensure that
the Channel A trace moves correctly vertically and horizontally with
respect to the Channel B trace. Adjust the X and Y position controls to
centre the traces about the screen centre.
2. If the CRT trace is not perfectly horizontal, adjust the Trace Rotation
control until the trace is parallel to the X axis. If the instrument is moved
to a new location readjustment of the Trace Rotation control may be
necessary.
3. Select the LOGIC RANGE and LOW FREQUENCY.
3-2
INSTALLATION AND SET-UP
3-4 Connecting the test cables
Single Channel Applications
With the Red probe connected to the Channel A (or Channel B) socket, and the Black
probe to the COM socket connect the probes across the device. When checking a
signature at a node in a circuit it will often be appropriate to connect the COM socket to
a convenient ground point using either a probe or test clip.
Dual Channel (Comparison) Applications
Connect the Red probe to the Channel A socket, the Black probe to the Channel B
socket, and a test clip to the COM socket. Connect the COM test clip to the ground of
the board under test.
If two boards are being compared, both grounds must be connected to the instrument's
COM sockets.
3-3
SECTION 4 – ASA DEVICE TESTING
TESTING COMPONENTS
Using the Analog Signature Analysis method, safe, low power drive voltages are
applied to components to produce "impedance signatures" on the oscilloscope screen.
Impedance signatures are graphs of current against voltage, plotted on a scale which
has its origin at the centre of the screen. This may be carried out on devices in or out of
circuit. Positive voltages and currents are displayed in the upper right quadrant on the
display. Negative voltages and currents are displayed in the lower left quadrant (see
Figure 4-1 Display X and Y Axis).
Figure 4-1 Display X and Y Axis
The graticule marks on the CRT face can be used for actual current and voltage
measurements. Table 4-1 shows the scales for each voltage range.
Range
Junction
Logic
Low
Med
High
Hi-Cap
Lo-Cap
Peak Voltage
Horizontal
Peak
Current
Vertical
1V
10V
10V
20V
50V
10V
10V
0.2V/div
2V/div
2V/div
4V/div
10V/div
2V/div
2V/div
500µA
5mA
150mA
1mA
1mA
150mA
20µA
0.125mA/div
1.25mA/div
37.5mA/div
0.25mA/div
0.25mA/div
37.5mA/div
5µA/div.
Table 4-1 Drive Ranges
4-1
T1500A OPERATOR MANUAL
All signatures are contained within the diamond shaped area formed by the “load lines”
joining the ends of the marked axes.
4-1 ASA techniques
The most effective way to use ASA is by comparison with a known good or reference
board.
The primary object of the comparison technique is to look for differences between
signatures. Using this technique will almost always prove the easiest and speediest
method of fault finding. There is often no need to analyse the shape of a signature in
detail.
To begin with, it is best to look at the signatures of nodes that connect to external
circuits (e.g. connectors), as any damage to the board is quite often the result of some
external influence.
Where reference boards are unavailable, it is important to try and obtain circuit
diagrams in order to predict with reasonable accuracy the signature of the nodes under
test. The user is again recommended to start by looking at the signatures on the
board’s connectors.
It is worth checking whether there are multiple similar circuits on the board under test,
(for example, a dual-channel modem or 4-channel audio amplifier).
These types of boards may have their individual channels compared just as if they
were separate boards. In such cases, exercise caution, however, when differences do
arise. Check that the separate channels are identical in the areas which show up
different signatures—circuit differences may be intentional.
Troubleshooting without documentation
If diagrams and reference board are not available, predicting signatures is obviously
more difficult, but it is still worth probing components like relays, known types of ICs
and 3-terminal devices and looking for the appropriate signature shape.
Even with no access to a reference board or to circuit diagrams the operator should be
suspicious of digital devices with signatures that are not diode-like, i.e. signatures with
sloping lines. Note, however, that some input protection diodes have a series
resistance which will produce a noticeable slope if tested with the LOW range.
Remember that on microprocessor buses, parallel ports, etc., the operator should
expect to see similar signatures on all the lines. If one is noticeably different from the
others treat this line with suspicion. However, if they all look a little strange, this is
probably not a fault.
4-2
TESTING COMPONENTS
Typical faults
It is worth keeping in mind that faults are not confined to the "high technology" areas of
the board. Most faults will be in the "low technology" sections, e.g. intermittent
connections, "dry" solder joints, reversed diodes.
For instance, the following faults are equally likely:
an open circuit relay coil,
a broken trace on a printed circuit board,
a broken transistor lead.
Remember that most service faults will be catastrophic device failures. Many faults
include broken connections of one sort or another (often resulting in open circuit
signatures) or short circuits (e.g. solder "bridges") so look particularly for unexpected
"straight line" signatures.
In general, when troubleshooting a complete circuit module such as a plug-in circuit
card, examine signatures in the following order:
edge connectors
other connectors
large ICs
small ICs
other components
4-3
T1500A OPERATOR MANUAL
4-2 Testing resistors
The signature produced by a pure resistance is an inclined straight line whose slope
(gradient) is dependent on the value of resistance.
Testing resistors is performed by connecting the probes across the device and
observing the slope of the displayed signature.
Select the voltage range which allows signatures and signature differences to be most
easily observed—a high value of resistance will only pass a small current if a low test
voltage is applied.
The resulting signature may not be easily distinguishable from the open circuit
horizontal trace. By selecting a higher voltage range a larger current flows and a more
recognisable sloping signature is displayed.
Table 4-2 gives the ranges against approximate resistor values for which signatures
can be distinguished from a short circuit (vertical trace) or an open circuit (horizontal
trace).
Range
Resistor Value (Ohms)
Junction
Logic
Low
Med
High
1K to 50K
300R to 6K
16.5R to 300R
5K to 60K
12K to 150K
Table 4-2 Resistance Ranges
Figures 4-2, 4-3 and 4-4 show typical signatures for three resistor values.
4-4
TESTING COMPONENTS
Figure 4-2
2K Resistor
Logic Range
Low Frequency.
Figure 4-3
10K Resistor
Med Range
Low Frequency
Figure 4-4
270K Resistor
Med Range
Low Frequency
4-5
T1500A OPERATOR MANUAL
4-3 Testing capacitors
Due to their energy storage characteristics, reactive components produce a phase shift
between voltage and current flow. This is displayed as a circular or elliptical signature.
See Figure 4-5.
Figure 4-5 Signature of a good capacitor
Note that for a pure reactance the major and minor axes of the ellipse align with the
vertical and horizontal graticule lines. The vertical deflection of the ellipse represents
the current through the capacitor, and will therefore increase with larger values of
capacitance (as the reactance decreases inversely with capacitance).
Table 4-3 shows the range of capacitors covered by each combination of drive voltage
and frequency.
Frequency
Range
LO-CAP
Logic
Low
Med
High
HI-CAP
Low
300nF – 6µF
6µF – 100µF
30nF – 300nF
10nF – 150nF
Med
56nF – 1µF
1µF – 20µF
5nF – 68nF
2nF – 30nF
500µF – 12000µF
Table 4-3 Capacitor Range
4-6
High
35pF – 800pF
15nF – 300nF
300nF – 5µF
1.5nF – 15nF
500pF – 7nF
TESTING COMPONENTS
Choosing capacitor testing ranges
Capacitor signature shapes can vary between a virtually horizontal line for low values of
capacitance through the range of ellipse shapes (including a circle) to a line that is
almost vertical for high capacitance values.
Using the higher frequency ranges will cause greater vertical deflection and make
subtle differences in signatures easier to detect.
For very small or large capacitors use the fixed frequency LO-CAP or HI-CAP ranges.
Figures 4-6 and 4-7 show typical signatures for capacitors on the Low Range and Low
Frequency settings.
Figure 4-6
22µF capacitor
Low Range
Low Frequency
Figure 4-7
10µF capacitor
Low Range
Low Frequency
4-7
T1500A OPERATOR MANUAL
Leakage current in capacitors.
In practice, because of leakage current, the capacitor will behave as if it were a perfect
capacitor with a resistor connected between its plates. Leakage currents represent a
power loss in capacitors and, if large enough, lead to problems in a circuit.
The amount of leakage current that is acceptable will depend on the circuit, but will
normally be very much smaller than capacitive current. Leakage current is greatest in
electrolytic capacitors because of their construction and impurities in the insulating
material.
Total current is thus made up of capacitive current and leakage current, and a real
capacitor can be considered as consisting of a pure capacitance in parallel with a high
value resistor.
C
RP
Figure 4-8 Equivalent circuit of a capacitor
Figure 4-8 illustrates the equivalent circuit of a "real" capacitor, comprising capacitance
C and parallel resistance RP.
In a good capacitor the value of RP will be high, so leakage current will be negligibly
small (RP is effectively open circuit).
The T1500A causes current to flow through both the capacitance and the parallel
resistance. The resulting signature will be the sum of the resistive and capacitive
components – Figure 4-9.
4-8
TESTING COMPONENTS
Figure 4-9 Signature of a "leaky" capacitor
Note the tilt in the ellipse. This is due to resistive current in the capacitor and indicates
a faulty capacitor.
In general, if the leakage current in a capacitor is significant compared with the
capacitive current, the elliptical signature will be tilted.
By choosing a different test frequency to vary the impedance, the effect of resistance
can be exaggerated or minimised as required.
Capacitors in circuit
In many applications capacitors are connected in parallel, with small capacitors
connected across large electrolytics. In such cases it will probably be necessary to
disconnect one of the capacitors from the circuit and test the capacitors separately.
Note that large capacitors for smoothing in power supplies are frequently connected
directly across a transformer secondary winding and also across a diode bridge — the
resulting signature can be difficult to predict so use comparison testing if a reference
circuit is available, or isolate the components from circuit.
Some types of capacitor are prone to short circuits between the plates, particularly
polystyrene capacitors which may have been subjected to excessive heat during
assembly or repair. The insulation in these components can easily melt at normal
soldering temperatures, bringing the plates into physical contact.
4-9
T1500A OPERATOR MANUAL
4-4 Inductors
Inductive reactance is dependent on the inductance value of the coil and the frequency
of the applied voltage, not the "ohmic" resistance of the coil.
The magnitude of the current flowing depends on:
1. The inductance of the coil
2. The frequency of the applied voltage
So reactance will rise and current will fall with rising frequency or inductance.
The result of inductive action is that, as in capacitors, the voltage and current do not
increase and decrease simultaneously.
This is reflected in their signatures.
Signatures of inductors
Inductors exhibit time delay between voltage and current in a manner similar to
capacitors, and similarly display elliptical signatures, but there are significant
differences.
Inductors are not simple inductances but are a combination of inductance and
resistance. Because they are coils of wire, they are able to pass current directly so at
low frequencies appear as very low value resistances. At high frequencies, reactance
increases compared with the resistance, so the signature looks more inductive than
resistive, taking on a more elliptical shape.
However, the signature can be difficult to predict (for example, inductors with iron cores
can "saturate") and signatures can show considerable distortion. For this reason, the
best technique for testing inductors is the comparison technique.
Frequency
Range
Low
Med
High
Logic
Low
Med
High
500mH – 11H
30mH – 500mH
10H – 110H
20H – 30H
100mH – 2H
6mH – 100mH
2H – 10H
4H – 50H
25mH – 500mH
1.5mH – 25mH
500mH – 5H
1H – 12H
Table 4-4 Inductor Range
4-10
TESTING COMPONENTS
Figure 4-10 shows the signature of a ferrite transformer primary winding with the test
voltage range set Low and test frequency set High. This demonstrates the effect of a
significant value of resistance causing the inductive ellipse to be tilted.
Figure 4-11 shows a similar (defective) transformer with a shorted turn.
Figure 4-10
Ferrite transformer
Primary winding
Low Range
High Frequency
Figure 4-11
Ferrite Transformer
Primary winding
Low Range
High Frequency
Shorted turn
4-11
SECTION 5 – ASA SEMICONDUCTOR TESTING
USING ASA TO TEST SEMICONDUCTORS
5-1 Diodes, LEDs and Zeners
When forward biased, a diode exhibits a low resistance and a voltage drop of
approximately 0.6V. This produces a signature that is an almost vertical trace close to
the Y axis (see Figure 5-1).
When reverse biased, the high resistance characteristics of the diode approaches that
of an open circuit, producing a horizontal trace close to the X axis. A light emitting
diode (LED) shows a similar signature to a conventional diode, except that the forward
voltage drop is approximately 1.5V.
A Zener diode exhibits the same signature as a conventional diode for voltages below
the Zener voltage. When the reverse bias exceeds the Zener voltage, a low resistance
signature is displayed. Figure 5-2 shows the signature of an 8.2V Zener diode.
When testing Zener diodes the graduations on the display X axis can be used to
measure the Zener voltage.
Suitable voltage ranges are:
Signal diodes
Power diodes
LEDs
Zener diodes
Junction
Low
Logic
Med up to 20V
NOTE: The signatures are inverted if the test probe and COM connections are
reversed.
5-1
T1500A OPERATOR MANUAL
Figure 5-1
Signal diode
Logic Range
Low Frequency
Figure 5-2
8.2V Zener diode
Med Range
Low Frequency
5-2
TESTING SEMICONDUCTORS
5-2 Testing transistors in circuit
In many fault finding situations it may not be possible or practicable to remove a device
from circuit. The T1500A can be used to test transistors as described in this section.
A transistor contains two semiconductor junctions connected "back-to-back" (one
between base and collector, the other between base and emitter).
Figures 5-3 – 5-5 show typical signatures for an NPN transistor (in which the collector
and emitter are N-type material and the base P-type) if the probes are connected
across the associated device terminals.
The base-emitter signature (Figure 5-3) is similar to that for a Zener diode. Highfrequency small signal transistors should not be operated in this mode for long periods.
Prolonged reverse-breakdown of the base-emitter junction may permanently affect the
characteristics of the device.
The base-collector (Figure 5-4) signature is similar to that of a conventional diode.
The collector-emitter signature (Figure 5-5) is similar to that of a diode in series with a
Zener diode. When the drive voltage is positive (right quadrant) the collector-base is
reverse biased and the base-emitter forward biased. The reverse biased collectoremitter prevents current flow, producing an open-circuit signature (a horizontal line).
When the drive voltage is negative (left quadrant), the collector-base is forward biased
and the base-emitter reverse biased. The base-emitter exhibits Zener breakdown as
described above, producing a Zener "tail" signature. Note the warning above about
operation of the transistor with the base-emitter reverse biased.
The signatures for a PNP transistor will be mirror images of those for an NPN
transistor.
Identifying Transistor Terminals
The terminals of an unknown transistor may be identified as follows:
Select Logic Range, Low Frequency.
Connect the COM clip to one lead of the transistor and probe the other two leads in
turn, looking for a match with the signatures shown in Figures 5-3 – 5-5. If the
signatures are mirror images of the figures, the transistor is a PNP device.
5-3
T1500A OPERATOR MANUAL
Figure 5-3
NPN Transistor
base-emitter
Med Range
Low Frequency
Figure 5-4
NPN Transistor
base-collector
Med Range
Low Frequency
Figure 5-5
NPN Transistor
emitter-collector
Med Range
Low Frequency
5-4
TESTING SEMICONDUCTORS
5-3 Field Effect Transistors (FETs)
Junction FETs (JFETs)
The junction field effect transistor (JFET) consists of a bar of semiconductor material
(the “channel”) and a region doped with material of the opposite semiconductor type to
the channel (the “gate”). The gate forms a diode junction with each end of the channel
(the “source” and “drain”) and these may be tested as conventional diodes.
Metal-Oxide Semiconductor FETs (MOSFETs)
CAUTION: Observe static precautions whenever handling MOSFETs. Use Logic
range for testing (or Low for power MOSFETs). Do not use the Med range.
MOSFETs are field effect transistors in which the gate is insulated from the channel.
The gate-drain and gate-source tests will usually produce an open-circuit signature,
although some MOSFETs have a protection diode between the gate and source. In
these cases the gate-source signature will be that for a Zener diode.
5-4 Integrated Circuits
The Logic and Junction ranges and Low frequency are recommended for use when
testing ICs. All integrated circuits can be tested by probing pairs of terminals. Most ICs
tested in this way display signatures similar to diodes or Zener diodes.
Note: ICs manufactured using different technologies can have distinctly different
signatures. This must be considered before diagnosing a device as faulty.
When testing an IC it is usually appropriate to connect COM to the ground pin of the
IC. Alternatively COM can be connected to Vcc.
In some circumstances unstable signatures can occur. Connecting
both ground and Vcc pins to COM can overcome this effect.
Figures 5-8, 5-7 and 5-8 show signatures for a 74LS00 IC.
The signature in Figure 5-8 is dominated by the input protection diode with its anode
connected to the COM probe via circuit ground. The signature in Figure 5-7 is more
complex as several output components within the IC influence the trace. The signature
in Figure 5-8 shows the effect of a network of components within the IC.
The corresponding signatures for an HC gate (74HC02) and 4000 series CMOS (4017)
are shown in Figures 5-9 to 5-11 and Figures 5-12 to 5-14 respectively.
5-5
T1500A OPERATOR MANUAL
Figure 5-6
74LS00
Logic Range
Low Frequency
Input to ground
Figure 5-7
74LS00
Logic Range
Low Frequency
Output to ground
Figure 5-8
74LS00
Logic Range
Low Frequency
Vcc to ground
5-6
TESTING SEMICONDUCTORS
Figure 5-9
74HC02
Logic Range
Low Frequency
Input to ground
Figure 5-10
74HC02
Logic Range
Low Frequency
Output to ground
Figure 5-11
74HC02
Logic Range
Low Frequency
Vcc to ground
5-7
T1500A OPERATOR MANUAL
Figure 5-12
4017
Logic Range
Low Frequency
Input to ground
Figure 5-13
4017
Logic Range
Low Frequency
Output to ground
Figure 5-14
4017
Logic Range
Low Frequency
Vcc to ground
5-8
TESTING SEMICONDUCTORS
An example of a defect in an IC is shown in Figures 5-15 and 5-16. Figure 5-15 shows
the signature between input and ground of a good IC type 7650 tested in circuit. Figure
5-16 shows the signature of a defective 7650 in the same circuit, where the input
protection diode has become leaky.
Note that in Figures 5-9, 5-11, 5-13, 5-15 and 5-16 loops are evident due to
capacitance within the ICs. Using Med or High frequency will exaggerate this effect. In
general Low frequency should be used when testing ICs.
Figure 5-15
7650 in circuit
Logic Range
Low Frequency
Input to ground
Figure 5-16
7650 in circuit
Logic Range
Low Frequency
Input to ground
Defective device
5-9
T1500A OPERATOR MANUAL
5-5 Testing Devices in Circuit
When testing a component in circuit, the signature is a composite of that device and
other components in parallel. This is most often the case when diagnosing faults in
service.
The characteristic signature at any probing point in a circuit is unique for that circuit.
Using Channels A and B to display the signatures of a suspect circuit and that of a
good circuit is the best way to identify a fault.
A faulty component may affect the signatures of several connected components. The
operator can 'home-in' on a fault by probing at several points in the circuit.
Circuit Example
Figure 5-18 is the signature of the power supply circuit shown in Figure 5-17 when
probed at the secondary winding of the transformer. The ranges used are Low voltage
and Low frequency. The looping of the signature is caused primarily by the smoothing
capacitor (C1). The slope of the axes of the ellipse arise from diode resistances in
circuit. The "knees" at either end of the ellipse are caused by the diodes in the rectifier
bridge.
Figure 5-17
Power
Supply
Circuit
5-10
TESTING SEMICONDUCTORS
Figure 5-18
Signature at
secondary winding
good circuit
Figure 5-19 shows the effect on the signature in Figure 5-18 when one diode (D3)
develops a short circuit.
During the positive half cycle of the drive voltage (right quadrant) the signature is
effectively that of diode D1. The remaining components are short circuited by D3.
During the negative half cycle (left quadrant) the signature is a composite of two current
paths, one through the transformer secondary, the other through the short circuited D3,
C1 and D2.
It is rarely necessary to analyse signatures in such detail. In this example the lack of
symmetry clearly indicates the fault. Moving the probes to test each component in the
circuit quickly leads to probing across the short circuited diode.
Figure 5-19
Signature at
secondary winding
D3 short circuit
5-11
T1500A OPERATOR MANUAL
Figure 5-20 shows the effect of superimposing two signatures to aid comparison
diagnosis.
Note: When comparing the signatures of two circuits ensure that both circuits are
connected to COM at the equivalent circuit points.
Figure 5-20
7650 in circuit
Input to ground
Channel 'A' (upper)
– defective
Channel 'B' (lower)
– good
5-12
TESTING SEMICONDUCTORS
5-6 Testing Bus-connected devices
When a number of devices are connected together on a common bus, the signatures
on the bus lines may be compared to look for differences. Lines on the same bus will
usually have similar signatures (e.g. all data lines will be similar to each other). If one
line has a different signature from other lines on the same bus, this suggests that a
device on that bus is faulty.
To isolate the fault to a specific device there are a number of methods:
1.
If any devices are socketed, remove them one by one until the defective line's
signature matches the other lines.
2.
Each device will have one or more pins that are not connected to a bus, e.g.
/CE (Chip Enable) or /OE (Output Enable). This provides a method for looking at
the ICs individually. Instead of connecting the T1500A's COM input to Vcc or
ground, connect it to the defective bus line. Probe each of the devices' /OE or
/CE pins, looking for a device whose signature differs from other similar devices.
3.
If neither of the above methods locate the fault, it may be necessary to unsolder
devices until the fault is cleared.
Figure 5-21 shows the signature of a data bus line of a microprocessor in circuit.
Figure 5-21
Microprocessor
data bus
in circuit
5-13
SECTION 6 – SIMPLE MAINTENANCE AND CLEANING
WARNING This instrument should only be serviced by a
qualified electronics technician.
Refer all servicing to qualified service personnel. Polar Instruments publishes a
T1500A Service Manual to assist the service technician.
Fault diagnosis
Symptom
Trace appears as a
vertical (short-circuit)
signature with channel
unconnected.
Cause
The problem most likely to be encountered is a
blown Channel Protection Fuse. Channels A
and B are each protected by fast blow fuses. If
the probes are connected to a powered board,
or a large, charged capacitor, the fuses open to
minimise damage to the T1500A. When a
channel is unconnected, its normal signature is
a horizontal line. If its protection fuse has
blown, then a vertical (short-circuit) signature
will be displayed. If a fuse needs to be replaced
refer the T1500A to qualified service personnel.
Trace unstable
Check that the COM lead is connected. If
testing ICs, connect the Vcc and ground pins
together.
Cleaning
Clean the T1500A with a cloth lightly moistened with water with a small amount of
mild detergent. Alternatively, a cloth lightly moistened with alcohol (ethanol or
methylated spirit) or isopropyl alcohol (IPA) may be used.
Do not spray cleaners directly onto the instrument.
Technical Support
For technical support contact your local Polar Instruments distributor or Polar
Instruments.
6-1