The Basics of Digital Multimeters

The Basics of Digital Multimeters
IDEAL INDUSTRIES INC.
The Basics of Digital Multimeters
A guide to help you understand the basic
Features and Functions of a Digital Multimeter.
Author:
Patrick C Elliott
Field Sales Engineer
IDEAL Industries, Inc
January 2010, Version 1
1
The Basics of Electricity
To better understand digital multimeters, it’s helpful to become clear on the basics of
electricity. After all, DMMs always measure some aspect of electricity.
Electricity passing through a conductor is similar to water flowing through a pipe. Every
pipe has force that creates a certain pressure, causing water to flow. In the case of
electricity, that force might be a generator, battery, solar panel or some other power
supply. The pressure created by that power supply is called voltage.
Voltage is the pressure applied to the circuit.
Current is the Flow of the electricity in the conductor.
Resistance is any restriction to the flow of the current in a conductor.
Voltage, current and resistance are the three most fundamental components of electricity.
Voltage is measured in volts, current in amps and resistance in ohms.
.
The three components in an electrical system are electrical
pressure, or voltage (measured in volts), the amount of
electricity flowing, or current (measured in amps), and
impedances within the system, or resistance (measured in
Ohms)
Voltage, Current and Resistance
Voltage is the pressure that is applied to a conductor. There are two common types of
power sources, Alternating Current (AC) and Direct Current (DC). Alternating Voltage is
the most common form of electricity. It is the power supplied by the utility or generators,
which flows through our electrical circuits. The symbol for AC voltage is
.
A generator creates
electricity from two opposite
magnetic fields, as the wire
turns between these two
fields, electrons are pulled
first in a positive, then in a
negative direction
DC Voltage is a constant level of stored energy. It is stored in batteries or converted
from alternating voltage through the use of electronic rectifiers. Electronic products like
TVs, VCRs and computer equipment run on DC power.
The symbol for DC voltage is
.
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Unlike alternating voltage,
direct voltage is a steady
flow of positive energy. It is
commonly stored in batteries
for use in electronic
equipment.
Current is the flow of electricity through a conductor. As with voltage, there are two
types of current, AC and DC. The symbol for current is the letter A.
The third component is resistance, measured in Ohms. Resistance in the circuit impedes
the flow of current through a conductor. The symbol for resistance is the Greek Omega,
Ω, sometimes referred to as the horseshoe.
Ohm’s Law
Together, voltage, current and resistance comprise Ohm’s Law. Ohm’s Law is an
important equation for electricians. By using a DMM, they can establish values for the
three variables which help in diagnosing electrical problems.
Ohm’s Law can be expressed in equation form in this way:
Ohm’s Law is expressed
in an equation: V=A x Ω
Tech Note: Voltage determines the flow of current; the greater the voltage, the greater
the current. If resistance is increased, the current will decrease. Lower the resistance,
and current will increase. The relationship of these three elements of Ohm’s Law; Volts,
Ohms and Amperes, must mathematically balance.
Let’s take an example; say we have a 120 Volt outlet and a hair dryer. If the hair dryer is
set on low, it would draw 7 amps. The load resistance is around 17 Ω, but if we change
the setting to high, the current draw would increase to 12 amps, and the load resistance
will decrease to 10 Ω.
If Ohms (Ω) increase, current (A)
decreases, and if Current (A) increases,
Ohms (Ω) decreases
For useful formulas See Appendix A
2
Electrical Circuits
In an Electrical system, there are two ways that loads are connected in a circuit, in Series
or in Parallel.
In a Series Circuit, each device is connected together in a line. Current flows through
each device connected to the circuit. If you were to increase the resistor in the Series
Circuit shown below, the light would dim. You have restricted the flow or available
current to the light.
In a series circuit, loads
within the circuit have an
impact on the flow of
electricity to the other loads.
In Parallel Circuit, the same amount of voltage is applied to each device. Current can
flow freely through each device without affecting another. Our homes are wired in
Parallel for this reason.
In a parallel circuit, loads
within the circuit will not
impact the flow of electricity
to the other loads.
When making measurements with a digital multimeter, it is important to remember that
Voltage measurements are made with the test leads connected in Parallel, and Current
measurements are made with the test leads connected in Series.
Tech Note: The number one mistake made when using modern multimeters is to try and
measure voltage with the test leads in the current input jacks. The input impedance of the
current inputs jacks is in the range of 0.1 ohm to around 8 ohms, depending on the
manufacturer. This low impedance is like a short circuit when making a voltage
measurement. Because of this low resistance and possible short circuit condition most
multimeters current input jacks are fused for protection. Well constructed meters will use
a high energy fuse for this protection but you will blow the fuse if you test in this manner.
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Types of Multimeters
There are two common types of Multimeters, Analog and Digital. Digital Multimeters
(DMMs) are the most common. They use a liquid crystal display (LCD) technology to
give more accurate readings. Other advantages include higher input impedances, which
will not load down sensitive circuits, and input protection.
Analog meters use a needle movement and calibrated scale to indicate values. These
were popular for years, but recently their numbers have declined. Every voltmeter has an
internal resistance or impedance. The input impedance of an analog meter is expressed in
“Ohms per Volt”
The input impedance of an analog meter is
expressed in “Ohms per Volt” In this
example the impedance for AC volts is
5000 ohms per volt. If I want to measure
120Vac the input resistance would be 5000
x 120 or 600,000 ohms.
Tech note: Analog Meters The internal impedance of the meter is in parallel to the
measured circuit. You want this impedance to have as little effect on the measurement as
possible so the higher the impedance the better. For most electrical measurements this
effect is minimal, but for sensitive electronics of today the effect of the added resistance
could be significant. This is just one of the disadvantages of an Analog meter. There are
however a few useful applications for analog meters, so they aren’t going away
tomorrow.
The Digital Multimeter (DMMs) feature a digital or liquid crystal display (LCD).
Measurement readings are displayed as numerical values on the LCD Display. The
display also alerts you to any pertinent symbols and warnings.
Tech Note: Digital Multimeters and ClampMeters use different techniques internally, to
measure AC, DC voltage, Resistance and Amperes. An advantage of a digital multimeter
is their accuracy and input protection. Their input resistance or impedance is very high,
in the range of 1,000,000 to 10,000,000 ohms, so there is little effect on the measurement.
On good quality meters, their inputs are also protected from faults and misuse. Test
instruments today devote a good deal of architecture to overload protection. Most digital
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meters meet some safety standard such as UL601010 or IEC (International Electrotechnical Commission).
DMMs at a Glance
The port panel is where you plug in your test leads. The diagram below explains where the
test leads go for specific tests.
Digital multimeters are more
commonly used because of a few
key features, including higher
accuracies, higher input
impedances and input protection.
Multimeter Safety
When making a meter selection look for a tester that is independently certified to some
safety standard, UL, IEC, CSA.
Pay close attention to how and where you are using the equipment. Never use equipment
that is outside of its manufacturer specified measurement range, or outside of its category
rating.
Over
Voltage
Category
Description of Category
CAT IV
Primary supply, Overhead or underground utility service.
CAT III
CAT II
Distribution level mains, fixed installation
Local level mains, appliances, portable equipment.
Signal level, special equipment or parts of equipment, telecommunication,
and electronics.
CAT I:
Tech Note: Multimeter Safety. The major issue addressed by the UL601010 standard
was to look at fault potential to available energy and define limited by category to each.
The most common fault was high voltage transients on high energy circuits. If a transient
were to cause a fault within an instrument with high energy present, it could result in a
cascading failure of meter, equipment, and possibly personal injury.
The easiest way to understand the different category ratings of the IEC standard is to
think of the potential Short Circuit energy. The higher available short circuit energy, the
higher the category.
5
For additional information on Meter Safety refer to the IDEAL whitepaper on METER
SAFETY
The Dial
Setting the Function
The dial of the DMM allows you to choose the function you’re interested in measuring.
Whether you intend to measure one of the three elements of Ohm’s Law, or a more
advanced function like frequency or capacitance, you must first set the dial to the
appropriate function.
Setting the Range
The dial also plays another essential role in measuring electricity – that of determining
the range of measurement. The range you select on the dial determines the placement of
the decimal point as it appears on the LCD. In turn, the position of the decimal point
determines how refined, or precise, your reading is. This is called resolution.
On a manual ranging
meter, the function and
range must be selected
Symbols
mV
A
mA
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Measurement
Functions
AC Voltage
DC Voltage
Milli Volts
Amperes
Milli Amperes
Ohms
Diode
Audible
Continuity
Capacitance
Auto ranging meters will
automatically choose the
measurement range
Descriptions
Measures amount of AC Electrical Pressure
Measures amount of DC Electrical Pressure
.00V or 1/1000V
Measures amount of electron flow
.001 or 1/1000A
Measurement of resistance to the flow of electron
Device used to control direction of electron flow
Audible indication of continuity for low
resistance
Device used to store electrical potential
For A complete listing See Appendix B
Auto vs. Manual Ranging
Tech note: Manual ranging multimeters force us to think about the measurement before
we select the range of the meter. As an example, if I want to measure 120V AC on a
manual ranging meter I would turn the Dial or switch to the VAC section and select the
200V Range. This gives you ample measurement range and the maximum resolution for
the measurement. If the voltage is unknown, start with the maximum or highest range and
step down to achieve the maximum resolution on the display. Note that OL or overload
means that you need to select a higher range and this should not damage the meter.
Tech Note: Auto ranging multimeters, only the measurement function needs to be
selected. The multimeters circuitry will “automatically” select the best range for the
measurement. There are two things to remember about an auto ranging meter. One thing
is that the timing for the meter to achieve and settle on a range can take a few seconds.
The other is the symbols and numerical expression used on the display. If a user fails to
pay close attention to what the display is telling them, an error can occur with the
interpretation of the displayed value. As an example, 240mV could be interpreted as
240V if the user doesn’t pay close attention to the little “m” in the “mV” icon on the
display.
Understanding Count, Resolution and Accuracy
The count is the maximum number of digits that can be shown on the display. In most
cases this value is one less that the Count of the display. For example if you have a 2000
count unit, the maximum reading per range is 1999 or one less that 2000.
To get a better understanding of resolution, let’s take an example. If you are using a
manual ranging unit that is set on 20V and you’re measuring an application that puts out
more than 20V, the display will read “OL”, or overload. You must reset the dial to a
higher range and take a new reading. The most refined reading, therefore, uses the range
that provides the best resolution without overloading. Select the range just higher than the
expected reading.
Range
Setting
2V
20V
200V
1000V
Maximum Range
and Resolution
1.999V
19.99V
199.9V
1000V
Meter Accuracy:
Most meter’s accuracy are expressed as a +/- percentage of input + a +/- number of
counts, expressed as +/-{ X% + No. of counts}. For example, the Ideal 61-342 is a 4000
count display with a basic DC Voltage accuracy of +/-{0.5% + 5} The +5 is called the
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count or floor and refers to the least significant digit of the display in reference to range
and resolution.
If we want to determine the maximum error of the meter that is measuring a source of
12V, first determine the percentage error and add the count or floor.
The % accuracy for a 12V source would be 12 x 0.005= 0.06
To determine the count, we must determine the meter’s range and resolution. If the
display is a 4000 count display, we need to determine the best range and resolution. For
12 V this would be the 40V range. The display maximum resolution is 39.99 and the least
significant digit would be 0.01 with a total count of 0.05
The accuracy of the meter is +/- (0.06 +.05) which is = +/- 0.11, so the Low limit is
11.89 and the High limit would be 12.11
Tech Note: Display Counts & Resolution
The display count is the maximum digital resolution of the multimeter. A 2000 count
display, has a maximum reading of 1999, one less than the display count. A 4000-count
display has a maximum reading of 3999. These two displays are the most common, 5000,
20,000 and even 50,000 count displays are also available. The display count determines
maximum range and resolution.
A 2000 count unit is often called a 3-½ digit display.
The 3 refers to the number of full digits, and the ½
refers to the capabilities of the most significant digit
(furthest to the left) which can be either a 1 or 0.
Most meters today are 4000 count units. This
means that the most significant would be 0 to 3 or
one less that the count of the analog to Digital
Converter.
The display count is important in determining the maximum resolution (number of digits
after the decimal point) of the reading. As an example, let’s look at the difference when
measuring a 240-volt supply with a 2000 count and 4000-count multimeter and what
range you would set the meter to.
The 2000 count display would be in the 600V range and display 280 volts. The maximum
resolution is 1 volt. The 4000-count multimeter would be in the 400V range and have a
maximum resolution of .1V. The unit would display the measurement as 280.0 volts.
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Range
600
200
20
2
200mV
Reading
600
199.9
19.99
1.999
199.9
Resolution
1V
.1V
.01V
.001V
0.1mV
The 2000 count unit
would need to be set on
the 600 volt range to
measure 280V. On the
600V range the maximum
resolution would be 1
Volt.
Range
600
400
40
4
400mV
Reading
600
399.9
39.99
3.999
399.9
Resolution
1V
.1V
.01V
.001V
0.1mV
The 4000 count unit
would be set on the 400V
range to measure 280V,
and have a maximum
resolution of .1V In this
case the 4000 count unit
would give you the best
resolution
It is important that we understand our numerical expressions to properly setup or read
the display of a Multimeter. In this example we have an auto ranging meter, measuring a
2,800,000 ohm resistor. The display reads 2.800 M Ω. M is the Symbol for Mega or one
million ohms.
In this example the
meter reading is
2.800 M Ω,
expressed as 2.8 Meg
Ohms or 2,800,000
ohms
Numerical Display notation
Terms
Giga
Mega
Kilo
Milli
Micro
Nano
Numerical Values
1,000,000,000
1,000,000
1,000
.001
.000001
.000000001
Symbol
G
M
K
m
m
n
Expression
x109
x106
x103
x10-3
x10-6
x10-9
Port Panel
The port panel is where you plug in your test leads. The diagram below explains where
the test leads go for specific tests.
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Instrument Input Jacks or Ports
The input jacks or ports of your meter are the working ends of the instrument. Use care
when connecting leads to your instrument. Pay close attention and be sure to connect the
leads into the correct port that is marked for the measurement selected on the dial.
DC Voltage Measurements: To measure DC voltage, we place the Red lead into the
VΩ
COM port. Turn the dial or switch to VDC or V
If it is a manual ranging meter set it for the proper range. As in the example below, we
want to measure a 9V battery so the best range would be the 20 V range. If you have an
auto-ranging meter you only need to set the function on the dial to VDC or V
.
Most Digital Multimeters are autopolarity sensing devices. This means that
we don’t have to worry about having the
Red lead on the hot or positive and the
Black Lead on the Neutral or negative.
If you do not pay close attention to
polarity when using an Analog meter the
meter movement or meter could be
damaged
AC Voltage Measurement: To measure AC voltage, we place the Red lead into the V Ω
port and black lead into the COM port. Turn the dial or switch to VAC or V
If it is a manual ranging meter set it for the proper range. As an example the meter would
be set to the 200 V range to measure a 120V outlet. . If you have an auto-ranging meter
you only need to set function to VAC or V
.
Remember that it is always a good practice to connect the black lead first then the red.
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Tech Note: Voltage Measurements
Voltage measurements are perhaps the most common function used on a multimeter.
Voltage is measured between two points so we must make sure that we have solid contact
at each point. The proper way to connect a meter is to connect the low or ground (black
lead) first and the High (Red lead) next. We remove the leads in reverse order, Red first
and then Black.
Whenever making live voltage measurements use the Three Point method. Measure a
known live circuit or source first, then the unknown circuit, then back to the known
circuit.
Average Responding vs. True RMS
The RMS or Root Mean Square value of an AC measurement is the “Effective Value” or
“Equivalent Value” of the waveform to do work in relationship to DC. Test Equipment
use two methods to measure an AC waveform. One is Average responding RMS
calibrated and the other is True RMS. Both are designed for periodic type perfectly
sinusoidal waveforms and most are AC coupled, meaning that is blocks any DC bias that
may effect the measurement.
Average Responding voltmeters use a simple circuit to provide a general-purpose voltmeter a
low cost method to calculate the RMS value of a sinusoidal waveform. The True effective
value can be obtained as long as the AC waveform is a periodic sinusoidal waveform.
When measuring complex waveforms with harmonics, such as square waves or AC signals
which have been rectified or electronically controlled in some way by devices like diodes,
SCR’s or triac’s, the True RMS or “effective heating value” cannot be accurately
measured using an Average responding meter. You must use a True RMS meter to make
an accurate measurement.
True-RMS voltmeters use an integrated circuit that computes the true root-mean-square value
of a complex waveform. Most are AC coupled, but in some higher end meters “AC + DC”
coupling is available which gives you the “effective heating value” of both the AC and
DC component of the waveform.
The Root Mean Square value is a
measurement of the “Effective Value”
of the waveform or the ability to do
work.
Average responding meters measure
the average value of a pure sine
waveform and calculate the RMS
value
Ave of .637 x 1.11= RMS of .707
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In commercial and industrial environments, loads like electronic lighting, computers,
variable speed drives and other electronic equipment draw current in short pulses. This
type of load is called non-linear because it doesn’t draw its current linearly with the load
voltage. The non-sinusoidal or distorted waveforms create harmonics. This distortion of
the waveform can cause an average responding meter to be as much as 10% to 40%
inaccurate. A DMM that is True RMS responding is more accurate in these situations
because it calculates the True Root Mean Square (RMS) value of the distorted waveform.
NEC and others now recommended the uses of True RMS meters on today’s electrical
power systems.
In this example of a common light
dimmer the power is turned down to
about 50% output. The Average value
measured was 45.5 volts AC. The True
RMS value was 70 volts AC. The error
between the average reading and the
true effective value was 35%.
Current Measurements
Current is the electron flow that causes electrical equipment to operate. When the
equipment is turned on, it is considered to be a “load” on the circuit. A load is any
electrical component, such as a lamp, stereo, motor or heating element, that draws
current. Current is measured in amperes, or amps.
Each load has a rated current limit that should not be exceeded. If a load pulls too much
current, excessive heat is produced that may cause insulation damage, component failure
and possible fire hazards. If the load is under its rated current limit, it may perform
poorly.
Testing current may be done in several ways, but the most common method, and the most
simple, is with a clamp meter.
This indirect measurement is inherently safer than using a multimeter in series with the
circuit. When making a measurement with a Clamp meter, clamp to either the Hot or
Neutral conductor but not both.
To measure using a meter we must open the circuit and make the measurement in Series
with the load. This is the most potentially hazardous measurement made with a
multimeter because the meter now is a part of the circuit.
12
To measure current with a
multimeter, turn the power off at the
breaker as close to the source as
possible.
Break the circuit, connect the
multimeter in series with the circuit,
and reestablish power.
Tech Notes: Good multimeters are now protected by a high-energy fuse. High energy
fusing is used to protect the meter and the user, but let’s not forget “Murphy’s Law”.
The most common mistake is to accidentally have the test leads in the current input jacks
and make a voltage or parallel measurement. Meters without fuse protection on the
current inputs should not be used on high energy electrical circuits.
From a practical standpoint, only small currents are measured with a multimeter. Most
multimeters have a maximum current capability of 10 amperes. It is also not practical to
shut down power and break the circuit to take a measurement. The most common
application for direct current measurements with a multimeter is small DC currents, like
4-20 mA control loops found in most process control systems.
Using a Clamp or Current Transformer.
Tech Note: When using a Clamp-meter, or a Multimeter with a clamp adapter. A
Clamp-or Current transformer (CT) measures the magnetic field around a conductor.
The strength of the magnetic field is determined by the amount of current flowing through
the conductor. This allows the clamp meter to measure the current flow indirectly.
It is also important that the Clamp be around either to Hot or Neutral. Current flows
through both wires but create magnetic fields in opposite directions. If you clamp around
both wires the meter would read “0”
In a household power cord,
current flows to the load
through the hot conductor, and
returns back through the
neutral conductor. As the
current flow is identical, the
magnetic fields would be
exactly the same strength, and
cancel each other out, resulting
in a measurement of 0.
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'Clamp-meters also allow a much higher level of current measurements. While most
multimeters have a maximum internal current measurement of 10 amps, clamp meters
are available that measure 400, 600 or even as much as 2000 amps. Meters with Clamp
adapters can be used to make high current, but Clamp-meters are much simpler to use.
DC current is measured through the use of a Hall Effect probe. A Hall Effect device is a
semiconductor that when subjected to a magnetic field responds with a voltage output
that is proportional to the field strength. Unlike standard Current Transformer Clamps,
Hall Effect current probes are electronic and powered in some way.
Clamp adapters differ from Clamp-meters in that they are designed to convert the AC or
DC current measurement to a smaller AC or DC signal. This small signal output is
either a millivolt or milliamp output. Most Clamp adapters are marked for the user.
This is an example of the label
on the Ideal 61-334, 600 Amp
Clamp adapter
Review the specifications of the adapter to determine the output signal and the ratio of
the measurement to the output signal. This is typically 1mV/Amp or 1mA/Amp. Be sure
to set the function switch on the meter to the appropriate measurement and place the test
leads in the appropriate ports. Note that the reading will be displayed in millivolts or
milliamps, not in Amps.
Continuity Measurement
Continuity is a quick check to see if a circuit is complete. Good fuses and closed
switches have continuity. During a continuity measurement, the multimeter sends a small
current potential through the circuit to measures the resistance of the circuit. The value
for the maximum resistance can vary from meter to meter. Most will indicate continuity
from 0 to 50 ohms. An audible alarm was added to aid in making fast go-no-go testing
without taking your eyes of your work.
Continuity is a great go no go test
for switches and fuses.
The audible beep gives you the
freedom to keep your eyes on the
work at hand.
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Resistance Measurements
). When you first place the meter in the ( )
function the meter will give a display of “OL” or “1____” indicating an infinite reading.
It is important when measuring Resistance that the circuit be de-energized or turned off,
or the circuit may damage the meter. Most meters have overload protection on all ranges
to prevent this, but you should check the specifications of your digital multimeter to be
sure.
For resistance measurements,
place the test leads on each side
of the resistor.
Other components in parallel with
the resistor being measured will
have an effect on the
measurement.
Diode Measurement
A diode is a semiconductor device which allows current to flow in only one direction.
The standard Ohms function on a digital multimeter does not supply enough energy to
test a diode. The diode function applies an appropriate amount of pressure, (or voltage
potential), and measures the voltage drop across the diode.
To test a diode, first measure the forward bias of the diode. For most silicon diodes the
voltage drop should measure around .5V +/- .2V.
Next, measure the reverse bias of the diode. You should see an “OL” or overload
condition on the display.
Some meters display the voltage potential applied to the diode. In this case, in the reverse
bias you would see the maximum voltage potential. This potential for most meters is
around 3 volts.
Measuring both the
forward and reverse
bias of the diode
ensures that current
will flow in only one
direction
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Capacitance Measurement
A capacitor is a device that stores energy. It is widely used to give a boost of energy at
start up when power is applied to lighting and motor systems. To test a capacitor, first
remove power from the device. Remember that a capacitor stores energy so the next step
is to discharge the device. Now you are ready to test. Never test without verifying that
the energy has been discharged from the capacitor.
Before making a
measurement on a
capacitor, make sure it is not
holding a charge.
Discharge the capacitor,
using a 10,000 to 20,000
ohm 2 or 5 watt resistor.
Frequency Measurement
Frequency is measured in Hertz. This is the number of cycles per second of an
Alternating waveform to complete one cycle or transition from 0 to max amplitude
positive back to 0 to max amplitude negative then back to 0..
Maintaining the right frequency
is crucial for devices that rely
on AC voltage and current.
Otherwise poor performance
and possible damage may
result.
In this example we have 4
cycles in one second so the
frequency is 4Hz
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Advanced Multimeter Functions
Many features are available on today’s advanced digital multimeters to make measuring
electrical systems and components easier. There are two common methods used for these
advanced features. Direct Key Selection or Menu Selection.
With the Direct Key function, “press and hold” for one second will activate the feature.
“Press and hold” for two seconds will disable the function.
Try this with the RANGE key on an Auto-ranging multimeter. Pressing the key for one
second turns manual ranging on and Auto-ranging off. Press again and you can manually
step through the ranges. Press for two seconds and Auto-range is activated.
Menu units use a list of options in the display and “F” keys directly under the options.
Pressing the “F” key below your selection will enable that function. Pressing the key
again will disable it.
Data Hold, Auto Hold and Max Hold
Data hold and auto hold locks the measurement on the display. These features are useful
when making hard to get to measurements like in a panel. They allow you to focus your
attention on the circuit under test instead of the multimeter display.
Data hold captures the display reading when pressed. Attach the common lead (black) to
the desired measurement point with an alligator clip or other type of attachment device.
Connect the red test lead to the circuit under test, press the data hold button then remove
the test leads. Be sure to allow time for the reading to stabilize before pressing the data
hold button to capture the measurement. Remove the leads and the display should hold
the last stable reading.
Auto hold waits to capture the reading until after it stabilizes. Press the auto hold button
and connect the test leads to the measurement circuit. After the reading stabilizes, the
multimeter gives an audible signal to notify the user that the measurement has been
captured. Remove the leads and the reading will stay for a few seconds before resetting to
no reading.
Max hold displays the highest value that the meter has seen during the measurement.
Connect the test leads to the measurement circuit and press the max hold button. The
meter monitors the circuit and gives an audible indication when a new maximum reading
has been obtained.
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Min/Max or Min/Max/Avg.
The min/max button captures the lowest, highest and average value that the meter has
seen during the duration of the measurement. Digital multimeters with a dual display
will show the real-time or instantaneous measurement on the main display, and show the
min-max- or avg. value on the secondary display. Pressing the min/max button will step
you through the minimum, maximum, and average readings recorded during the duration
of the test. As with max hold, most meters will give an audible indication when a new
minimum or maximum value has been captured. “Press and hold” for > 2 seconds will
disable the min/max function.
A typical application would be monitoring a motor for over voltage and under voltage
conditions. Remember to turn off the Auto Power Off function if you will be leaving the
multimeter connected to the circuit for extended periods of time.
Some advanced multimeters will add an averaging function to this feature. It calculates
the average reading over the duration of the test period.
Relative Mode
Relative mode, usually displayed as ▲REL, stores the instantaneous measurement as a
reference value, and sets the display to zero. Measurements are now shown as a
differential to this reference value. To use this function, select the measurement, and
attach the test leads to measurement circuit. Allow the reading to stabilize, and then
press the ▲REL button.
The display should read zero. Take a new measurement. The difference from the new
measurement and the original should be displayed.
This feature is often used when taking very low resistance measurements. The test leads
of every multimeter have some resistance (0.1 to 0.2 Ohms). You can compensate by
measuring the test leads using the relative mode feature. After the test lead resistance has
18
been set as the reference value, all new measurements will be the resistance of the circuit
or component without the test lead resistance.
Peak Hold and Peak Min/Max
Unlike True RMS measurements, which calculate the effective value of the voltage or
current waveforms, Peak measurements capture the highest amplitude of the waveform.
Peak hold is often used to measure in-rush current caused by a motor start-up. Pressing
the Peak Hold button on a clamp meter and clamping the jaws around one leg of the
motor just prior to start-up will capture the highest peak, or in-rush current.
Another way to get this information is with a Peak min/max feature. This feature
captures the highest and lowest amplitudes of the wave form.
A Peak min/max
feature captures the
highest and lowest
points on the sine
wave. When new
minimum or maximum
values are measured,
the multimeter will
capture new readings
and notify the user with
an audible beep.
Duty Factor or [% DF] is a measurement of relative time between positive and negative
parts of one cycle or pulse. How long on versus how long off.
In this TTL square wave
the % of duty factor, time
on versus time off, is
equal or 50%
19
Appendix A
Ohms Law Pie Chart
Direct Current calculation
Symbolic
V
W
Ω
A
C
Hp
EFF
PF
kW
kWh
VA
kVa
Term
Voltage
Watts
Ohms
Amperes
Capacitance
Horsepower
Efficiency
Power Factor
Kilowatts
Kilowatt hour
Voltage Amperes
Kilovolt-Amperes
Symbolic
E
P
R
I
Term
Voltage
Watts
Resistance
Amperes
Real Power
Apparent Power
Direct Current Ohms Law.
Amperes= Watts / Voltage
Watts = Voltage x Amperes
Volts = Watts / Amperes
Horsepower= (Voltage x Amperes x Efficiency) / 746
Efficiency = (746 x Horsepower)/ (Voltage x Amperes)
20
Ohms Law Pie Chart
Alternating Current calculation
AC Single Phase calculations
Amperes = Watts / (Voltage x Power Factor)
Watts = Voltage x Amperes x Power Factor
Voltage = Watts / Amperes
Volt – Amp = Voltage x Amperes
Power Factor = Watts / Volt-Amp
HP = (V x A x Efficiency x PF) / 746
Efficiency = (746 x HP) / (V x A x PF)
A= W/(V x PF)
W=V x A x PF
V= W / A
VA = V x A
PF= W / VA
HP = (V x A x EFF x PF)/ 746
Eff= (746 x HP) / (V x A x PF)
AC Three Phase Calculations
Amperes = Watts / (1.732x Volt x Power Factor)
Watts = 1.732 x Volts x Amperes x Power Factor
Voltage = Watts / Amperes
Volt – Amp = 1.732 Volts x Amperes
Power Factor = Watts / (1.732 x Volt-Amp)
HP = (1.732 x V x A x Efficiency x PF) / 746
Efficiency = (746 x HP) / (1.732 x V x A x PF)
21
A= W/(1.732 x V x PF)
W=1.732 x V x A x PF
V= W / A
VA = 1.732 x V x A
PF= W / (1.732 x VA)
HP = (1.732 x V x A x EFF x PF)/ 746
Eff= (746 x HP) / (1.732 x V x A x PF)
Appendix B
Measurement Functions
Symbols
VAC
VDC
Measurement
Functions
AC Voltage
DC Voltage
Measures amount of AC
Electrical Pressure
Measures amount of DC
Electrical Pressure
Milli Volts
.00V or 1/1000V
Amperes
Measures amount of electron
flow
mA
Milli Amperes
.001 or 1/1000A
µA
Micro Amperes
.000001A or 1/1,000,000A
mV
A
Ohms
Diode
Audible Continuity
Capacitance
HZ
o
o
22
Descriptions
Hertz
Measurement of resistance to
the flow of electron
Device used to control
direction of electron flow
Audible indication of
continuity for low resistance
Device used to store
electrical potential
Measurement of Frequency
or number of cycles per/sec
F
Degrees Fahrenheit
Temperature measurement
C
Degrees Celsius
Temperature measurement
Glossary
A, ampere or amp — The basic unit of electric current.
AC, alternating current — An electric signal in which the current and voltage vary in a
repeating pattern over time; the most common type of voltage.
analog meter — A mechanical measuring device using a needle moving across a
graduated scale or dial.
APO- Auto-Power — Off Automatically shuts down unit after a certain amount of time
to preserve battery life. Most meters with APO may be disabled or set to a certain amount
of time before shutting off.
auto ranging — A DMM that automatically selects the range with the best resolution
and accuracy in response to the sensed values.
calibration — To adjust the meter measured value to a recognized artifact or standard.
capacitance — Ability of a component to hold an electrical charge, usually stated in
microfarads.
capacitor — Electronic component which stores energy and then discharges it rapidly;
blocks DC and allows AC to pass through.
clamp-on — DMM with jaws that allow it to fit around a conductor to measure AC or
DC current without breaking the circuit.
contact — A connection between two conductors that allows a flow of current.
continuity — A continuous path for current flow in a closed circuit.
current — The flow of an electrical charge through a conductor; measured in amperes or
amps.
DC, direct current — a direct, steady voltage; typically produced through
electromagnetism, chemicals (batteries), light, heat or pressure.
data hold — Feature of a DMM that allows continued display of the last reading taken
after probes have been removed.
diode — Electronic device in circuits that allows current to flow easily in only one
direction and blocks flow in the opposite direction.
DMM, digital multimeter — An instrument that uses an LCD display typically capable
of measuring voltage, current and resistance.
F, farad — The basic unit of capacitance.
frequency — The number of cycles per second that a wave form repeats; measured in
hertz. (Line voltage in the U.S. is 60 Hz.)
ground — A large conducting body (earth) used as a common return for fault current in a
circuit.
H, hertz — One cycle per second; the unit of frequency.
harmonics — A signal with a frequency which is an integer multiple of the fundamental
frequency (60Hz); may damage or degrade the performance of electrical devices.
harmonic distortion — Diminishes power quality; caused by non-linear loads such as
variable speed motor drives, electronic lighting ballasts and computers.
impedance — Total opposition to current flow; includes resistance, capacitance and
reactance.
load — Any device which consumes power in a circuit.
manual ranging — DMM that requires the user to manually select the range, using the
meter’s dial.
23
min/max — Feature that allows a meter to capture and store the highest and lowest
readings during a specific measurement.
ohm — The basic unit of resistance, specified as equal to that of a conductor in which
one amp of current is produced by one volt of potential across its terminals.
OL, overload — Signal amplitudes or frequencies above the specified limits of the
instrument; typically displayed as “OL” on the display of a DMM.
peak hold — Feature of DMM that allows retention of highest reading in a series of
measurements.
polarity — The positive or negative direction of DC voltage or current.
resolution — Increments in value that can be displayed by a DMM; the greater the
resolution the more precise the readout.
resistance — Opposition to current; measured in ohms.
Sleep mode — Automatically shuts down unit not in use to preserve battery life.
short — Any connection that has relatively low resistance or any resistance between two
points below a preselected threshold. Typically, this is unintended.
True RMS meter — DMM that has the True RMS feature, allowing for accurate
measurement of AC voltage in environments with harmonics (see harmonics).
V, volt — The unit of electrical pressure; one volt is the potential difference needed to
cause one amp of current to pass through one Ohm of resistance.
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