Application - Fluke Connect™ ABCs of DMMs
ABCs of DMMs
Multimeter features and
functions explained
Application Note
Digital multimeters offer a wide selection of features. Choosing the
right meter for the job can be challenging unless you know what
the features do. This application note explains some of the most
common features and how they can be used in actual applications.
Choosing your DMM
Multimeters. They’ve been
described as the tape measure
of the new millennium. But what
exactly is a digital multimeter
(DMM) and what can you do with
it? How do you make measurements safely? What features do
you need? What is the easiest
way to get the most out of your
meter? Which meter is best
suited to the environment you’re
working in? These and other
questions are answered in this
application note.
Technology is rapidly changing our world. Electrical and
electronic circuitry seems to permeate everything, and continues
to get more complex and smaller
in size. The communication
industry booms with cell phones
and pagers, and Internet connections have put more pressure on
the electronics technician. Servicing, repairing, and installing
this complex equipment requires
diagnostic tools that provide
accurate information.
Let’s begin by explaining
what a DMM is. A DMM is simply
an electronic tape measure for
making electrical measurements.
It may have any number of
special features, but mainly a
DMM measures volts, ohms, and
Fluke DMMs are used for
examples in this application
note. Other DMMs may operate differently or offer different
features from the ones shown.
However, this application note
explains common uses and tips
for using most DMMs. In the next
few pages, we will discuss how
to use a DMM to make measurements, and how DMMs differ
from one another.
Choosing a DMM for the job
requires not only looking at basic
specifications, but also looking
at features, functions, and the
overall value represented by
a meter’s design and the care
taken in its production.
Reliability, especially under
tough conditions, is more important than ever today. Another
important factor is safety. Providing adequate component spacing,
double insulation, and input
protection helps prevent injury
and meter damage when they
are used improperly. Choose a
DMM designed to the latest, most
demanding safety standards.
Productivity is also critical.
The equipment being serviced
today is more complex than ever.
The right DMM can make your
job faster, safer and easier.
Some basics
Resolution, digits and counts
Resolution refers to how fine a
measurement a meter can make.
By knowing the resolution of a
meter, you can determine if it is
possible to see a small change in
the measured signal. For example, if the DMM has a resolution
of 1 mV on the 4 V range, it is
possible to see a change of 1 mV
(1/1000 of a volt) while reading
1 V.
You wouldn’t buy a ruler
marked in one-inch (or one-centimeter) segments if you had to
measure down to a quarter inch
(or one millimeter). A thermometer that measures only in whole
degrees isn’t much use when
your normal temperature is
98.6 °F. You need a thermometer with one-tenth degree
The terms digits and counts
are used to describe a meter’s
resolution. DMMs are grouped by
the number of counts or digits
they display.
A 31⁄2-digit meter can display
three full digits ranging from 0
to 9, and one “half” digit which
displays only a 1 or is left blank.
A 31⁄2-digit meter will display up
to 1,999 counts of resolution. A
41⁄2-digit meter can display up to
19,999 counts of resolution.
It is more precise to describe
a meter by counts of resolution
than by digits. Today’s 31⁄2-digit
meters may have enhanced resolution of up to 3,200, 4,000, or
6,000 counts.
For certain measurements,
3,200-count meters offer better
resolution. For example, a 1,999count meter won’t be able to
measure down to a tenth of a
volt if you are measuring 200
volts or more. However, a 3,200count meter will display a tenth
of a volt up to 320 volts. This is
the same resolution as a more
expensive 20,000-count meter
until you exceed 320 volts.
Accuracy is the largest allowable error that will occur under
specific operating conditions. In
other words, it is an indication
of how close the DMM’s displayed measurement is to the
actual value of the signal being
Accuracy for a DMM is usually expressed as a percent of
reading. An accuracy of one
percent of reading means that for
a displayed reading of 100 volts,
the actual value of the voltage
could be anywhere between 99
volts and 101 volts.
Specifications may also
include a range of digits added
to the basic accuracy specification. This indicates how many
counts the digit to the extreme
right of the display may vary. So
the preceding accuracy example
might be stated as ± (1 % + 2).
Therefore, for a display reading
of 100 volts, the actual voltage
would be between 98.8 volts
and 101.2 volts.
Analog meter specifications
are determined by the error at
full scale, not at the displayed
reading. Typical accuracy for an
analog meter is ± 2 % or ± 3 %
of full scale. At one-tenth of full
scale, these become 20 percent
or 30 percent of reading. Typical basic accuracy for a DMM is
between ± (0.7 % + 1) and ±
(0.1 % + 1) of reading, or better.
(V) Voltage
(A) Current
Ohm’s law
(Ω) Resistance
Voltage, current, and resistance
in any electrical circuit can be
calculated by using Ohm’s Law,
which states that voltage equals
current times resistance (see
Figure 1). Thus, if any two values
in the formula are known, the
third can be determined.
A DMM makes use of Ohm’s
Law to directly measure and
display either ohms, amps, or
volts. On the following pages,
you will see just how easy it is to
use a DMM to find the answers
you need.
Digital and analog displays
For high accuracy and resolution, the digital display excels,
displaying three or more digits
for each measurement.
The analog needle display
is less accurate and has lower
effective resolution because you
have to estimate values between
the lines.
A bar graph shows changes
and trends in a signal just like an
analog needle, but is more durable and less prone to damage.
Saving and sharing results
As the equipment
you service has
grown more complex
and more powerful,
so have the DMMs
available to service
it. Wireless test tools can send
test results to each other and
to smartphones, where you can
share data, images and notes
with coworkers. Wireless DMMs,
other related test tools, and
smart phone apps, like Fluke
Connect ™ let you make the best
decisions faster than ever before,
saving time and increasing your
V = Volts
A = Current in Amps
Ω = Resistance in Ohms
Ohm’s Law explains the relationship between voltage,
current and resistance.
Put your finger over the value you want to find. Multiply
the remaining values if side-by-side; divide if one is over
the other. But it really is much easier just to use your DMM.
Figure 1.
DC and AC voltage
Measuring voltage
One of the most basic tasks of
a DMM is measuring voltage.
A typical dc voltage source is
a battery, like the one used in
your car. AC voltage is usually
created by a generator. The
wall outlets in your home are
common sources of ac voltage.
Some devices convert ac to dc.
For example, electronic equipment such as TVs, stereos, VCRs,
and computers that you plug into
an ac wall outlet use devices
called rectifiers to convert the ac
voltage to a dc voltage. This dc
voltage is what powers the electronic circuits in these devices.
Testing for proper supply voltage is usually the first step when
troubleshooting a circuit. If there
is no voltage present, or if it is
too high or too low, the voltage
problem should be corrected
before investigating further.
2 Fluke Education Partnership Program ABCs of DMMs: Multimeter features and functions explained
The waveforms associated
with ac voltages are either
sinusoidal (sine waves), or
non-sinusoidal (sawtooth,
square, ripple, etc.). True-rms
DMMs display the “rms” (root
mean square) value of these voltage waveforms. The rms value
is the effective or equivalent dc
value of the ac voltage.
Many DMMs are “average
responding,” giving accurate rms
readings if the ac voltage signal
is a pure sine wave. Average
responding meters are not capable of measuring non-sinusoidal
signals accurately. Non-sinusoidal signals are accurately
measured using DMMs designated “true-rms” up to the DMM’s
specified crest factor. Crest factor
is the ratio of a signal’s peak-torms value. It’s 1.414 for a pure
sine wave, but is often much
higher for a rectifier current
pulse, for example. As a result,
an average responding meter
will often read much lower than
the actual rms value.
A DMM’s ability to measure
ac voltage can be limited by the
frequency of the signal. Most
DMMs can accurately measure ac
voltages with frequencies from
50 Hz to 500 Hz, but a DMM’s ac
measurement bandwidth may be
hundreds of kilohertz wide. Such
a meter may read a higher value
because it is “seeing” more of a
complex ac signal. DMM accuracy specifications for ac voltage
and ac current should state the
frequency range along with the
range’s accuracy.
How to make voltage
Figure 3. Accessories, such as high-voltage probes,
extend the voltage measurement range of a DMM.
Figure 2. Three voltage signals: dc, ac sine wave, and
non-sinusoidal ac signal.
Note: For dc readings of the correct polarity
(±), touch the red test probe to the positive
side of the circuit, and the black probe to the
negative side or circuit ground. If you reverse
the connections, a DMM with autopolarity
will merely display a minus sign indicating
negative polarity. With an analog meter, you
risk damaging the meter.
Note: 1/1000 V = 1 mV
1000 V = 1 kV
High-voltage probes are available for TV and
CRT repair, where voltages can reach 40 kV
(see Figure 3).
Caution: These probes are not intended for
electrical utility applications in which high
voltage is also accompanied by high energy.
Rather, they are intended for use in low-energy applications.
Resistance, continuity
and diodes
Resistance is measured in ohms
(Ω). Resistance values can vary
1 Select V~ (ac) or V (dc), as
greatly, from a few milliohms
(mΩ) for contact resistance to bil2. Plug the black test probe into
lions of ohms for insulators. Most
the COM input jack. Plug the
DMMs measure down to
red test probe into the V input
0.1 Ω, and some measure as
high as 300 MΩ (300,000,000
3. If the DMM has manual rangohms). Infinite resistance (open
ing only, select the highest
circuit) is read as “OL” on the
range so as not to overload the
Fluke meter display, and means
the resistance is greater than the
4. Touch the probe tips to the
meter can measure.
circuit across a load or power
Resistance measurements
source (in parallel to the
must be made with the circuit
power off—otherwise, the meter
5. View the reading, being
or circuit could be damaged.
sure to note the unit of
Some DMMs provide protection
in the ohms mode in case of
accidental contact with voltages.
The level of protection may vary
greatly among different DMM
For accurate, low-resistance
measurements, resistance in the
test leads must be subtracted
from the total resistance measured. Typical test lead resistance
is between 0.2 Ω and 0.5 Ω. If
the resistance in the test leads is
greater than 1 Ω, the test leads
should be replaced.
If the DMM supplies less than
0.6 V dc test voltage for measuring resistance, it will be able to
measure the values of resistors
that are isolated in a circuit by
diodes or semiconductor junctions. This often allows you to
test resistors on a circuit board
without unsoldering them (see
Figure 4).
3 Fluke Education Partnership Program ABCs of DMMs: Multimeter features and functions explained
Auto Range Manual Range 610000mV
Figure 4. For measuring
resistance in the presence
of diodes, DMM test voltages
are kept below 0.6 V so the
semiconductor junctions do
not conduct current.
How to make resistance
1. Turn off power to the circuit.
2. Select resistance (Ω).
3. Plug the black test probe into
the COM input jack. Plug the
red test probe into the Ω input
4. Connect the probe tips across
the component or portion of
the circuit for which you want
to determine resistance.
5. View the reading, being sure
to note the unit of measurement—ohms (Ω), kilohms (kΩ),
or megohms (MΩ).
Note: 1,000 Ω = 1 kΩ
1,000,000 Ω = 1 MΩ
Make sure the power is off before making
resistance measurements.
Continuity is a quick go/no-go
resistance test that distinguishes
between an open and a closed
A DMM with a continuity
beeper allows you to complete
many continuity tests easily
and quickly. The meter beeps
when it detects a closed circuit,
so you don’t have to look at the
meter as you test. The level of
resistance required to trigger
the beeper varies from model to
model of DMM.
Diode test
A diode is like an electronic
switch. It can be turned on if the
voltage is over a certain level,
generally about 0.6 V for a silicon diode, and it allows current
to flow in one direction.
When checking the condition
of a diode or transistor junction, an analog VOM not only
gives widely varying readings
but can drive currents up to
50 mA through the junction (see
Table 1).
Some DMMs have a diode test
mode. This mode measures and
displays the actual voltage drop
across a junction. A silicon junction should have a voltage drop
less than 0.7 V when applied
in the forward direction and an
open circuit when applied in the
reverse direction.
DC and AC current
This causes a direct short across
the source voltage through a
Measuring current
low-value resistor inside the
Current measurements are
DMM, called a current shunt. A
different from other DMM
high current flows through the
measurements. Current meaDMM and if it is not adequately
surements taken with the DMM
protected, can cause extreme
alone require placing the meter
damage to both the DMM and
in series with the circuit being
the circuit, and possible injury to
measured. This means opening
the operator. Extremely high fault
the circuit and using the DMM
currents can occur if industrial
test leads to complete the circuit. high-voltage circuits are involved
This way all the circuit current
(240 V or higher).
flows through the DMM’s cirA DMM should therefore have
cuitry. An indirect method of
current input fuse protection of
measuring current on a DMM can high enough capacity for the
be performed using a current
circuit being measured. Meters
probe. The probe clamps around without fuse protection in the
the outside of the conductor, thus current inputs should not be used
avoiding opening the circuit and on high-energy electrical circuits
connecting the DMM in series.
(> 240 V ac). Those DMMs that
do use fuses should have a fuse
How to make current
with sufficient capacity to clear
a high-energy fault. The voltage
1. Turn off power to the circuit.
2. Cut or unsolder the circuit, cre- rating of the meter’s fuses should
ating a place where the meter be greater than the maximum
voltage you expect to measure.
probes can be inserted.
For example, a 20 A, 250 V fuse
3. Select A~ (ac) or A (dc) as
may not be able to clear a fault
inside the meter when the meter
4. Plug the black test probe into
is across a 480 V circuit. A 20 A,
the COM input jack. Plug the
red test probe into the amp or 600 V fuse would be needed to
milliamp input jack, depending clear the fault on a 480 V circuit.
on the expected value of the
Current probe accessories
Sometimes you may have to
5. Connect the probe tips to the
make a current measurement
circuit across the break so that that exceeds the rating of your
all current will flow through
DMM or the situation does not
the DMM (a series connection). allow you to open the circuit to
6. Turn the circuit power back
measure the current. In these
higher current applications
7. View the reading, being
(typically over 2 A), where high
sure to note the unit of
accuracy is not needed, a current
probe is very useful. A current
Note: If the test leads are reversed for a dc
probe clamps around the conmeasurement, a “–” will show in the display.
ductor carrying the current, and
it converts the measured value to
Input protection
a level the meter can handle.
A common mistake is to leave
the test leads plugged into the
current input jacks and then
attempt a voltage measurement.
Diode Test
35 mA to 50 mA
0.5 mA to 1.5 mA
Junction Current
0.5 mA to 1 mA
Germanium8 Ω to 19 Ω200 Ω to 300 Ω
0.225 V to 0.225 V
Silicon8 Ω to 16 Ω450 Ω to 800 Ω
0.4 V to 0.6 V
Table 1.
4 Fluke Education Partnership Program ABCs of DMMs: Multimeter features and functions explained
There are two basic types of
Never attempt a voltage
Always make sure the
current probes: current transmeasurement with the test
power is off before cutting
formers, which are used to
probes in the current jack.
or unsoldering the circuit
measure ac current only, and
Meter damage or personal
and inserting the DMM for
Hall-Effect probes, which are
injury may result.
current measurements.
used to measure ac or dc current.
Even small amounts of
The output of a current transcurrent can be dangerous.
former is typically 1 milliamp per
amp. A 100 amp value is reduced
to 100 milliamps, which can be
safely measured by most DMMs.
The probe leads are connected to
the “mA” and “COM” input jacks,
and the meter function switch is
set to mA ac.
The output of a Hall-Effect
probe is 1 millivolt per amp, ac
or dc. For example, 100 A ac is
converted to 100 mV ac. The
probe leads are connected to the
“V” and “COM” jacks. Set the
A transformer-type current probe, such as the one
A Hall-Effect probe safely measures high-current
meter function switch to the “V”
depicted above, scales down the current being
ac or dc values by scaling down the current being
measured and converting this reduced current to a
measured. The DMM displays 1 mA for every amp
or “mV” scale, selecting V~ for
being measured.
voltage. The DMM displays 1 mV for every amp.
ac current or V for dc current
measurements. The meter
Figure 5.
displays 1 millivolt for every amp
been designed to IEC standards,
Look for these safety
but has been independently
features in a DMM:
tested and meets those stan1. Fused current inputs.
dards. (See Independent Testing
2. Use of high-energy fuses
Multimeter safety
sidebar on page 6.)
(600 V or more).
Making measurements safely
High-voltage protection in
Common situations that lead
starts with choosing the proper
resistance mode (500 V or
meter for the application as well to DMM failure:
1. Contact with ac power source
as the environment in which
Protection against voltage
while test leads are plugged
the meter will be used. Once the
transients (6 kV or more).
into current jacks
proper meter has been chosen,
5. Safety-designed test leads
you should use it by following
with finger guards and
while in resistance mode
good measurement procedures.
shrouded terminals.
3. Exposure to high voltage
Carefully read the instrument
Independent safety organizatransients
user manual before use, paying
tion approval/listing (e.g., UL
particular attention to the WARN- 4. Exceeding maximum input
or CSA).
limitations (voltage and
ING and CAUTION sections.
Keep yourself away from
The International Electrotechnidangerous panels
cal Commission (IEC) established Types of DMM protection
Your DMM can also protect you
safety standards for working on
be keeping you away from
electrical systems. Make sure you 1. Protection with automatic
hazardous situations. DMMs that
are using a meter that meets the
recovery. Some meters have
communicate wirelessly with
IEC category and voltage rating
circuitry that detects an overpersonal computers, smartphones
approved for the environment
load condition and protects
and other wireless test tools can
where the measurement is to be
the meter until the condibe placed safely inside electrical
made. For instance, if a voltage
tion no longer exists. After
panels with the power shut off.
measurement needs to be made
the overload is removed, the
When the panel is closed and
in an electrical panel with
DMM automatically returns to
480 V, then a meter rated Catenormal operation. Usually used reenergized, measurements can
be made remotely, saved and
gory III 600 V or 1000 V should
to protect the ohms function
shared, all without putting yourbe used. This means the input
from voltage overloads.
self in front of a live electrical
circuitry of the meter has been
2. Protection without
panel. Diagnosing and solving
designed to withstand voltage
automatic recovery. Some
problems has never been easier.
transients commonly found in
meters will detect an overthis environment without harmload condition and protect the
ing the user. Choosing a meter
meter, but will not recover
with this rating which also has a
until the operator performs an
UL, CSA, VDE or TÜV certification
operation on the meter, such
means the meter not only has
as replacing a fuse.
5 Fluke Education Partnership Program ABCs of DMMs: Multimeter features and functions explained
Measurement categories
A critically important concept to understand about
electrical safety is the measurement category.
Standards define Categories 0 through IV, often
abbreviated as CAT 0, CAT II, etc.
The division of a power distribution system
into categories is based on the fact that a dangerous high-energy transient such as a lightning
strike will be attenuated or dampened as it travels
through the impedance (ac resistance) of the
system. A higher CAT number refers to an electrical environment with higher power available
Measurement category In brief
Three-phase at
utility connection,
any outdoor mains
including singlephase commercial
connected loads
Safety checklist
and higher energy transients. Thus a multimeter
designed to a CAT III standard is resistant to much
higher energy transients than one designed to
CAT II standards.
Within a category, a higher voltage rating
denotes a higher transient withstand rating, e.g.,
a CAT III 1000 V meter has superior protection
compared to a CAT III 600 V rated meter. The real
misunderstanding occurs if someone selects a
CAT II 1000 V rated meter thinking that it is superior to a CAT III 600 V meter.
Refers to the “origin of installation,” i.e., where low-voltage connection is made to utility power
Electricity meters, primary overcurrent protection equipment
Outside and service entrance, service drop from pole to building, run between meter and panel
Overhead line to detached building, underground line to well pump
Equipment in fixed installations, such as switchgear and poly-phase motors
Bus and feeder in industrial plants
Feeders and short branch circuits, distribution panel devices
Lighting systems in larger buildings
Appliance outlets with short connections to service entrance
Appliance, portable tools, and other household and similar loads
Outlet and long branch circuits
– Outlets at more than 10 meters (30 feet) from CAT III source
– Outlets at more than 20 meters (60 feet) from CAT IV source
Protected electronic equipment
Equipment connected to (source) circuits in which measures are taken to limit transient overvoltages to an appropriately low level
Any high-voltage, low-energy source derived from a high-winding resistance transformer, such
as the high-voltage
3 Use a meter that meets accepted safety standards
for the environment in which it will be used.
3 Use a meter with fused current inputs and be
sure to check the fuses before making current
3 Inspect test leads for physical damage before
making a measurement.
3 Use the meter to check continuity of the test
3 Use only test leads that have shrouded connectors and finger guards.
3 Use only meters with recessed input jacks.
3 Select the proper function and range for your
3 Be certain the meter is in good operating
3 Follow all equipment safety procedures.
3 Always disconnect the “hot” (red) test lead first.
3 Don’t work alone.
3 Use a meter that has overload protection on the
ohms function.
3 When measuring current without a current
clamp, turn the power off before connecting into
the circuit.
3 Be aware of high-current and high-voltage situations and use the appropriate equipment, such
as high-voltage probes and high-current clamps.
Meter ratings and capabilities vary by manufacturer.
Before working with a new meter, be sure to familiarize
yourself with all operating and safety procedures for
that meter contained in the users manual.
Independent testing is the key to safety compliance
How can you tell if you’re getting a genuine CAT III or CAT
II meter? It’s not always easy. It is possible for a manufacturer to self-certify its meters as CAT II or CAT III without
any independent verification. Beware of wording such as
“Designed to meet specifications...” Designer’s plans are
never a substitute for an actual independent test. The IEC
(International Electrotechnical Commission) develops and
proposes standards, but it is not responsible for enforcing
the standards.
Look for the symbol and listing number of an independent testing lab such as UL, CSA, TÜV or other recognized
approval agency. That symbol can only be used if the
product successfully completed testing to the agency’s
standard, which is based on national/international standards. UL 61010, for example, is based on IEC 61010.
In an imperfect world, that is the closest you can come
to ensuring that the multimeter you choose was actually
tested for safety.
6 Fluke Education Partnership Program ABCs of DMMs: Multimeter features and functions explained
Accessories and glossary Annunciator. A symbol that
DMM accessories
One very important requirement
of a DMM is that it can be used
with a wide variety of accessories. Many accessories are
available that can increase your
DMM’s measurement range and
usefulness, while making your
measurement tasks easier.
High-voltage probes and
current probes scale down high
voltages and currents to a level
the DMM can safely measure.
Temperature probes convert
your DMM into a handy digital
thermometer. RF probes can be
used to measure voltages at high
Furthermore, a selection of
test leads, test probes, and test
clips can help you easily connect
your DMM to the circuit. Soft and
hard carrying cases protect your
DMM and conveniently store
your accessories with your DMM.
Accuracy. How close the DMM’s
displayed measurement is to
the actual value of the signal
being measured. Expressed as
a percentage of reading or as a
percentage of full scale.
Analog meter. An instrument
that uses a needle movement to
display the value of a measured
signal. The user judges the reading based on the position of the
needle on a scale.
identifies a selected range or
Average responding DMM. A
DMM that accurately measures
sinusoidal waveforms, while
measuring non-sinusoidal waveforms with less accuracy.
Count. A number used to specify
a DMM’s resolution.
Current-shunt. A low-value
resistor in a DMM for measuring
current. The DMM measures the
voltage drop across the current
shunt and, using Ohm’s Law, calculates the value of the current.
DMM, digital multimeter. An
instrument that uses a digital
display to show the value of a
measured signal. DMMs feature
greater durability, resolution, and
far more accuracy than analog
Non-sinusoidal waveform. A
distorted waveform such as a
pulse train, square waves, triangular waves, sawtooth waves
and spikes.
Resolution. The degree to
which small changes in a measurement can be displayed.
rms. The equivalent dc value of
an ac waveform.
Sinusoidal waveform. A pure
sine wave without distortion.
True-rms DMM. A DMM that
can accurately measure both
sinusoidal and non-sinusoidal
Special features
The following special features
and functions may make it easier
to use your DMM.
• Annunciators show at a
glance what is being measured (volts, ohms, etc.).
• One-switch operation makes
it easy to select measurement
• Overload protection prevents
damage to both the meter and
the circuit, while protecting
the user.
• Special high-energy fuses
provide extra protection for
user and meter during current
measurements and overloads.
• Autoranging automatically
selects proper measurement
range. Manual ranging lets
you lock into a specific range
for repetitive measurements.
• Autopolarity indicates negative readings with a minus
sign, so even if you connect
the test leads in reverse you
won’t damage the meter.
• Low battery indicator.
Fluke Corporation
PO Box 9090, Everett, WA USA 98206
©2006-2014 Fluke Corporation. All rights reserved.
Printed in U.S.A. 4/2014 2100079D_EN
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7 Fluke Education Partnership Program ABCs of DMMs: Multimeter features and functions explained
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