Fluke 83V Application Note

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Application Note

Introduction

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 measure-

ments 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 chang-

ing our world. Electrical and elec-

tronic circuitry seems to permeate

everything, and continues to get

more complex and smaller in

size. The communication indus-

try booms with cell phones and

pagers, and Internet connections

have put more pressure on the

electronics technician. Servicing,

repairing, and installing this com-

plex equipment requires diag-

nostic tools that provide accurate

information.

Let’s begin by explaining what

a DMM is. A DMM is simply an

electronic tape measure for mak-

ing electrical measurements. It

may have any number of special

features, but mainly a DMM mea-

sures volts, ohms, and amperes.

Fluke DMMs are used for

examples in this application

note. Other DMMs may oper-

ate 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 measure-

ments, and how DMMs differ from

one another.

ABCs of DMMs

Multimeter features and

functions explained

Choosing your DMM

Choosing a DMM for the job

requires not only looking at basic

specifications, but also looking at

features, functions, and the over-

all value represented by a meter’s

design and the care taken in its

production.

Reliability, especially under

tough conditions, is more impor-

tant than ever today. Another

important factor is safety. Provid-

ing adequate component spac-

ing, 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.

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 exam-

ple, 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-cen-

timeter) segments if you had to

measure down to a quarter inch

(or one millimeter). A thermom-

eter 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 resolution.

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.

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.

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 reso-

lution of up to 3,200, 4,000, or

6,000 counts.

For certain measurements,

3,200-count meters offer better

resolution. For example, a 1,999-

count meter won’t be able to

measure down to a tenth of a volt

if you are measuring 200 volts or

more. However, a 3,200-count

meter will display a tenth of a

volt up to 320 volts. This is the

same resolution as a more expen-

sive 20,000-count meter until

you exceed 320 volts.

Accuracy

Accuracy is the largest allowable

error that will occur under spe-

cific operating conditions. In other

words, it is an indication of how

close the DMM’s displayed mea-

surement is to the actual value of

the signal being measured.

Accuracy for a DMM is usually

expressed as a percent of read-

ing. An accuracy of one percent

of reading means that for a dis-

played 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 preced-

ing 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. Typi-

cal basic accuracy for a DMM is

between ± (0.7 % + 1) and ±

(0.1 % + 1) of reading, or better.

Ohm’s law

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 Fig-

ure 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 dis-

play 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, dis-

playing 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.

DC and AC voltage

Measuring voltage

One of the most basic tasks of a

DMM is measuring voltage. A typ-

ical 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 comput-

ers 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 volt-

age 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 volt-

age problem should be corrected

before investigating further.

The waveforms associated

with ac voltages are either sinu-

soidal (sine waves), or non-sinu-

soidal (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 capa-

ble 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-to-rms

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.

(Ω) Resistance

(V) Voltage

(A) Current

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.

(A)

Current

(Ω)

Resistance

(V)

Voltage V = A x Ω

Where:

V = Volts

A = Current in Amps

Ω= Resistance in Ohms

Figure 1.

2 Fluke Education Partnership Program ABCs of DMMs: Multimeter features and functions explained

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 accu-

racy specifications for ac voltage

and ac current should state the

frequency range along with the

range’s accuracy.

How to make voltage

measurements

1 Select V~ (ac) or V (dc), as

desired.

2. Plug the black test probe into

the COM input jack. Plug the

red test probe into the V input

jack.

3. If the DMM has manual rang-

ing only, select the highest

range so as not to overload the

input.

4. Touch the probe tips to the

circuit across a load or power

source (in parallel to the

circuit).

5. View the reading, being sure

to note the unit of measure-

ment.

Note: For dc readings of the cor-

rect 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 con-

nections, a DMM with autopolar-

ity 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 avail-

able 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 volt-

age is also accompanied by high

energy. Rather, they are intended

for use in low-energy applications.

Resistance, continuity

and diodes

Resistance

Resistance is measured in ohms

(Ω). Resistance values can vary

greatly, from a few milliohms

(mΩ) for contact resistance to bil-

lions of ohms for insulators. Most

DMMs measure down to

0.1 Ω, and some measure as high

as 300 MΩ (300,000,000 ohms).

Infinite resistance (open circuit) is

read as “OL” on the Fluke meter

display, and means the resistance

is greater than the meter can

measure.

Resistance measurements must

be made with the circuit power

off—otherwise, the meter or cir-

cuit could be damaged. 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 models.

For accurate, low-resistance

measurements, resistance in the

test leads must be subtracted

from the total resistance mea-

sured. 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 measur-

ing 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).

Figure 2. Three voltage signals: dc, ac sine wave, and

non-sinusoidal ac signal.

k

1000 Ω

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.

Figure 3. Accessories, such as high-voltage probes,

extend the voltage measurement range of a DMM.

3 Fluke Education Partnership Program ABCs of DMMs: Multimeter features and functions explained

How to make resistance

measurements:

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

jack.

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 measure-

ment—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

Continuity is a quick go/no-go

resistance test that distinguishes

between an open and a closed

circuit.

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 junc-

tion 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

Measuring current

Current measurements are dif-

ferent from other DMM measure-

ments. Current measurements

taken with the DMM alone

require placing the meter in

series with the circuit being mea-

sured. This means opening the

circuit and using the DMM test

leads to complete the circuit. This

way all the circuit current flows

through the DMM’s circuitry. An

indirect method of measuring

current on a DMM can be per-

formed using a current probe.

The probe clamps around the

outside of the conductor, thus

avoiding opening the circuit and

connecting the DMM in series.

How to make current

measurements

1. Turn off power to the circuit.

2. Cut or unsolder the circuit, cre-

ating a place where the meter

probes can be inserted.

3. Select A~ (ac) or A (dc) as

desired.

4. Plug the black test probe into

the COM input jack. Plug the

red test probe into the amp or

milliamp input jack, depending

on the expected value of the

reading.

5. Connect the probe tips to the

circuit across the break so that

all current will flow through

the DMM (a series connection).

6. Turn the circuit power back on.

7. View the reading, being

sure to note the unit of

measurement.

Note: If the test leads are

reversed for a dc measurement, a

“–” will show in the display.

Input protection

A common mistake is to leave

the test leads plugged into the

current input jacks and then

attempt a voltage measurement.

This causes a direct short across

the source voltage through a

low-value resistor inside the

DMM, called a current shunt. A

high current flows through the

DMM and if it is not adequately

protected, can cause extreme

damage to both the DMM and

the circuit, and possible injury to

the operator. Extremely high fault

currents can occur if industrial

high-voltage circuits are involved

(240 V or higher).

A DMM should therefore have

current input fuse protection of

high enough capacity for the

circuit being measured. Meters

without fuse protection in the

current inputs should not be used

on high-energy electrical circuits

(> 240 V ac). Those DMMs that

do use fuses should have a fuse

with sufficient capacity to clear

a high-energy fault. The voltage

rating of the meter’s fuses should

be greater than the maximum

voltage you expect to measure.

For example, a 20 A, 250 V fuse

may not be able to clear a fault

inside the meter when the meter

is across a 480 V circuit. A 20 A,

600 V fuse would be needed to

clear the fault on a 480 V circuit.

Current probe accessories

Sometimes you may have to

make a current measurement that

exceeds the rating of your DMM

or the situation does not allow

you to open the circuit to mea-

sure the current. In these higher

current applications (typically

over 2 A), where high accuracy

is not needed, a current probe

is very useful. A current probe

clamps around the conductor car-

rying the current, and it converts

the measured value to a level the

meter can handle.

Table 1.

VOM VOM DMM

Range Rx1 Rx100 Diode Test

Junction Current 35 mA to 50 mA 0.5 mA to 1.5 mA 0.5 mA to 1 mA

Germanium 8 Ω to 19 Ω 200 Ω to 300 Ω 0.225 V to 0.225 V

Silicon 8 Ω to 16 Ω 450 Ω to 800 Ω 0.4 V to 0.6 V

4 Fluke Education Partnership Program ABCs of DMMs: Multimeter features and functions explained

Always make sure the

power is off before cutting

or unsoldering the circuit

and inserting the DMM for

current measurements.

Even small amounts of

current can be dangerous.

Never attempt a voltage

measurement with the test

probes in the current jack.

Meter damage or personal

injury may result.

There are two basic types of

current probes: current transform-

ers, which are used to measure

ac current only, and Hall-Effect

probes, which are used to mea-

sure ac or dc current.

The output of a current trans-

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 con-

verted to 100 mV ac. The probe

leads are connected to the “V” and

“COM” jacks. Set the meter func-

tion switch to the “V” or “mV”

scale, selecting V~ for ac current

or V for dc current measure-

ments. The meter displays 1 milli-

volt for every amp measured.

Safety

Multimeter safety

Making measurements safely

starts with choosing the proper

meter for the application as well

as the environment in which

the meter will be used. Once the

proper meter has been chosen,

you should use it by following

good measurement procedures.

Carefully read the instrument user

manual before use, paying par-

ticular attention to the WARNING

and CAUTION sections.

The International Electrotechni-

cal Commission (IEC) established

safety standards for working on

electrical systems. Make sure you

are using a meter that meets the

IEC category and voltage rating

approved for the environment

where the measurement is to be

made. For instance, if a voltage

measurement needs to be made in

an electrical panel with

480 V, then a meter rated Cat-

egory III 600 V or 1000 V should

be used. This means the input

circuitry of the meter has been

designed to withstand voltage

transients commonly found in this

environment without harming the

user. Choosing a meter with this

rating which also has a UL, CSA,

VDE or TÜV certification means

the meter not only has been

designed to IEC standards, but

has been independently tested

and meets those standards. (See

Independent Testing sidebar on

page 6.)

Common situations that lead

to DMM failure:

1. Contact with ac power source

while test leads are plugged

into current jacks

2. Contact with ac power source

while in resistance mode

3. Exposure to high voltage

transients

4. Exceeding maximum input

limitations (voltage and

current)

Types of DMM protection

circuits:

1. Protection with automatic

recovery. Some meters have

circuitry that detects an over-

load condition and protects

the meter until the condition

no longer exists. After the

overload is removed, the DMM

automatically returns to nor-

mal operation. Usually used to

protect the ohms function from

voltage overloads.

2. Protection without

automatic recovery. Some

meters will detect an over-

load condition and protect the

meter, but will not recover

until the operator performs an

operation on the meter, such

as replacing a fuse.

Look for these safety

features in a DMM:

1. Fused current inputs.

2. Use of high-energy fuses

(600 V or more).

3. High-voltage protection in

resistance mode (500 V or

more).

4. Protection against voltage

transients (6 kV or more).

5. Safety-designed test leads

with finger guards and

shrouded terminals.

6. Independent safety organiza-

tion approval/listing (e.g., UL

or CSA).

Figure 5.

A transformer-type current probe, such as the one

depicted above, scales down the current being

measured. The DMM displays 1 mA for every amp

being measured.

A Hall-Effect probe safely measures high-current ac

or dc values by scaling down the current being mea-

sured and converting this reduced current to a volt-

age. The DMM displays 1 mV for every amp.

5 Fluke Education Partnership Program ABCs of DMMs: Multimeter features and functions explained

Safety checklist

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 cur-

rent measurements.

3 Inspect test leads for physical

damage before making a mea-

surement.

3 Use the meter to check conti-

nuity of the test leads.

3 Use only test leads that have

shrouded connectors and fin-

ger guards.

3 Use only meters with recessed

input jacks.

3 Select the proper function and

range for your measurement.

3 Be certain the meter is in good

operating condition.

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 func-

tion.

3 When measuring current with-

out 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 equip-

ment, such as high-voltage

probes and high-current

clamps.

Accessories and glossary

DMM accessories

One very important requirement

of a DMM is that it can be used

with a wide variety of acces-

sories. Many accessories are

available that can increase your

DMM’s measurement range and

usefulness, while making your

measurement tasks easier.

High-voltage probes and cur-

rent probes scale down high

voltages and currents to a level

the DMM can safely measure.

Temperature probes convert your

DMM into a handy digital ther-

mometer. RF probes can be used

to measure voltages at high fre-

quencies.

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.

Glossary

Accuracy. How close the DMM’s

displayed measurement is to the

actual value of the signal being

measured. Expressed as a per-

centage of reading or as a per-

centage of full scale.

Analog meter. An instrument

that uses a needle movement to

display the value of a measured

signal. The user judges the read-

ing based on the position of the

needle on a scale.

Meter ratings and capabilities vary by manufacturer.

Before working with a new meter, be sure to familiar-

ize 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 inde-

pendent verification. Beware of wording such as

“Designed to meet specifications...” Designer’s

plans are never a substitute for an actual inde-

pendent test. The IEC (International Electro-

technical 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 success-

fully completed testing to the agency’s standard,

which is based on national/ international stan-

dards. UL 3111, for example, is based on IEC

1010. In an imperfect world, that is the closest

you can come to ensuring that the multimeter

you choose was actually tested for safety.

Annunciator. A symbol that

identifies a selected range or

function.

Average responding DMM. A

DMM that accurately measures

sinusoidal waveforms, while

measuring non-sinusoidal wave-

forms 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, cal-

culates 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

meters.

Non-sinusoidal waveform. A

distorted waveform such as a

pulse train, square waves, tri-

angular 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

waveforms.

LISTED

R

6 Fluke Education Partnership Program ABCs of DMMs: Multimeter features and functions explained

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

functions.

• Overload protection prevents

damage to both the meter and

the circuit, while protecting the

user.

• Special high-energy fuses pro-

vide extra protection for user

and meter during current mea-

surements 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, 2007 Fluke Corporation. All rights reserved.

Printed in U.S.A. 7/2006 2100079 A-EN-N Rev C

Web access: http://www.fluke.com

7 Fluke Education Partnership Program ABCs of DMMs: Multimeter features and functions explained