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.

Summary of content (7 pages)

PAGE 1
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. Introduction Choosing your DMM Multimeters. They’ve been described as the tape measure of the new millennium.
PAGE 2
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.
PAGE 3
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.
PAGE 4
How to make resistance measurements: DC and AC current Input protection A common mistake is to leave the test leads plugged into the Current measurements are difcurrent input jacks and then ferent from other DMM measureattempt a voltage measurement. ments. Current measurements This causes a direct short across taken with the DMM alone the source voltage through a require placing the meter in low-value resistor inside the series with the circuit being mea- DMM, called a current shunt. A sured.
PAGE 5
There are two basic types of current probes: current transformers, which are used to measure ac current only, and Hall-Effect probes, which are used to measure ac or dc current. The output of a current transformer 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.
PAGE 6
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 current measurements. 3 Inspect test leads for physical damage before making a measurement. 3 Use the meter to check continuity of the test leads. 3 Use only test leads that have shrouded connectors and finger guards. 3 Use only meters with recessed input jacks.
PAGE 7
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 measurements and overloads.