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

electronic circuitry seems to per-

meate everything, and continues

to get more complex and smaller

in size. The communication

industry booms with cell phones

and pagers, and Internet connec-

tions have put more pressure on

the electronics technician. Ser-

vicing, 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

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

overall value represented by

a meter’s design and the care

taken in its production.

Reliability, especially under

tough conditions, is more import-

ant than ever today. Another

important factor is safety. Provid-

ing 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 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 thermome-

ter that measures only in whole

degrees isn’t much use when

your normal temperature is

98.6 °F. You need a thermom-

eter with one-tenth degree

resolution.

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
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.
PAGE 3
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.
PAGE 4
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 measurement—ohms (Ω), kilohms (kΩ), or megohms (MΩ).
PAGE 5
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.
PAGE 6
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.
PAGE 7
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.