Application Note

A 3
1
⁄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 3
1
⁄2-digit meter will display up
to 1,999 counts of resolution. A
4
1
⁄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 3
1
⁄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