Application Note

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 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 res-
olution 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
expensive 20,000-count meter
until you exceed 320 volts.
Accuracy
Accuracy is the largest allow-
able error that will occur under
specific operating conditions. In
other words, it is an indication
of how close the DMM’s dis-
played measurement is to the
actual value of the signal being
measured.
Accuracy for a DMM is usu-
ally 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 specifica-
tion. 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. 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
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 resolu-
tion, 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 dura-
ble 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
productivity.
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 equip-
ment 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 elec-
tronic 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 voltage
problem should be corrected
before investigating further.
(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