Application Note

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 volt-
age 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-sinu-
soidal signals are accurately
measured using DMMs desig-
nated “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.
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
measurement.
Note: For dc readings of the correct 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 connections, a DMM with autopolarity
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 available 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
voltage is also accompanied by high energy.
Rather, they are intended for use in low-en-
ergy 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 circuit 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 junc-
tions. 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.
mVA
DCAC
MAXMINAVG
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kHz
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Auto Range Manual Range mV
0 610000
1000
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