Application Note

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 measure-
ment.
Note: For dc readings of the cor-
rect 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 con-
nections, a DMM with autopolar-
ity 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 avail-
able 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 volt-
age is also accompanied by high
energy. Rather, they are intended
for use in low-energy 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 cir-
cuit 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 junctions. 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.
k
1000 Ω
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