Application Note

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 measure-
ment—ohms (Ω), kilohms (kΩ),
or megohms (MΩ).
Note: 1,000 Ω = 1 kΩ
1,000,000
Ω = 1 MΩ
Make sure the power is off before
making resistance measurements.
Continuity
Continuity is a quick go/no-go
resistance test that distinguishes
between an open and a closed
circuit.
A DMM with a continuity
beeper allows you to complete
many continuity tests easily and
quickly. The meter beeps when
it detects a closed circuit, so you
don’t have to look at the meter as
you test. The level of resistance
required to trigger the beeper
varies from model to model of
DMM.
Diode test
A diode is like an electronic
switch. It can be turned on if the
voltage is over a certain level,
generally about 0.6 V for a silicon
diode, and it allows current to
flow in one direction.
When checking the condition
of a diode or transistor junction,
an analog VOM not only gives
widely varying readings but
can drive currents up to 50 mA
through the junction (see Table 1).
Some DMMs have a diode test
mode. This mode measures and
displays the actual voltage drop
across a junction. A silicon junc-
tion should have a voltage drop
less than 0.7 V when applied
in the forward direction and an
open circuit when applied in the
reverse direction.
DC and AC current
Measuring current
Current measurements are dif-
ferent from other DMM measure-
ments. Current measurements
taken with the DMM alone
require placing the meter in
series with the circuit being mea-
sured. This means opening the
circuit and using the DMM test
leads to complete the circuit. This
way all the circuit current flows
through the DMM’s circuitry. An
indirect method of measuring
current on a DMM can be per-
formed using a current probe.
The probe clamps around the
outside of the conductor, thus
avoiding opening the circuit and
connecting the DMM in series.
How to make current
measurements
1. Turn off power to the circuit.
2. Cut or unsolder the circuit, cre
-
ating a place where the meter
probes can be inserted.
3. Select A~ (ac) or A
(dc) as
desired.
4. Plug the black test probe into
the COM input jack. Plug the
red test probe into the amp or
milliamp input jack, depending
on the expected value of the
reading.
5. Connect the probe tips to the
circuit across the break so that
all current will flow through
the DMM (a series connection).
6. Turn the circuit power back on.
7. View the reading, being
sure to note the unit of
measurement.
Note: If the test leads are
reversed for a dc measurement, a
“–” will show in the display.
Input protection
A common mistake is to leave
the test leads plugged into the
current input jacks and then
attempt a voltage measurement.
This causes a direct short across
the source voltage through a
low-value resistor inside the
DMM, called a current shunt. A
high current flows through the
DMM and if it is not adequately
protected, can cause extreme
damage to both the DMM and
the circuit, and possible injury to
the operator. Extremely high fault
currents can occur if industrial
high-voltage circuits are involved
(240 V or higher).
A DMM should therefore have
current input fuse protection of
high enough capacity for the
circuit being measured. Meters
without fuse protection in the
current inputs should not be used
on high-energy electrical circuits
(> 240 V ac). Those DMMs that
do use fuses should have a fuse
with sufficient capacity to clear
a high-energy fault. The voltage
rating of the meter’s fuses should
be greater than the maximum
voltage you expect to measure.
For example, a 20 A, 250 V fuse
may not be able to clear a fault
inside the meter when the meter
is across a 480 V circuit. A 20 A,
600 V fuse would be needed to
clear the fault on a 480 V circuit.
Current probe accessories
Sometimes you may have to
make a current measurement that
exceeds the rating of your DMM
or the situation does not allow
you to open the circuit to mea-
sure the current. In these higher
current applications (typically
over 2 A), where high accuracy
is not needed, a current probe
is very useful. A current probe
clamps around the conductor car-
rying the current, and it converts
the measured value to a level the
meter can handle.
Table 1.
VOM VOM DMM
Range Rx1 Rx100 Diode Test
Junction Current 35 mA to 50 mA 0.5 mA to 1.5 mA 0.5 mA to 1 mA
Germanium 8 Ω to 19 Ω 200 Ω to 300 Ω 0.225 V to 0.225 V
Silicon 8 Ω to 16 Ω 450 Ω to 800 Ω 0.4 V to 0.6 V
4 Fluke Education Partnership Program ABCs of DMMs: Multimeter features and functions explained