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 sili-
con diode, and it allows current
to flow in one direction.
When checking the condition
of a diode or transistor junc-
tion, 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
different from other DMM
measurements. Current mea-
surements taken with the DMM
alone require placing the meter
in series with the circuit being
measured. 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 cir-
cuitry. An indirect method of
measuring current on a DMM can
be performed 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
measure 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 con-
ductor carrying 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