Lesson 3: Digital Multimeter

L
e
s
s
o
n

3
:


D
i
g
i
t
a
l

M
u
l
t
i
m
e
t
e
r
Introduction
This lesson covers basic functions and operation of the digital
multimeter. Although an analog multimeter and test light may be
used by a service technician, the digital multimeter performs the
more complex measurements on the newer electronic systems. In
order to make it easier to work with large numbers, digital
multimeters use the metric system.
Objectives
At the completion of this lesson, the student will be able to:
Given a digital multimeter and an electrical circuit, connect the meter
leads to the electrical circuit and adjust the meter to correctly
measure:
Voltage
Current
Resistance
COM
A
10A MAX
FUSED
FLUKE 87
TRUE RMS MULTIMETER
MIN MAX RANGE HOLD
REL Hz
Peak Min Max
OFF
A
mA
A
V
V
mV

- - -
- - -
~
V-
1000V MAX
400mA MAX
FUSED
mA A
DC AUTO
V
0 1 2 3 4 5 6 7 8 9 0
4
0 0 0 0
Fig. 2.3.0 Digital Multimeter
Digital Multimeter
The digital multimeter is highly accurate and used to find the precise
value of any type of voltage, current or resistance. Powered by a 9-
volt alkaline battery, the meter is sealed against dirt, dust and
moisture.
The meter has four main areas: the liquid-crystal-display, push
buttons, rotary dial function switch, and inputs for the meter leads.
Unit 2 2-3-2 Electrical Fundamentals
Lesson 3
FLUKE 87
TRUE RMS MULTIMETER
0 1
AUTO RECORD MAX MIN AVG AC DC 100ms
H
2 3 4 5 6 7 8 9 0
4
V
Liquid Crystal Display
The meter's liquid crystal display, or LCD, uses display segments and
indicators. Digital readings are displayed on a 4000-count display
with polarity () indication and automatic decimal point placement.
When the meter is turned ON, all display segments and annunciators
appear briefly during a self test. The display updates four times per
second, except when frequency readings are taken. Then the update
is three times per second.
The analog display is a 32-segment pointer that updates at 40 times
per second. The display segments have a pointer that "rolls" across
them indicating a measurement change. The display also uses
indicators to abbreviate various display modes and meter functions.
COM A
10A MAX
FUSED
FLUKE 87
TRUE RMS MULTIMETER
MIN MAX RANGE HOLD
REL Hz
Peak Min Max
OFF
A
mA
A
V
V
mV

- - -
- - -
~
V-
1000V MAX
400mA MAX
FUSED
mA A
DC AUTO
V
0 1 2 3 4 5 6 7 8 9 0
4
0 0 0 0
DISPLAY
PUSH BUTTONS
ROTARY SWITCH
TEST LEAD JACKS
S
Fig. 2.3.2 Liquid Crystal Display
Fig. 2.3.1 Digital Multimeter
Push Buttons
The buttons on the meter are used to perform additional functions.
This lesson will cover only the range button. The additional buttons
will be covered later in the course as they apply to the type of
measurement taken.
When it is first switched on and a measurement is made, the meter
automatically selects a range and displays the word AUTO in the
upper left. Pressing the range button will put the meter in manual
range mode and display the range scale in the lower right. With each
additional press of the range button, the next increment will be
displayed. Press and hold the range button to return to the auto range
mode. The yellow button can be used to back light the meter display.
Rotary Switch
Various meter functions are selected by turning the meter's rotary
switch. Each time the rotary switch is moved from OFF to a function
setting, all display segments and indicators turn on as part of a self-
test routine. Moving clockwise from the OFF switch, the first three
positions on the rotary switch are used for measuring AC voltage,
DC voltage and DC millivolts. The top position is used for
measuring resistance. The next position will allow the meter to check
diodes. The last two positions are used for measuring AC and DC
current in amperes, milli-amperes and micro-amperes.
Unit 2 2-3-3 Electrical Fundamentals
Lesson 3
FLUKE 87
TRUE RMS MULTIMETER
0 1
AUTO RECORD MAX MIN AVG AC DC 100ms
H
2 3 4 5 6 7 8 9 0
4
V
MIN MAX RANGE HOLD
REL Hz
RANGE
PUSH
BUTTONS
BACK
LIGHTING
Peak Min Max
Fig. 2.3.3 Push Buttons
Meter Lead Inputs
Depending on the measurement you wish to make, the meter leads
will have to be placed in the correct terminals. Notice the insides of
the input terminals are color-coded red or black. The positive lead
can go in any of the red inputs.
The COM or common terminal is used for most measurements. The
black lead will always remain in the COM terminal.
The first input terminal, on the left side of the meter is for measuring
amps. This input is fused at 10 amps continuous (20A for 30
seconds).
The next position to the right is for measuring milliamps or
microamps . No more than 400 milliamps can be measured when the
rotary switch is in this position. If you are unsure of a circuit's
amperage, you may want to start out with the red meter lead in the
10-amp input jack (highest range).
The input terminal on the right side of the meter is for measuring
voltage, resistance and diode test.
Unit 2 2-3-4 Electrical Fundamentals
Lesson 3
COM A
10A MAX
FUSED
V-
1000V MAX
400mA MAX
FUSED
mA A
METER LEAD
INPUT JACKS
Fig. 2.3.4 Multimeter Input Jacks
Overload Display Indicator
While making some measurements you may see OL displayed. OL
indicates that the value being measured is outside the limits for the
range selected. The following conditions can lead to an overload
display:
In autorange, a high resistance reading indicates an open circuit.
In manual range, a high resistance reading indicates an open
circuit or incorrect scale selected.
In manual range, a voltage reading that exceeds the range
selected.
When performing a diode check, voltage readings greater than 3.0
volts or open test leads.
Input Terminal and Limits
The following chart shows the meter functions, the minimum display
reading, maximum display reading and maximum input for the digital
multimeter.
Unit 2 2-3-5 Electrical Fundamentals
Lesson 3
Fig. 2.3.5 Overload Display
Function Min Reading Max Reading Max Input
AC Volts 0.01 mV 1000V 1000V
DC Volts 0.0001V 1000V 1000V
mVolts 0.01mV 400.0 mV 1000V
Ohms 0.01 40.00 M 1000V
AC/DC Amps 1.0 mA 10.0 A (cont) 600V
mA/A 0.01 mA 400.0 mA 600V
0.1A 4000 A 600V
FLUKE 87
TRUE RMS MULTIMETER
0 1
DC
H
2 3 4 5 6 7 8 9 0
4
V
When using the multimeter to make voltage measurements it is
important to remember that the voltmeter must always be connected
in parallel with the load or circuit under test. The accuracy of the
multimeter is approximately 0.01% in the five ac/dc voltage ranges
with an input impedance of approximately 10 M when connected in
parallel.
To measure voltage perform the following tasks:
- Make sure the circuit is turned ON.
- Place the black meter lead in the COM input port on the meter
and the red lead in the VOLT/OHM input port.
- Place the rotary switch in the desired position AC or DC.
- Place the black meter lead in the on the low side or the ground
side of the component or circuit being measured.
- Place the red meter lead in the on the high side or the positive
side of the component or circuit being measured.
Unit 2 2-3-6 Electrical Fundamentals
Lesson 3
COM A
10A MAX
FUSED
FLUKE 87
TRUE RMS MULTIMETER
MIN MAX RANGE HOLD
REL Hz
Peak Min Max
OFF
A
mA
A
V
V
mV

- - -
- - -
~
V-
1000V MAX
400mA MAX
FUSED
mA A
DC AUTO
V
0 1 2 3 4 5 6 7 8 9 0
4
0 2 1
Fig. 2.3.6 Digital Multimeter
Measuring AC/DC Voltage
Unit 2 2-3-7 Electrical Fundamentals
Lesson 3
COM A
10A MAX
FUSED
FLUKE 87
TRUE RMS MULTIMETER
MIN MAX RANGE HOLD
REL Hz
Peak Min Max
OFF
A
mA
A
V
V
mV

- - -
- - -
~
V-
1000V MAX
400mA MAX
FUSED
mA A
DC AUTO
V
0 1 2 3 4 5 6 7 8 9 0
4
0 2 1
12V
Load
Fig. 2.3.7 Measuring Voltage Drop
Observe the circuit in Fig. 2.3.7. The tests leads are connected in
parallel across the circuit load. With a 12 volt power source
connected to the load, the meter should read a voltage drop equal to
the source voltage or 12 volts.
If the meter reads a voltage drop less than 12 volts, it would indicate
that an un-wanted resistance was present in the circuit. A logical
process would be to measure the voltage drop across the closed
switch contacts. If a voltage reading was present it would indicate
that the switch contacts were corroded, requiring the switch to be
replaced.
The digital multimeter is a high impedance meter. This means the
meter will not significantly increase the current flow in the circuit
being measured. Voltage measurements should always be made with
the circuit under power. The digital multimeter is ideal for use in
circuits controlled by solid state devices such as, electronic
components, computers and microprocessors.
When using the multimeter to make current measurements it is
necessary that the meter probes must be connected in SERIES with
the load or circuit under test. To toggle between alternating and
direct current measurements, use the BLUE pushbutton.
When measuring current, the meters internal shunt resistors develop
a voltage across the meters terminals called burden voltage. The
burden voltage is very low, but could possibly affect precision
measurements.
When measuring current flow, the Fluke 87 multimeter is designed
with low resistance to not affect the current flow in the circuit. When
measuring current in a circuit, always start with the red lead of the
multimeter in the Amp input (10 A fused) of the meter. Only move
the red lead into the mA/A input after you have determined the
current is below the mA/A input maximum current rating (400 mA).
The meter has a "buffer" which allows it to momentarily measure
current flows higher than 10A. This buffer is designed to handle the
"surge" current when a circuit is first turned on. As stated earlier, the
meter is capable of reading 20 amps for a period not to exceed 30
seconds.
Unit 2 2-3-8 Electrical Fundamentals
Lesson 3
COM A
10A MAX
FUSED
FLUKE 87
TRUE RMS MULTIMETER
MIN MAX RANGE HOLD
REL Hz
Peak Min Max
OFF
A
mA
A
V
V
mV

- - -
- - -
~
V-
1000V MAX
400mA MAX
FUSED
mA A
DC AUTO
A
0 1 2 3 4 5 6 7 8 9 0
4
5 3 1
Red Lead
(+)
Black Lead
(-)
Fig. 2.3.8 Digital Multimeter
Measuring AC/DC Current
To measure current, perform the following tasks:
- Place the black multimeter input lead in the COM port and the
red input lead in the A (amp) port.
- Create an open in the circuit, preferably by pulling the fuse, or
by "opening" the switch.
- Place the leads in SERIES with the circuit, so that the circuit
amperage is flowing through the meter.
- Apply power to the circuit.
Caution: If the current flow exceeds the rating of the fuse in the
meter, the fuse will "open."
Unit 2 2-3-9 Electrical Fundamentals
Lesson 3
COM A
10A MAX
FUSED
FLUKE 87
TRUE RMS MULTIMETER
MIN MAX RANGE HOLD
REL Hz
Peak Min Max
OFF
A
mA
A
V
V
mV

- - -
- - -
~
V-
1000V MAX
400mA MAX
FUSED
mA A
DC AUTO
A
0 1 2 3 4 5 6 7 8 9 0
4
5 3 1
12V
Load
Switch
Open
Fig. 2.3.9 Measuring Current Flow
Measuring Resistance
Unit 2 2-3-10 Electrical Fundamentals
Lesson 3
COM A
10A MAX
FUSED
FLUKE 87
TRUE RMS MULTIMETER
MIN MAX RANGE HOLD
REL Hz
Peak Min Max
A
mA
A
V
V
mV

- - -
- - -
~
OFF
V-
1000V MAX
400mA MAX
FUSED
mA A
AUTO
0 1 2 3 4 5 6 7 8 9 0

L O M
40
~
- - -
- - -
- - -
~
Fig. 2.3.10 Digital Multimeter
When using the multimeter to make resistance measurements it is
necessary to turn off the circuit power and discharge all capacitors
before attempting in-circuit measurements. If an external voltage is
present across the component being tested, it will be impossible to
record an accurate measurement.
The digital multimeter measures resistance by passing a known
current through the external circuit or component and measures the
respective voltage drop. The meter then internally calculates the
resistance using the Ohms Law equation R = E/I. It is important to
remember, the resistance displayed by the meter is the total resistance
through all possible paths between the two meter probes. To
accurately measure most circuits or components it is therefore
necessary to isolate the circuit or component from other paths.
Additionally, the resistance of the test leads can affect the accuracy
when the meter is in its lowest (400 ohm) range. The expected error
is approximately 0.1 to 0.2 ohms for a standard pair of test leads. To
determine the actual error, short the test leads together and reads the
value displayed on the meter. Use the (REL) mode on the digital
multimeter to automatically subtract the lead resistance from the
actual measurements.
To accurately measure resistance, perform the following tasks:
- Make sure the circuit or component power is turned OFF.
- Place the red lead in the jack marked Volt/Ohms and the black
lead in the jack marked COM.
- Place the rotary selector in the position.
- Place the meter leads ACROSS the component or circuit being
measured.
NOTE: It is important that your fingers are not touching the tips
of the meter leads when performing resistance measurements.
Internal body resistance can affect the measurement.
Unit 2 2-3-11 Electrical Fundamentals
Lesson 3
12V
COM
A
10A MAX
FUSED
FLUKE 87
TRUE RMS MULTIMETER
MIN MAX RANGE HOLD
REL Hz
Peak Min Max
A
mA
A
V
V
mV

- - -
- - -
~
OFF
V-
1000V MAX
400mA MAX
FUSED
mA A
AUTO
0 1 2 3 4 5 6 7 8 9 0

57.3

40
~
~
- - -
- - -
- - -
~
57.3
Switch
Open
Fig. 2.3.11 Measuring Resistance
NOTE: In the circuit under test in Fig. 2.3.11, the power source is
isolated from the circuit by "opening" the switch. It also, isolates
the resistor from any other path that may affect the accuracy of the
measurement.