DEE 30052 ELECTRONIC EQUIPMENT REPAIR
PRACTICAL WORK: 3
TITLE: DIAGNOSIS TECHNIQUE TO IDENTIFY FAULT USING MULTIMETER.
Learning Outcomes(LO) :
Upon completion of this course, students should be able to:
CLO1: diagnose fault of electronic equipment related to electronic equipment repair using the
correct diagnosis technique and tools (C4, PLO2)
CLO2: fix the post-consumer’s electronic equipment fault using the correct diagnosis technique
(P4, PLO5)
CLO3: demonstrate good social responsibility in solving well defined engineering problems
during performing electronic system and appliances maintenance task (A3, PLO6)
Objectives:
1. Measure resistance to determine the functionality of a passive and active component
using multimeter.
2. Recognize the terminal of active components using data sheet.
Apparatus:
1. Electronic Components.
2. Analog/Digital Multimeter.
Resistor
The resistor is a passive two pole component used to limit the amount of current that is allowed
to flow at any given time as well as the voltage. Different resistors can resist more than others
the way you work out a resistors resistance is by looking at the 4 stripes on the main body of the
resistor the different colors show the resistance, using the color code you can calculate a
resistance.
Procedures:
1. Select a resistance range. Connect the two probes together to give a short circuit and then
adjust the zero control to give a zero ohms (full scale deflection) reading. This process
needs to be repeated if the range is changed.
2. Measure and record the resistance value given below by using suitable range in analogue
multimeter.
3. Repeat step 1 and 2 by using digital multimeter.
4. Fill up your result in Table 1.
1
Colour Reading Multimeter Reading
Resistance Value Resistance value Condition
Resistance Value
Colour Code (analogue (digital
@ Tolerance
multimeter) multimeter)
1
Table 1 ( 3 mark )
Inductor, Transformer, Relay and Speakers
The Inductor, Transformer, Relay and Speakers should read a very low resistance across its
terminals, only a few ohms. If an inductor reads a high resistance, it is defective and should be
replaced in the circuit. If is reading very small resistance, less than an ohm (very close to 0Ω),
this may be a sign that it's shorted. Functional inductors normally read a few ohms, greater than
1Ω and normally less than 10Ω. Outside this range and this is normally a sign the inductor is
bad.
Procedures:
1. Select a suitable range using analog/digital multimeter, measure and record the resistance
of the inductor, transformer, relay and speakers below.
2. Fill up your result in the Table 2.
Item Resistance Value Condition
Inductor
Primary -
Transformer
Secondary -
Relay
Speakers
Table 2 ( 5 marks )
Continuity of Fuses / Switches
Continuity is the presence of a complete path for current flow. A circuit is complete when its
switch is closed. A multimeter’s Continuity Test mode can be used to test switches, fuses,
electrical connections, conductors and other components. A good fuse, for example, should have
continuity (0 Ohm).
Procedures:
1. Select a suitable range using analog/digital multimeter, measure and record the resistance
of the fuse, switch below.
2. Fill up your result in the Table 3.
2
Item Resistance Value Condition
Fuse
Switch
Table 3 ( 2 marks )
Diode
1. Set the red probe of analogue multimeter is negative source and the black probe of the
analogue multimeter is positive source.
2. Select a resistance range (X1 or X10). Measure and record the forward bias and reverse
bias resistance in Table 4.
3.
Diode 1 Diode 2
Types of diode
Forward diode resistance
Reverse bias resistance
Diode condition
Table 4 ( 4 marks )
Transistor
1. Finding the Base Terminal of Transistor.
i. Set the red probe of analogue multimeter is negative source and the black probe of the
analogue multimeter is positive source
ii. Set the multimeter to ohm meter range X10. Measure any 2 transistor leads to get the low
resistance. There are only two places where a non-infinite (low value resistance) reading
is obtained. The probe which is stayed is the base of the transistor.
iii. Determine types of transistor (NPN or PNP). If you see the red probe on the base, it
means PNP type of transistor but if you see the black probe on the base, it means NPN
type of transistor.
2. Finding the Collector and Emitter Terminals of Transistor.
i. Set the base of transistor as a reference. Set the multimeter to ohm meter range X10K.
ii. Change the probe at the base of transistor. (Black for PNP and Red for NPN).
iii. Measure the resistance of both terminals. High resistance reading is the collector terminal
and low resistance reading is emitter terminal.
iv. Fill the result in Table 5.
3
Transistor’s Physical diagram of Resistance (ohm) Type of Condition
number transistor transistor
(NPN/PNP)
Table 5 ( 7 marks )
Terminal of SCR
A multimeter can be used to test SCRs quite effectively. The first procedure is to check the diode
action between the gate and cathode terminals of the SCR. This test is just like what you have
done in the case of testing a silicon diode (see testing a silicon diode).
Now put the multimeter selector switch in a high resistance position. Connect the positive lead of
multimeter to the anode of SCR and negative lead to the cathode. The multimeter will show an
open circuit. Now reverse the connections and the multimeter will again show an open circuit.
Then connect the anode and gate terminals of the SCR to the positive lead of multimeter and
cathode to the negative lead. The multimeter will show a low resistance indicating the switch ON
of SCR. Now carefully remove the gate terminal from the anode and again the multimeter will
show a low resistance reading indicating the latching condition. Here the multimeter battery
supplies the holding current for the triac. If all of the above tests are positive we can assume the
SCR to be working fine.
4
Gate (G) Anode (A) Cathode (K) Reading( H or L) Condition
Positive Positive Negative
None Positive Negative
Positive Negative Positive
None Negative Positive
Table 6 ( 3 marks )
Optoisolator
Optoisolator IC or Optocoupler (board location marked as “IC”) is widely used in many
electronic circuits such as in the Power supplies (TV, Monitor, Computer), Modem, Medical
Equipment and etc. The Opto-isolator is simply a package that contains both an infrared light-
emitting diode (LED) and a photodetector such as a photosensitive transistor, silicone diode,
Triac, transistor Darlington pair, or silicon controlled rectifier (SCR)
Procedure
1. Set your analog meter to X10 K ohm. Place your test probes on pin 1 and 2 (measuring
the internal LED) and you should get one reading when testing it either way. If you get
two readings or no reading at all, then the internal LED have problem and need to be
replaced. You can use this method too to test any silicone diode.
2. From the datasheet, you would know which pin is the base, collector and emitter. Now
set your meter to X1 Ohm and place the black probe to the base (pin 6) and the red probe
to collector and then to the emitter. You should get a low resistance when the red probe
touches on the collector and emitter pin. If you do not get any reading or you get only one
reading, that means the internal phototransistor have problem. Now, set your meter to
X10 K ohm and measure the collector and emitter pin and you should get one high
5
resistance reading. If you get one high resistance reading when testing it either way, that
means the phototransistor is good.
Type of Opto- Pin input Pin output Condition
isolator
Diode Opto-
isolator
Transistor
Opto-isolator
Table 7 ( 6 marks )
6
Discussion:
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Conclusion :
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