AIM
Use of multimeter to () identify base of transistor, (i) distinguish between n-p-n and p-np type transistors, (üi) see the
unidirectionalflow of current in case of a diode and an LED, (iv) check whether a given electronic component (e.g.,
diode, transistor or IC) is in working order.
APPARATUS
1. Given transistors 2. Diode 3. LED. 4. IC 5. Multimeter.
THEORY
1. Emitter-collector junction in a transistor always has a high resistance in both directions ie., when forward or
reverse biased.
2. The resistance of the emitter base or collector base junction is high in one direction and low in the opposite
direction.
PROCEDURE (Stepwise)
(a) To identify the base of transistor
1. Set the multimeter as an ohmmeter.
2. Usually, the base lead of a transistor lies between emitter and collector leads. But in some cases, it may not be so.
In order to identify the base lead of a transistor, touch the two probes of the multimeter to the extreme two legs of
the transistor. Note whether the resistance of the transistor between these two legs is low or high. Now, interchange
the probes touching the two extreme legs of the transistor. Again note whether the resistance of the transistor is low
or high.
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ACTIVTES9 147
() If the resistance of the transistor is high in both the cases, then the central leg of the transistor is base and the
two extreme legs are emitter and collector. This is because emitter collector junction offers high resistance in
both the directions.
(ii) If the resistance of the transistor is low in one direction and high in the other direction, then one of the two
extreme legs of the transistor is base.
3. In order to find out which of the twO extreme legs is the base, call the two extreme legs as the left and the right leg
of the transistor.
4. Touch one of he two probes of the multimeter to the left leg and the other to the central leg. Note whether the
resistance of the transistor between these two legs is low or high. Now interchange the probes. Note again whether
the resistance of the transistor is low or high.
(0 If the resistance of the transistor is high in both the cases, then the right leg is the base of the transistor.
(ii) If the resistance of the transistor is low in one direction and high in other direction, then the left leg is the base
of the transistor.
(b) To distinguish between n-p-n and p--p type transistors
5. Afer identifying the base of the transistor, touch the black lead to the base and the red lead to any one of the oher
two legs. Note whether the resistance of the transistor is low or high.
() If the resistance of the transistor is low, then it is n-p-n transistor.
(iü) If the resistance of the transistor is high, then it is p-n-p transistor.
(c) To see the unidirectional flow of current in case of a diode and LED
For diode
6. Touch the two leads of the multimeter to the two end terminals of the junction diode. Note whether the resistance
of the diode is low or high. Now, interchange the two leads. Again note whether the resistance of the diode is low
or high.
7. It will be observed that if the resistance of the diode is high in the first case, it will be low in the second case and
vice versa. This shows the unidirectional flow of current in a diode.
For LED
8. Proceced as in step 6 and repeat the observation for LED.
It will be observed that if the resistance of the LED is high in the first case, it will be low in the second case and vice
versa. Moreover, the LED will glow by emitting light when its resistance is low. This shows the unidirectional flow
of current in an LED.
(d) To check whether the given diode or transistor is in working order
For junction diode
9. Set the multimeter as an ohmmeter. Touch the two leads to he two terminals of the diode. Note whether the
resistance of the diode is low or high. Now, interchange the two leads. Again note whether the resistance of the
diode is low or high.
10. If the resistance of the diode is high in one case and low in the other case, then the diode is in working order. If the
resistance of the diode is low in both the cases, then the diode is damaged.
48 PHYSICS ACTIVIMESCUASS X1
For transistor
the transistor between base and emitter, Also fda
11. Set the multimeter as an ohmneter. Find the resistanceof ofbase
resistance between base and collector. If he resistances emiter junction and collector base junction are le
in one direction and high in the other direction, then the given transistor
in working order.
12. If resistance is low in both the directions, then the transistor is a damaged one.
OBSERVATIONS AND RESULT
With naked eye
S. No. Number qf legs or pins Component
of the compon
1. More than three
Three Transistor
'3. Two May be resistos, capacitor, junction
diode or LED
Using multimeter s ohmmeter
S. No Deflections before and ater
Interchanging he probes
Component
1. Sume constant deflection Resistor
Large deflection which falls to zero Capacitor
3. Unequal deflections Junction diode
4. Unequal deflections along with
emission of light. LED
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