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050

Transistor Tester

H. Kemp

carry out a brief check, when there is no time or equipment

to carry out a thorough test.

The operation is simple: in the position ‘battery test’ (S2

closed), the 10 mA moving coil meter M1 in series with a

600

This tester is intended to quickly check whether a transis-

tor is functional or not and possibly also select two or more

transistors with (approximately) equal gains. This is about

the simplest conceivable test circuit, so don’t expect super

accuracy. The circuit has been designed only to quickly

resistor (R4 + R5) is connected to a 6 V battery. A

current of 10 mA will flow, resulting in full-scale deflection

of the meter.

82

Elektor Electronics

7-8/2001

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SUMMER CIRCUITS COLLECTION

When a transistor is being tested (S2 open, S3 in position

2 or 3) a current will flow through the base-emitter junction

of the transistor under test, the value of which can be com-

puted by dividing the voltage across R1 or R2 by its resis-

tance. With S3 in position 2 this will be (6 V – 0.6 V)/560 k

S1a

R1

R2

R3

R4

M1

Ω

= approx. 10 µA. If the transistor has a gain of 1000 it will

cause a collector current (and therefore a meter current) of

10 mA, causing full-scale deflection of the moving coil

instrument. Therefore, the value indicated by the meter,

when S3 is in position 2, has to be multiplied by a factor of

100 to obtain the gain of the transistor. In position 3 the base

resistor is 10 times lower (R1 = 56 k

R5

NPN

10mA

2

Bt1

3

1

PNP

S3

c

TUT

6V

S2

b

e

), so in this case the

reading has to be multiplied by 10 to obtain the gain.

It will be clear that position 2 of S3 is intended for high

gains of up to 1000 and position 3 for gains of 0 to 100. The

purpose of S1 is to reverse the polarity: the upper position

drawn is for NPN transistors, the bottom for PNP types. If

you have no moving coil instrument available, it is of course

S3:

1 = LEAKAGE TEST

2 = α = 0 ...1000

α = 0 ...100

3 =

S1b

014051 - 11

also possible to replace M1 with a digital meter.

(014051-1)

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