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Figure 1. With
the meter set to measure
ohms, clip one meter
lead to the base
connection of the
transistor.
Touch
the other lead first
onto the collector lead
and then onto the
emitter lead.
The readings should both
be the same, either both
high resistance or both
low resistance.
Now reverse the leads
and repeat the
procedure.
The results should be
the opposite of those
obtained before.
If they were both high
before they should now
be both low.
If they were both low
before they should now
both be high.
Now measure the
resistance between
emitter and collector.
It should read high
resistance in both
directions.
If you dont know the
transistor connections
consult a data book.
If you cant find the
data then measure
between the three
connections in both
directions.
You should now be able
to identify the base
connection and then
decide if the transistor
is OK.
Note that for this to
work the internal
battery of the meter
must supply a voltage
high enough to overcome
the forward resistances
of the transistors.
Many meter have a
position marked with a
diode symbol which must
be selected when
checking transistors or
diodes.
Note that NPN
transistors have low
resistances where PNP
have high, and vice
versa.
Figure 2. shows a
simple power transistor
tester. With the switch
open both lamps are off.
With the switch closed
both lamps are on.
If different results are
obtained the transistor
is faulty.
The transistor shown is
an NPN. Reverse the
battery for PNP
transistors.
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