The TIP125 are Darlington Transistors, you
can't test them very good with
your Diode Tester.
Better use a small battery (between4 and 10 Volts) a samll lightbulb from
the collector to the negative pole from the battery, connect the emitter
to
the positive pole and a resistor with approx
10-20Kohms from the basis to
the negative pole. The light bulb should light up then, w/o the resistor
not.
Regards,
Holm
Holm,
Thanks for the procedure for testing the Darlington transistors. One thing I
wanted to check though. You say that the collector should be connected to
the negative pole of the battery, I thought that, conventionally at least,
collectors were connected to the more positive side and emitters to the
negative?
It depends on the ploarity of the transsitor. With an NPN transsitor (or
darlington), the colelctor is +ve wrt the emitter. With a PNP transistor
it's -ve. I'd guessed the TIP125 was NPN, sounds like I got it wrong. If
it's PNP, the collector should be -ve (and in the circuit I gave i nan
earlier reply, you need to swap the PSU connections round).
-tony