testing power transistors

[mich]

im trying to test an NPN power transistor (MJ802),and i get a closed circuit between C and E.after some reading i learned that some power transistors of this type have built in dioded in them that makes this testing imposibble..how can i know if this is the case?(didnt find any clue on the datasheet),and if that is the case,how can i test such transistor?
thanks

 

[CJ]

Most power transistors will do this. It is leakage.

Put it in the circuit to test.

 

[PRR]

> i get a closed circuit between C and E.

Switch the ohm-meter leads and check the other way.

A good transistor junction, tested with an analog voltmeter or a diode tester, will read open-circuit one way and low-resistance the other way.

The most common way for power transistors to fail in audio use is to develop a dead-short in the collector junction: low-R both ways. (Or to go "open" after a shorted junction causes melt-down of the internal bond wires.)

 

[mich]

Switch the ohm-meter leads and check the other way.

i read low R both ways.(closed circuit)
i guess that means its fried?

 

[bcarso]

Highly likely fried.

If it were a MOSFET then sometimes you get the gate charged and the drain-source is looking like a short in either direction, but this doesn't last and you can just tie gate to source to be sure. A bipolar is not going to do this however.

 

[mich]

i tested it's PNP brother transistor and yes,-between E to C i get low R and fliping the leads of the meter i get high R.but never a closed circuit.so its definitly fried
.thanks for the help all.

 

[Mark Burnley]

Simple tester that can be knocked up in seconds using croc clip leads, or made into small plastic box with sockets/clips for permanent bench use:

Rb seems to be ok even with low hfe power transistors- Ib = ~800uA

Mark