transistor measure ok but not working

Thread Starter

TAKYMOUNIR

Joined Jun 23, 2008
352
i have circuit it has transistor is bad but this transistor measure ok when i check it by meter, but when i change the transistor the circuit work ok and i compare the reading of the 2 transistors is the same so my question is why the transistor measure ok but not working is this could be the gain is different or what?
 
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SgtWookie

Joined Jul 17, 2007
22,220
Did you measure:
1) Base to emitter, both forwards and backwards, reading ~0.7 one way and open the other?
2) Base to collector, both forwards and backwards, reading ~0.7 one way and open the other?
3) Emitter to collector and collector to emitter, both indicating open?
 

thatoneguy

Joined Feb 19, 2009
6,359
/The "two diode test" of a transistor isn't much of a test. You need to essentially check every pin to every other pin to make sure there isn't leakage or shorts, as well as looking for open pins. Checking a transistor by treating it like 2 diode tests pretty much gives you an idea of the pinout and type (PNP or NPN)

Some multimeters (for some reason, the cheap ones) will have a gain measurement circuit for transistors, so if it passes that, it is more than likely fine.

There are a lot of simple build yourself Transistor testing circuits, I've built a simpler version of this one, I didn't use thee 4066 switch, just 3 momentary switches instead, and bi-color LEDs instead of 2 for each leg:

http://www.redcircuits.com/Page83.htm Has the info and description, I'll try hotlinking the schematic:

 

Thread Starter

TAKYMOUNIR

Joined Jun 23, 2008
352
Did you measure:
1) Base to emitter, both forwards and backwards, reading ~0.7 one way and open the other?
2) Base to collector, both forwards and backwards, reading ~0.7 one way and open the other?
3) Emitter to collector and collector to emitter, both indicating open?
it measure .7v in forward only and open in backward
 

Adjuster

Joined Dec 26, 2010
2,148
Apparently correct forward and backward results for the collector-base and base-emitter junctions tested as diodes does not prove that a transistor is good.

There could be excessive collector-emitter leakage, or even a dead short between them. The current gain could be very low (or excessively high - but that's less likely to be a problem).

Measuring the two junctions separately does not even prove that you have a transistor at all - two separate diodes in the same encapsulation would "pass" this test, but could not provide any amplification.
 
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