out of circuit test for transistors

[neil]

I was working on a couple of battery powered flourescent lights. I don't
know the make / model number because i had to peel the label off to get
at the circuit, which consisted of one transistor (S8040 npn) , a
transformer and a few r's and c's.
In the first unit , the transistor was open b-e, so i replaced it
with a BFY50, and the light worked.
In the second unit, the transistor tested OK (out of circuit) so i
resolered it and the light still didnt work. Replacing the S8050 fixed
the problem. Presumably the transistor was breaking down at the battery
voltage (6v).
My question is, are there more effective ways of testing transtors
that will pick up faulty ones that test OK with an out of circuit DMM
diode test? Or are there DMMs out there that will detect this kind of
fault with a higher test voltage?

PS. I just tested the s8050 with my old Beckman meter which used a
higher test voltage (because i can test LEDs with that one), and it
indicated a Vb-e drop of over 1v so i have just answered my own
question. I suppose i need a better DMM.

cheers
Neil.

 

[Homer J Simpson]

I suppose i need a better DMM.

A DMM isn't much good for this sort of testing. Trying in circuit is usually
the only way.

 

[Charles Schuler]

I was working on a couple of battery powered flourescent lights. I don't
know the make / model number because i had to peel the label off to get
at the circuit, which consisted of one transistor (S8040 npn) , a
transformer and a few r's and c's.
In the first unit , the transistor was open b-e, so i replaced it
with a BFY50, and the light worked.
In the second unit, the transistor tested OK (out of circuit) so i
resolered it and the light still didnt work. Replacing the S8050 fixed
the problem. Presumably the transistor was breaking down at the battery
voltage (6v).
My question is, are there more effective ways of testing transtors
that will pick up faulty ones that test OK with an out of circuit DMM
diode test? Or are there DMMs out there that will detect this kind of
fault with a higher test voltage?

PS. I just tested the s8050 with my old Beckman meter which used a
higher test voltage (because i can test LEDs with that one), and it
indicated a Vb-e drop of over 1v so i have just answered my own
question. I suppose i need a better DMM.

There is no general-purpose, out-of-circuit transistor tester that is always
reliable, perhaps short of a curve tracer. With a curve tracer, one can
sweep Vce over the working range and choose the base current steps and view
the collector family of characteristic curves. But, curve tracers also have
a few limitations (e.g. high-frequency devices). In-circuit testing
requires some knowledge, but is mostly the best way to go