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out of circuit test for transistors

Discussion in 'Electronic Repair' started by neil, Feb 15, 2007.

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  1. neil

    neil Guest

    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.

  2. A DMM isn't much good for this sort of testing. Trying in circuit is usually
    the only way.
  3. 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
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