BJT Type and Lead ID Procedure

Discussion in 'General Electronics Chat' started by tracecom, Aug 27, 2012.

  1. tracecom

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    Apr 16, 2010
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    After spending much of the morning trying to understand how to use my DMM to ID the type and leads of bipolar junction transistors, I decided to document the procedure. It is attached for review and corrections. Thanks.
     
    Last edited: Aug 27, 2012
  2. ifixit

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    How did you come to the conclusion in step 5 that the higher voltage is the base-emitter junction?
     
  3. tracecom

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    I should have said in my first post that the procedure is based on what I have read. My only contribution is to try to simplify and proceduralize the information that I found.

    I read it somewhere today, but I don't recall exactly where. I have been looking at information on various web sites, in some books that I have, and even some you-tube videos. I tried it out on a 2N2222A, 2N3904, 2N2907A, 2N3055, 2N3906, 2N4403, BC547B, BC546, and BD139; it worked every time. As I recall the explanation, it was that the emitter was doped heavier than the collector, and thus had more charge carriers.

    Do you think it's wrong? Thanks.
     
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  4. ifixit

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    Sounds reasonable to me. I'll have to try it out.

    Thanks,
    Ifixit
     
  5. tracecom

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    Here's the data from my (limited) testing. As you can see, sometimes there is only a tiny difference, but in this data, it's 100% correct in identifying the emitter.

    I hope you will (and others) will try it, and will post your findings.
     
    Last edited: Aug 27, 2012
  6. Ron H

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    The B-E junction is typically smaller (in area) than the B-C junction. (From this web site.)
    [​IMG]
    Assuming equal doping of the junctions, and assuming you multimeter applies a current that is approximately constant, then the current density in the B-E junction will be higher than that of the B-C junction, so the B-E voltage will be higher.
    The doping assumption is probably invalid, but It also probably doesn't change the relative fwd voltage drops (for equal areas) very much.
     
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  7. tracecom

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    I do hope that some of you will try this out. Here is a sheet with 10 spaces on it for your data. The instructions are in post #1 in this thread.

    Thanks.
     
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