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Transistor question

Discussion in 'Electronic Equipment' started by Dave, Sep 29, 2004.

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  1. Dave

    Dave Guest

    Was trying to test a 2N3906 (PNP 2N2222 substitute) and found something
    strange. My DMM has a transistor built into it with a socket marked EBCE
    and I wasn't sure which configuration the transistor had. When plugging
    what should be the emitter/base/collector into the appropriate slots, it
    came up with an hfe reading of 150, which is a little low but in the
    ballpark. But if I move it down one slot and plug what should be the
    collector into the bottom emitter slot/base into the collector slot/emitter
    into the base slot, the hfe reads 1500! Anybody have any idea what gives
    with this? Is it possible that the transistor has two operable
    configurations? Or do I have a faulty tester built into my DMM?

    Crossposted to sci.electronics.equipment, just in case

    Thanks much,

    Dave
     
  2. Dave

    Dave Guest

    Oh yeah, no doubt. I am just trying to figure out why I got an hfe reading
    of 1500 with the transistor basically reversed. Any ideas on that?

    Thanks,

    Dave
     
  3. Jamie

    Jamie Guest

    150 is more like it.
     
  4. Dave

    Dave Guest

    Hmm. Okay. Makes more sense than anything I can think of. My wife says
    that 20 years ago I told her about this effect and explained it. Only, that
    was 20 years ago. :)

    Thanks for the feedback. It is appreciated.

    Dave
     
  5. Norm Dresner

    Norm Dresner Guest

    She remembered and you didn't. What does that tell you about memory and
    aging and gender?
     
  6. Jamie

    Jamie Guest

    short effects due to improper polarity throwing off your meter.
    if you put the vcc in the Base the emitter will sure get a lot for
    output with no current on the base leg.
    this will give the illusion of high gain.
     
  7. Dave

    Dave Guest

    YEAH! And testosterone and Estrogen. It's not fair...

    (But what is?)

    Dave
     
  8. guy pastuzak

    guy pastuzak Guest

    Hi Dave
    The transistor tester is made of a known current generator, connected to the
    base.
    Emitter is grounded.
    Collector has a voltage generator, probably a resistor to limit the current, and
    a current measure.

    If you shift the plugs, you connect E to the base generator, B to collector, and
    C to ground. So, the current generator is ignored because reverse biased. The
    collector/base junction of the tested transistor has, in serie, the voltage
    generator, the milliameter and the protective resistor, direct biased. The
    current is MAXIMUM limited by R. Usually, the indication is overflow (no value),
    but on your DMM it is the same as the current for hfe 1500.
     
  9. << I am just trying to figure out why I got an hfe reading
    of 1500 with the transistor basically reversed. Any ideas on that? >>

    Dave-

    As others have pointed out, you may not have it exactly reversed. Finite
    output divided by zero input equals infinite apparent gain!

    If you did have the transistor connected with collector and emitter reversed,
    it should still function as a transistor, but most likely would have much lower
    gain. You would also expect it to have a lower breakdown voltage.

    At one time in the development of computer circuits, a transistor was designed
    and produced that was somewhat symmetrical. According to its specifications,
    it worked equally well connected either way. I do not recall a specific part
    number or the circuit in which it was used. This was probably around 40 years
    ago.

    Fred
     
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