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How to identify PNP or NPN device?

Discussion in 'Electronic Design' started by cfgl2001, Jul 15, 2005.

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  1. cfgl2001

    cfgl2001 Guest

    I have a 2-terminal solid-state (pressure) switch device which is
    known to be a bipolar PNP or NPN device. The 2 terminals are emitter
    and collector with known pin positions. My question is:

    In the case of INACCESSIBLE base terminal of a bipolar device, is
    there any easy way to identify whether it is a PNP or NPN type of

    Thanks for any help.
  2. Ted Wilson

    Ted Wilson Guest

    Depends what's connected to the base.

    If the base were effectively open-circuit and you know which is the
    collector and which the emitter, you could check
    which way round the device exhibits the lower breakdown voltage.

    Use a voltage of say 12V, with a series resistor to limit current -
    something like 22k-ohm, but it's not particularly critical.

    If the device exhibits a breakdown voltage less than 12V with the collector
    negative, then it's an NPN, otherwise it's a PNP.

    (Note that this test relies on reverse-breakdown of the b-e junction which
    is not particularly good for a transistor and causes progressive degradation
    several parameters, so don't take any longer than necessary).

    Do you know what's connected to the base? It could be that a test along the
    lines described above would give adequate indication, depending what base
    connection is present.

    Ted Wilson
    Senior Circuit Design Engineer
    BAE Warton

    "The gods do not deduct from man's
    allotted span the hours spent in fishing."
  3. John  Larkin

    John Larkin Guest

    One trick that I like, that often (not always) works: grab the tip end
    of a scope probe with the fingers of one hand; you'll usually see a
    volt or more of 60 Hz hum. Now touch the node in question with a
    finger of your other hand. If you now see positive half-wave humps,
    it's an NPN, and vice versa. The gadget should be grounded, of course.

  4. Joerg

    Joerg Guest

    Hello John,
    Unless you live too close to Telegraph Hill.

    Regards, Joerg
  5. Guest

    In the case of INACCESSIBLE base terminal of a
    If you know where it goes in the circuit, you can
    just check the voltage polarity from emitter to collector.
    NPN will be positive on collector, and PNP will be negative.

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