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pnp test

Discussion in 'Beginner Electronics' started by NoRk1, Jun 10, 2005.

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  1. NoRk1

    NoRk1 Guest

    Hi all,

    Just wandering should a continuity test with a DMM across
    the E and C terminals of a pnp transistor return true for

  2. Pi

    Pi Guest

    Hi, it should not conduct. Here is an overview:
    Whyen you measure PNP transistors you measure a diode conducting from
    C to B, and from E to B. NPN has the diodes conducting from B to E,
    and from B to C.

    C to E, and E to C, should never conduct, but do not touch the base. A
    very small base current is enough to let the transistor conduct.

    Pieter Hoeben
  3. This is the way I test unknown transistors, takes about 10 seconds.

    Take the positive DVM probe and connect to one terminal. Touch the other
    probe on the other two terminals in turn. If both show a typical diode
    reading, the positive probe is on the B terminal and it in NPN. Then put
    the probes on the C and E and try them both ways round - you shouldn't get
    anything at all as they are 2 back to back diodes.
    If you don't get 2 diode readings, move the probe to another terminal and
    try again. If you still don't get 2 diodes, move to the third terminal. If
    still no 2 diodes, reverse the probes and do the same again. When you find
    2 diodes, the negative probe is on the base and it is PNP. Again you should
    get no conduction at all on the C and E terminals. Any reading other than a
    typical diode means a blown transistor.

    Unfortunately this method doesn't tell you which of the C and E terminals is
    which. I have a transistor tester for that or look it up in my equivalents
    book. This method is really for checking quickly for blown devices.

    You should also be aware of Darlington transistors that are actually more
    than one transistor on the same chip. These will read differently to the

  4. NoRk1

    NoRk1 Guest

    Thx Pieter,

    I did what u said and the pnp test worked fine. But that leaves me
    with one more question... according to textbooks i've read pnp
    should allow current to pass through E to C when there is no
    current at B..

    So why wouldn't the continuity test accross E to C work?

    I constructed the most simple npn which works fine replacing the
    npn with a pnp i excpected the reverse result but got nothing.

  5. Thx Pieter,
    Either the textbook is wrong or you have misread the text. Read it again.

  6. Pi

    Pi Guest

    The current between C-E is the gain factor * Base current. If there is
    no base current, there will be no CE/EC current.
    Because the currents flow in the other direction. This will not work.

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