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Transistor in cutoff mode

  1. Apr 25, 2016 #1
    1. The problem statement, all variables and given/known data
    Hi! I learned how to work with transistors a long time ago. However, it is good to fortify youre knowledge. I was checking a circuit board for issues and found a npn bjt, wich Colector-emitter resistance while in cutoff mode, is arround 1k ohms. So my knowledge of bjt's says me that that is a pretty low resistance for a transistor in cutoff mode (it should be almost infinite). I mean, that thing is damaged for sure, right?

    2. Relevant equations


    3. The attempt at a solution
     
  2. jcsd
  3. Apr 25, 2016 #2
    Probably... depends on how you "measured" it.

    Most accurate of course is out-of-circuit with a transistor tester.
    Out-of-circuit again, a few ohmmeter tests will usually spot a bad one. First, check that the junctions are actually diodes, that Collector-Emitter are not shorted, check Base-Emitter when forward biased then short Collector to Base and the indicated resistance should drop slightly.

    In-circuit is tough because you have to account for EVERY component that the transistor is connected to, and if the circuit is powered-up you also have to take in to account any signal present and even if there is a parasitic oscillation throwing the measurements off.

    Have fun!
     
  4. Apr 25, 2016 #3
    Thank you :-)
     
  5. Apr 26, 2016 #4

    rude man

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    How did you determine the collector-emitter resistance? Include details such as powered-on or not, instrumentation, schematic, etc.
     
  6. May 12, 2016 #5
    Thank you everyone for the replies, but the situation is already under control :-)
     
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