transistor measure ok but not working

Discussion in 'General Electronics Chat' started by TAKYMOUNIR, Sep 16, 2011.

  1. TAKYMOUNIR

    Thread Starter Active Member

    Jun 23, 2008
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    i have circuit it has transistor is bad but this transistor measure ok when i check it by meter, but when i change the transistor the circuit work ok and i compare the reading of the 2 transistors is the same so my question is why the transistor measure ok but not working is this could be the gain is different or what?
     
    Last edited: Sep 16, 2011
  2. hgmjr

    Moderator

    Jan 28, 2005
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    Supply your schematic.

    hgmjr
     
  3. Veracohr

    Well-Known Member

    Jan 3, 2011
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    How are you 'measuring' the transistor? What kind of meter, what kind of measurement?
     
  4. TAKYMOUNIR

    Thread Starter Active Member

    Jun 23, 2008
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    i use fluke meter and i measure the diode junction(vd)
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Did you measure:
    1) Base to emitter, both forwards and backwards, reading ~0.7 one way and open the other?
    2) Base to collector, both forwards and backwards, reading ~0.7 one way and open the other?
    3) Emitter to collector and collector to emitter, both indicating open?
     
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  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    /The "two diode test" of a transistor isn't much of a test. You need to essentially check every pin to every other pin to make sure there isn't leakage or shorts, as well as looking for open pins. Checking a transistor by treating it like 2 diode tests pretty much gives you an idea of the pinout and type (PNP or NPN)

    Some multimeters (for some reason, the cheap ones) will have a gain measurement circuit for transistors, so if it passes that, it is more than likely fine.

    There are a lot of simple build yourself Transistor testing circuits, I've built a simpler version of this one, I didn't use thee 4066 switch, just 3 momentary switches instead, and bi-color LEDs instead of 2 for each leg:

    http://www.redcircuits.com/Page83.htm Has the info and description, I'll try hotlinking the schematic:

    [​IMG]
     
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  7. TAKYMOUNIR

    Thread Starter Active Member

    Jun 23, 2008
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    it measure .7v in forward only and open in backward
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Did you check both the base-collector and base-emitter junctions, as well as measuring the emitter-collector forwards and backwards?
     
  9. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Apparently correct forward and backward results for the collector-base and base-emitter junctions tested as diodes does not prove that a transistor is good.

    There could be excessive collector-emitter leakage, or even a dead short between them. The current gain could be very low (or excessively high - but that's less likely to be a problem).

    Measuring the two junctions separately does not even prove that you have a transistor at all - two separate diodes in the same encapsulation would "pass" this test, but could not provide any amplification.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    I can't seem to get an answer from our OP if they tested all possible combinations. :confused:
     
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