KN2222A works reverse bias

Discussion in 'General Electronics Chat' started by cincir, Sep 22, 2014.

  1. cincir

    Thread Starter New Member

    Sep 22, 2014
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    Hello all. I have a relay driver circuit which I used a KN2222A NPN transistor to switch 5V relay grounding. According to the datasheet facing to the flat side of the transistor the squence from left to right is Collector - Base - Emitter. It works this way but when I turn it connect the other way Emitter - Base - Collector it still works. Could anyone explain what I am doing wrong? Circuit is supplied with +5V and the IC is PC817 optocoupler. Thanks.

    relay_bb.jpg
     
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    This is actually normal behavior. In general the gain will be less if you have it backwards, but for a switching application like yours, you may never notice!

    ERic
     
  3. MrChips

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    Oct 2, 2009
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    Last edited: Sep 22, 2014
  4. cincir

    Thread Starter New Member

    Sep 22, 2014
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    Thanks ERic. but when I change the NPN to BC237 it works just it needs to be not the other way. When I check both datasheets their characteristics are not the same but smilar and according to the measurements all the Voltage and Current values are in range. I am a little confused.
     
  5. cincir

    Thread Starter New Member

    Sep 22, 2014
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    You are correct but as you indicated what confuses me is I am observing this with only one transistor used in different ways. Thanks anyway.
     
    Last edited by a moderator: Sep 22, 2014
  6. KL7AJ

    AAC Fanatic!

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    Just two? I've seem all three possibilities just from Radio Shack parts of the same part number! Caveat emptor
     
  7. KL7AJ

    AAC Fanatic!

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    In general, the emitter and collector are interchangeable on small transistors, though the dissipation capacity of the collector is more...so a transistor may not last long reverse biased. And depending on the type, the gain can be anything from moderately reduced to drastically reduced. If you have an actual transistor tester or curve tracer, you can know for sure. (Both of which are great homebrew projects, by the way. Check older editions of the ARRL Handbook for these!)
     
  8. MrChips

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    The impurity doping concentration profile at the base-emitter junction is different from that of the base-collector junction. Hence the collector and emitter are not interchangeable.

    FETs are sometimes symmetrical and hence you can interchange the drain and source on some FETs.
     
  9. crutschow

    Expert

    Mar 14, 2008
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    If you interchange the collector and emitter of a modern planer BJT transistor, the Beta will be much less and the (collector) breakdown voltage is now equal to the base-emitter breakdown which is usually only about 5V.
     
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