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Transistor Help

Discussion in 'Electronic Components' started by Alex Hunter, Jan 10, 2004.

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  1. Alex Hunter

    Alex Hunter Guest

    Hello. I am building a circuit where a PNP transistor is used to switch
    a 12v relay. I was using a general purpose transistor but the 12v was
    enough to pass through the transistor even when the base was not

    My question is what kind of transistor should I use that will work with
    12v? I know very little about different kinds of transistors, so any
    help would be appreciated.

    Alex Hunter
  2. The classic way is to Ground the relay coil with an NPN transistor.

    If you insist on a high side switch the PNP should have a pullup
    Base to emitter resistor. You ARE using the emitter to the +12 supply
    All transistors have leakage, so if your relay coil is sensitive enough,
    and the transistor is leaky enough, you can't shut it off!.

    I prefer Power FETs, so a P channel FET would do this job as well.

    | __O Thomas C. Sefranek
    |_-\<,_ Amateur Radio Operator: WA1RHP
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  3. If the 12V (hopefully this is DC, not AC) is passing thru the
    transistor, even tho the base is not connected, then you may have the
    transistor connected incorrectly, or else the transistor is breaking
    down. Whatever the case, it shouldn't allow current without the base
    being connected. I would remove it from the circuit, and not use it
    but replace it with a new transistor. Chances are that it is damaged.

    You should have a resistor in series with the base lead to limit the
    current, and it's possible that if you did not have this, the
    transistor may have been damaged by excessive current.

    Also, you should have a 1N4002 rectifier across the relay coil, with
    the cathode or banded end towards the positive supply. If you didn't,
    then the relay may have generated high voltages that could damage the

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