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Diode to protect transistor

  1. Jan 12, 2010 #1
    i want to protect my transistor(working in saturation) from high incoming voltage....
    can a diode be used to protect it from the high voltage?
  2. jcsd
  3. Jan 12, 2010 #2


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    It would help to have more information. Normally you would use a Zener diode (reverse biased) as a uni-directional transient supressor, but there are other options available as well.
  4. Jan 12, 2010 #3


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    Yes - a circuit would help.If a transistor is saturated, it is unlikely to have a high voltage across it! You'd expect less than 1V.
  5. Jan 12, 2010 #4
    Well, if it's really always saturated why not be done with and replace it with a wire? Seriously though, as others have said we need more information. What is the circuit, what does it do?

    If you are actually referring to protecting the device when it is OFF, or in the process of turning off, that's another matter. Properly chosen protection devices (TVSs etc.) can often safely absorb more energy from such things as switching transients. Note however that these things are no substitute for transistors with adequate voltage ratings.

    In fact, it may be necessary to use a higher voltage transistor than might otherwise seem to be necessary, in order to be sure that the protector absorbs the energy without the transistor avalanching. Some otherwise excellent protection devices have (in my opinion) surprisingly vague voltage ratings.
  6. Jan 13, 2010 #5
    further information:-
    the transistor is being used to control an electronic lock.the device switches between cut off and saturated region.
    the collector is connected to a lock which is further connected to 12 volt dc.
    when there is no current on base the transistor is in cut off- infinite impedence.
    it can be said that the lock is being used as a relay. wen the microcontroller provides 5 volt at the base , the transistor goes into saturation and conducts.
    the lock requires 12vdc and 3A CURRENT to operate.
    how to protect the transistors against such ratings?
  7. Jan 13, 2010 #6


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    So you want to protect against transients when yo try to turn off an inductive load.
    One way to do it is to put the diode across the load (in parallel with the lock) but 'upside down', so it is reverse biased, normally and will take no current. When the transistor 'tries' to turn off (reducing the current) a voltage spike will occur, due to the inductance, which would take the collector volts way above Vcc (positive). The diode will stop this happening by shorting out the inductance when its volts are above Vcc. As long as the transistor will survive Vcc, it will not be harmed.
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