zener and pn diode

Discussion in 'Homework Help' started by bhuvanesh, Mar 16, 2015.

  1. bhuvanesh

    Thread Starter Member

    Aug 10, 2013
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    The transfer characteristics of zener and pn diode looks same in reverse bias, i have asked few why in many rectifier circuits zener is used.i got answer like, it wont burn up in high reverse current like pn diode . why zener is special than pn diode.
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    The difference is in the doping of the P and N layers. Changing the concentration of the dopant atoms that are added to the respective layers can change the characteristics of the diode so that it can conduct in both directions. In the transfer characteristic you should notice that the slope of the curve beyond the knee is much steeper for a Zener diode.
     
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  3. bhuvanesh

    Thread Starter Member

    Aug 10, 2013
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    your saying that zener conduct in both direction ,but pn also conducts in both direction.Though after short period pn will be bursted with reverse magnitude ,yet it conducts in reverse direction for that short duration ,right?
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    I hardly think working for a short time and then blowing up counts as working. Maybe in your universe, working for a short period is the same as working. Not in mine. The point is that a pn diode is not designed to conduct in the reverse direction, but a zener is designed to do that. It should also be obvious that if you limit the reverse current you can prevent the pn diode form destroying itself, just as you do for a zener. Using a pn diode in this way, you have a poor man's zener with terrible performance. Why would you do that? You wouldn't, you'd go get a zener diode and be done with it.
     
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  5. bhuvanesh

    Thread Starter Member

    Aug 10, 2013
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    my universe is scaled to get sharp changes ;),thanks i got u
     
  6. WBahn

    Moderator

    Mar 31, 2012
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    Whether the pn diode or the zener diode is destroyed when conducting in the reverse direction is entirely dependent on the circuitry that it is a part of. The same is true in the forward direction, for that matter.

    The big difference is that the reverse knee of a rectifier diode is generally guaranteed to go into breakdown ABOVE some voltage whereas a zener diode is specified to go into breakdown AT some voltage. Plus, the knee is designed to be sharper for a zener so that it will hold the desired voltage within spec over a larger current range.
     
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  7. bhuvanesh

    Thread Starter Member

    Aug 10, 2013
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    forward direction of zener also gives constant 0.7 volt ,but why everyone talking about the constant voltage given in reverse bias region of diode alone?
     
  8. WBahn

    Moderator

    Mar 31, 2012
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    The normal forward voltage drop is seldom a desired thing -- while there are exceptions, in most cases we would like to have 0V forward drop but accept that we must live with 0.7V or so (or whatever it is for the flavor diode used). But when we operate a diode in reverse breakdown it is usually because we want to get a constant voltage of a certain amount for some purpose, usually to provide a reference voltage for some circuit.
     
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  9. bhuvanesh

    Thread Starter Member

    Aug 10, 2013
    268
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    okay
    content of book.

    At breakdown voltage its supplying constant voltage with increase in reverse current,its apparent from transfer characteristics ,Then how do you say that zener dont have any reverse current .
     
  10. WBahn

    Moderator

    Mar 31, 2012
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    A zener DOES have a reverse current in almost all applications in which it is used. But in those applications it is NOT a rectifier design.
     
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