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Zener Diode

Discussion in 'General Electronics Discussion' started by ecomsteve, Jul 20, 2011.

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  1. ecomsteve


    Jul 20, 2011

    I have been working on a simple voltage regulator circuit using a zener diode. The circuit has been designed by someone else, the zener is connected using a resistor for the voltage drop and also a transistor to pass current.

    Next to the zener diode there is a standard diode wired in series, I am not sure why this standard diode is being used? I understand that it will add additional 0.6V to the output but there any other reason besides that?


  2. poor mystic

    poor mystic

    Apr 8, 2011
    Hi Steve

    First, here's a bit about zener diodes.
    Zener diodes conduct in both directions.
    They're just like ordinary diodes, except that when a large enough reverse voltage is applied they "break down" and stop the reverse voltage from getting any higher by taking more and more current. Of course that process has a finite limit, if they get too much current they'll melt!
    So a series resistor is used to limit current, and that makes them useful voltage references.

    In the particular circuit you've got, it might be necessary to ensure that the zener does not conduct in the forward direction.

    Or, post high quality photos of the circuit board (both sides would be good) so we can see for sure what's happening.

  3. Resqueline


    Jul 31, 2009
    It could also be in order to negate (cancel) the negative temperature coefficient (-2mV/C) of the output transistor b-e junction.
    Zeners around 5V have approx zero temperature coefficient, higher voltages have a positive coefficient, and lower voltages have a negative coefficient.
  4. poor mystic

    poor mystic

    Apr 8, 2011
    That's a thought. :)
    In that case the diode should be mounted in good thermal contact with the o/p transistor. That helps it track the voltage on the base-emitter junction more closely.
    Can we see the circuit diagram please?
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