voltage drop across the zener

Discussion in 'General Electronics Chat' started by samjesse, Aug 23, 2009.

  1. samjesse

    Thread Starter Active Member

    Sep 14, 2008
    202
    0
    Hi
    what is the voltage drop across a 9.1v Zener connected in a circuit which has a battery of 18v.
    is it 8.9 i.e. (18 - 9.1) or is it 9.1v which is what the Zener suppose to maintain across its terminals all the time regardless of the source voltage?

    thx
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Connecting a 9.1 volt zener directly across an 18V battery will most likely damage/destroy the zener. A zener, like a diode, is intended to be used with a current limiting resistance in series with it and the voltage source.

    hgmjr
     
  3. samjesse

    Thread Starter Active Member

    Sep 14, 2008
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    0
    I understand the connection but the questions was about something else. it is weather the voltage drop across the zener is 9.1v or it drops the voltage by 9.1v and lets the rest passes.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    The short answer is that the voltage across the zener is the zener voltage rating.

    The drop across the zener will be nominally 9.1V, but even if you have the zener current set exactly at spec, a 5% tolerance part could drop between 8.65V and 9.55V.
     
  5. rspuzio

    Active Member

    Jan 19, 2009
    77
    0
    The voltage drop has a slight dependence on current (but much
    less than a resistor or a regular diode, which is why Zener diodes are
    used as voltage references) so, to get 9.1 volts (or whatever the
    exact value happens to be for your unit), you need to operate
    the diode at close to the recommended current. (This is the Zener
    current referred to in the previous post.) In the scenario you
    describe with the battery, an easy way to do this would be to put an
    appropriate resistor in series with the diode.
     
    Last edited: Aug 23, 2009
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