voltage drop in zener diode

Discussion in 'General Electronics Chat' started by ect_09, Sep 4, 2014.

  1. ect_09

    Thread Starter Member

    May 6, 2012
    180
    1
    Hello
    can anybody tell me that what is the voltage drop across zener diode.??
    as we know in simple diode there is 0.3 and 0.7 for germanium and silicon respectively..

    secondly , over voltage protection is also zener diode application???

    because i seen
    1-voltage regulator
    2-zener limiter

    Thanks...
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,100
    3,034
    The data sheet for any zener will provide the Vf, including how it is affected by current and maybe temperature.

    Not sure I understand your second question. Zener diodes are frequently used as a voltage regulator for "low" power applications. The diode passes excess current to ground as needed to maintain the desired voltage at its opposite pole. A downstream device attached to that pole sees a steady voltage.
     
  3. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Zeners are made of silicon or (very old) germanium, just like other diodes.

    In the forward voltage connection they therefore offer a similar voltage drop - 0.5 to 1 volt for silicon and 0.35 - 0.8 for germanium. The higher values reflect high power devices.

    Note that zeners are usually used in the reverse mode connection where they offer a stated voltage drop of the zener voltage.

    In voltage protection mode,reverse mode is used; a zener of higher value than the normal circuit working voltage will be selected, so in normal voltage conditions in the circuit the zener will be off or non conductive.
    If the circuit voltage exceeds the zener voltage for some reason the zener will then conduct, clamping the circuit to a safe value.
     
  4. ect_09

    Thread Starter Member

    May 6, 2012
    180
    1
    can you please post the schematics for over voltage protection using zener??
     
  5. wayneh

    Expert

    Sep 9, 2010
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    All you need is a resistor to limit current, in between V+ and the zener, with the other end of the zener to ground, reverse biased. The resistor is sized to - at the least - protect the zener from ever carrying more current than it is rated for. Of course it can be a higher value to further limit the current, to save power for instance.

    As long as even a tiny current is flowing thru the zener (because V+ exceeds the breakdown or "zener" voltage), it will have a nearly constant voltage drop across it. Placing a small load at the junction of the resistor and the zener will not upset this situation, and the load will see only the steady zener voltage. If the voltage goes up (or drops), the zener draws more (or less) current and this causes a larger (or smaller) ∆V across the resistor, so that the voltage there stays steady.

    Check the wiki for zener diodes for basic information.
     
  6. ian field

    Distinguished Member

    Oct 27, 2012
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    782
    With most European parts; the zener voltage is indicated by the type number, with American and Asian parts most of the part number is only a device serial number - you have to look in the data sheet for the rated zener voltage.

    Most data sheets should also include a figure for dynamic resistance, you can calculate how far Vz will "flex" if the current through it varies.
     
  7. ramancini8

    Member

    Jul 18, 2012
    442
    118
    Zener diodes must be protected by an external resistor in all cases or the user risks a blown zener diode. I disagree with Wayneh to a small degree, the zener voltage varies with the zener current, and thus a zener makes a poor regulator unless the zener current is fixed as is done in reference applications. The zener voltage is temperature sensitive, so beware.
     
  8. wayneh

    Expert

    Sep 9, 2010
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    3,034
    No real disagreement. A zener is a low precision, or "poor", regulator that is nevertheless good enough for many applications including "over voltage protection" mentioned by the OP. The OP's question was a bit unclear, to me anyway.
     
  9. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
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    Look up zener dynamic resistance - the voltage *DOES* vary with the amount of current flowing through it.

    But it is rather more precise than just 2 resistors in a voltage divider.

    Some of the better designed equipments feed the zener with a constant current source to make the practical Vz as predictable as possible.
     
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