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Zener Voltage Regulator

Discussion in 'General Electronics Chat' started by Sharpiedeluxe, Feb 8, 2010.

  1. Sharpiedeluxe

    Sharpiedeluxe New Member

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    Hi, I'm really new to diodes, and I'm trying to solve a circuit problem creating a voltage regulator that can operate given four extreme conditions. As far as I have calculated, not all of the conditions can be met at the same time, but I'm not quite sure so that's why I'm asking! I am using a 1N4735 zener diode, with a Zener Voltage of 6.2V, the Pz is 1.0W and I need to find a resistance value given that Vin(min) is 10V and Vin(max) is 14V. the min Load amperage is zero, and the maximum is 70mA.

    http://img8.imageshack.us/img8/1698/82118367.png

    If you could tell me if I'm doing this right that'd be great.

    I calculated the max Amperage based off of the Pz to be
    Pz = Vz*Iz , so 1.0W = (6.2V)*Iz , Iz = 161mA

    If I was to find the worst case scenario to deal with Power dissipation and stay below 1.0W,
    I believe that having 14Vin and 0mA load would require 7.8V drop over a 48ohm resistor
    in order to keep the zener voltage. This I assume would be the maximum that R could be.

    However in order to keep the Zener Voltage, then the scenario in which there is Vin = 10V and
    70mA current on the load means, that 91mA goes through the diode and the V drop over the resistor
    would be 3.8V meaning the smallest R could be is 42ohms.

    Therefore, I think if I design a circuit in which the Resistance is between 42 and 48 ohms I should be
    able to meet the conditions required.

    Does this information seem correct or am I missing something critical? Any help would be appreciated.
    Thanks so much!
     
  2. Diver300

    Diver300 Well-Known Member

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    Where Vin = 10 V and there is a 70 mA load, you want basically zero zener current. That gives your maximum resistance as (10 - 6.2) / 0.07 = 54 Ohms. Any more resistance and there isn't enough current at 10 V to supply the load.

    The 91 mA is just wrong.

    You calculated 48 Ohms correctly, but that is the minimum resistance. Any less and there is too much heat dissipated in the zener at 14V and no load.

    So you need between 48 and 54 Ohm.

    In practice, that is cutting things a bit fine. You have little spare current at 10 V and you are running the zener at the limit of it rating.
     
  3. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Is this for a practical circuit or is it homework?
     
  4. dave

    Dave New Member

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  5. Grossel

    Grossel Member

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    I would recomend you to search for "constant current source" on the web. You'll get lower power dissipation and a lot more stable system. Also do a search for "voltage follower" on the web becayse you'll need that.
     

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