Zener Shunt Regulator Current

Discussion in 'General Electronics Chat' started by tracecom, Aug 26, 2012.

  1. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
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    In the circuit below, assume that ZD1 is a 1N4733A, 5.1V, 1W, zener. What should the value of R_z be if R_load could be disconnected during operation? Am I correct that R_z should be sized to limit the total power to .5 watts, which means that the maximum current should be 72mA, and R_z should be 96 Ω and 1 watt?
     
    Last edited: Aug 26, 2012
  2. #12

    Expert

    Nov 30, 2010
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    Zener diodes kind of creep up to their rated voltage as the curent increases. I have found that running them at half their wattage limit works well.

    P=IE
    1/2W = 5.1V x I
    I=98 ma

    12-5.1 = 6.9V
    R=E/I
    R=6.9/.098
    R=70 ohms

    P=E^2/R
    P= 6.9^2/70
    P= .68 watts actual
    size the resistor at more than .68 watts
     
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  3. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
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    #12,

    While waiting for a reply, I redid my calculations and came up with the same numbers you did! :D Thanks. I understand it (for the moment, at least.)

    That's the part that was left out of the textbook.
     
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  4. Ron H

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    Apr 14, 2005
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    Note that the 1N4733A's test current is 49mA - half the max current.
     
  5. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
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    Thanks,

    I hadn't looked at the datasheet, but I should have. I now see the test current, but I don't see the max current. Why wouldn't the max current be 1 W / 5.1 V = 196 mA?
     
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  6. Ron H

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    Apr 14, 2005
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    Oops, I thought it was a half watt zener.:(
    I guess I should pay attention.
     
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