# 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?

• ###### Zener Shunt Regulator Schematic.png
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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! 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 AAC Fanatic!

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 AAC Fanatic!

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