Re: Zener Diode

On Wed, 10 May 2006 00:30:45 GMT, Robert Baer
<robertbaer@xxxxxxxxxxxxx> wrote:

Gerhard Nerge wrote:
Hi

I'm in need of a 18 Volt 5 Watt Zener diode, I have only 18 Volt 1 Watt
Zener diodes.
My question is, can I put 5 one Watt Zener diodes parallel to make it 5 Watt
?

Thank you in advance
Gerhard.


No.
They are not exactly the same voltage at all currents of interest and
so will not "share" equally.
However, one can add in a transistor in two different ways:
1) Use resistor from supply tozener so it is a shunt regulator, and
connect top end of zener to the base of te transistor, the collector
goes to the supply and the emitter goes to the load. Use an NPN for
positive volage, and a PNP for negative voltage.
The regulation is not as good as a direct zener, and the output
voltage is about 650mV lower than the zener.
2) Make a power shunt "zener" by connecting the 1W zener fromthe base of
a transistor to its collector; the emitter-collector of that transistor
acts like a power zener.
Again, the regulation is not as good as a direct zener, but the output
(shunt) voltage is about 650mV higher than the zener.

Those are the simplist methods; others add an amplifier of sorts.

Robert's replies are good, but you didn't exactly say how you want to
use this 5w, 18v zener.

Try searching "shunt regulator circuit"

The following sites might help -

http://www.elexp.com/t_shunt.htm
http://www.tpub.com/content/neets/14179/css/14179_212.htm
http://en.wikipedia.org/wiki/Linear_regulator

Eric Sears


.

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