[sdiy] zener question

[Scott Bernardi]

How much current do you need out of it?  Will the current be fairly
constant?
Basic circuit is your +12v supply hooked to a resistor, to a 9v zener,
to ground. You tap your voltage between the resistor and the zener.
Choose you resistance so that the current through it (12v-9v)/R is
greater than the current you need to supply to external circuitry. 
This is only suitable for low current demands.
If you need something well regulated with a fair amount of current (over
50mA or so), you should go to an adjustable voltage regulator - like the
uA723, or LM317 (positive reg) and LM337 (negative reg).  With heat
sinks you can get up to 1 amp out of these, 100mA with no heat sink.

Ken MacBeth wrote:
> 
> Hi! I'm wanting to convert -+12V into -+9V using zener
> doides, how do I do this, and do I use resistors as
> well?
> Thanks in advance, Ken....
> 
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-- 
Scott Bernardi
sbernardi at home.net