[sdiy] which zener and current limiting resistor?

[John Luciani]

On 8/10/06, keith sterling <keithsterling at sbcglobal.net> wrote:
> I have a synth with a +-15V supply and need 16 pots to wipe from 10V to
> GND and 2 pots to wipe from 5V to GND for cv generation.  So I think I
> need 2 zener diodes to generate the 10V and the 5V.  How do I calculate
> the size of the current limiting resistor and current rating of the
> diodes?  My supply doesn't have a lot of headroom so less current drawn
> is better.
>
> I tried using a 2 resistor voltage divider but then every control
> affected all the other controls!  Pretty hard to dial in a sound that
> way!

If you buffer the voltage divider with an op-amp you won't load your resistor
divider. If you use a zener you should also buffer the zener voltage
with an op-amp.

The zener current, Iz, will be in the zener datasheet. For your 15V supply the
series resistance required would be ---

     R = (15 - Vz) / Iz

(* jcl *)

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