zener volt ratings

[coreybox]

Quick question.  I have a mosfet boost that i've built that uses a 9.1v zener diode.  All the caps are rated at 16volts.  Is it safe to run this at a higher voltage (say 12v), or does the voltage rating on the zener not allow this.

thanks
corey

[R.G.]

It depends on where the zener is in the circuit.

[coreybox]

not the same circuit, but it is connected to the mosfet (G and S) in the same way as jacks circuit here http://www.muzique.com/schem/mosfet.htm

I don't think this would limit the pedal to what voltage it could use, but i just wanted to make sure.

thanks again
corey

[George Giblet]

The easy answer is:  The zener is for static protection, it prevents the gate source voltage going above the maximum allowed for the MOSFET which is about 20V max.  It also prevents the gate going negative.  In normal linear operation the gate will be about 2V above the source so the zener doesn't conduct.

A more detailed answer: If you use multiple MOSFET stages the input voltage, from a previous MOSFET stage, can drive the gate hard negative and positive.   If you raise the rails voltages it is possible that the zener could conduct (due to either positive and negative swings) and when it does the zener would contribute to the sound (that's neither good nor bad, it just is).  If you chose a zener voltage that was equal to the supply then that would prevent the zener conducting for positive swings.  However if you raised the supply voltage to say 24V then using a 24V zener would be unwise since that would allow the drive from the previous MOSFET stage to exceed the maximum allowed gate source voltage - in other words at high supply voltages the zeners must conduct and hence have an impact on the circuit's sound.

For the circuit you gave there is no previous MOSFET stages it's only driven from a guitar or effects pedal so in most cases keeping the 9.1V zener would be fine, and would have no effect.