Author Topic: 9.1v zener safe to use with 9.5v PSU?  (Read 930 times)


  • Electron Doctor
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9.1v zener safe to use with 9.5v PSU?
« on: June 09, 2017, 07:06:38 PM »
Pretty straight forward question. I am assembling a fairly simple circuit (SHO-ish) which uses a 9.1v zener, presumably as protection for the MOSFET (though its placement in the circuit is a bit odd to me). However, the power supply it will be fed with puts out about 9.5v. Will this lead to failure? Its a PCB circuit, but is there some other way that this could be implemented a bit more elegantly?

Or am I just over thinking things and it will all work out fine?

The circuit in question:


  • Solder Ninja
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  • TJ
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Re: 9.1v zener safe to use with 9.5v PSU?
« Reply #1 on: June 22, 2017, 01:36:13 AM »
The zener protects the mosfet from abuse; it also protects against overvoltage. 9.5v is no problem at all, and my guess is that higher voltages won't hurt anything either (but you would probably need some other adjustments to make it function optimally).


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  • Jon P
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Re: 9.1v zener safe to use with 9.5v PSU?
« Reply #2 on: June 22, 2017, 03:18:05 PM »
The short answer is yes.

The long answer is:

Any time a zener shunts voltage, particularly zeners rated above 3.6V (<= which is true zeners ... but that part's complicated), it will create noise, because it must dissipate heat.

You can help keep the zener and the circuit happy by determining the current draw of the circuit, and then putting a current limiting resistor before the zener such that you will drop the voltage BEFORE the zener shunt to as close to the zener's rating as possible under normal conditions. If there's ever a situation where the zener is asked to do more of it's job, or if the wrong polarity is hooked up, then the CLR will help keep it from blowing up as well. You have to make sure you use a high enough wattage for the CLR. Start with Ohm's law, and you'll naturally be led to the formula you need for the second part.

While the noise increase is a real thing, you are unlikely to notice unless it's a very high gain effect and a very quiet rig.
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