# Zener diode and input voltage noise

I want to use a zener diode to clamp the input voltage. Input voltage is 12V with a +-4V noise. I want to know that if the noise in input voltage can affect zener voltage or not? If I have noise in input, will I have noise in zener too? Is this true?

• It depends how dynamic your load is relative to no-load and zener load then impedance ratio determines noise Attenuation ratio Jan 23 at 7:02
• 4.7V Zeners are not that "rigid" with their output voltage (their dyn. resistance is quite high). A 431 shunt reference will give you much more stable voltage at quite low cost. I find it hard to see application potential for normal Zeners (not TVS) Jan 23 at 7:59
• Have you tried to simulate it? Jan 23 at 11:07

In AC, the usual circuit having a resistor R and a zener in series forms a voltage divider with the diode's dynamic resistance.

Check zener datasheet for Rd, "Dynamic resistance" or "Differential resistance" of the zener (it depends on current).

Example: BZX84C-6V2 at 5mA Rd=6 to 10 ohms

Therefore, output voltage variation will be input voltage variation multiplied by Rd/(R+Rd).

The PSRR is not usually that good with zeners, unless a current source is used.

Here's an input voltage DC sweep. Top graph is voltage on zener. Middle graph is zener dynamic resistance. Bottom graph is PSRR in dB.

Whether this matters or not depends on what you want to do.

Yes, you will have some noise left, even if the Zener removes most of it.

Zeners only have the rated voltage over them at rated current. So with a simple resistor-Zener circuit, varying the input voltage will cause the Zener current to vary too, which varies the output voltage.

• ...and using an additional constant current source with a discrete Zener blows it out of proportion compared to a 431. Jan 23 at 8:27

Zener is used to clamp excess voltage (noise) in the circuit. So I guess the answer to your question is no, Zener won't have noise and can effectively remove the circuit's noise. But, you have to select a Zener with voltage rating according to what you want.

• I think you're making a common mistake about Zener diodes: seeing them as 'magic voltage drops' that produce a fixed and exact voltage drop across themselves, whatever the weather. Actually, they have quite dynamic behaviour and the drop across them varies with current, temperature etc. Downvoting for that reason, I'm afraid. Jan 23 at 11:44
• @TonyM I guess I'm wrong. Thank you for the information.
– Momo
Jan 24 at 4:56
• Glad to help and appreciate your spirit of co-operation here, it's welcome asset to the site. You can delete your answer if you prefer to, which I think removes the downvotes from your reputation. Jan 24 at 19:36
• @TonyM Nah, my mistake might actually help someone so they don't have to make the same mistake.
– Momo
Jan 29 at 6:01