# Do I need >5V power supply to adequately protect against overvoltage >5V? Seems contradictory

My circuit requires 5 volts and roughly 4 amps. I am using a 5V/10A power supply. I would like to protect against overcurrent and overvoltage (>5V).

Protecting against overcurrent seems straightforward using a polyfuse. My circuit should never require > 5A, so I am sizing my polyfuse accordingly.

But, protecting against overvoltage seems almost impossible while using a 5 volt supply. Nearly all the overvoltage protection methods use Zener diodes, which typically have a 0.6V voltage drop, which would provide less than the 5 volts I need downstream.

Increasing the voltage of my power supply (>5V) to ensure a safer lower voltage (5V) seems wrong to me. It's like increasing the danger in order to decrease the danger. Am I missing something?

I am pairing a 5V power supply with my 5 volt circuit. I'm just worried than someone else could accidently plug in a 12V (or worse) supply and cause damage.

• See also "crowbar circuit" - where zener diode gets placed anti-parallel not series, so its the zener (reverse) voltage that matters not the forward voltage drop. Though simple circuits have their pitfalls. Commented Oct 18, 2020 at 19:45
• Why would a zener diode reduce voltage downstream or introduce a 0.6V drop for that matter? I think you are not using it correctly. It sounds like you are using zener diodes as if it were a reverse polarity diode. Show us your schematic. Commented Oct 18, 2020 at 19:46
• Decide what you are trying to protect. The surge through a fuse will be much more than rated before it opens. Overvoltage protection might allow the voltage to get to 6 or 7 V before it tames the spike. If your circuit can die at 5.501V it may be inadequate. Commented Oct 18, 2020 at 19:51

Zener diodes are used in the reverse direction and have a specific breakdown voltage, eg a 5.1V Zener will start taking significant current at 5.1V, not 0.6V.

However, they have limited accuracy and a finite slope. They don't prevent a higher voltage across the, they just take an ever increasing current at higher voltages.

Back to the main question.

Whether an over-voltage protection is needed depends upon the particular scenario.

For your case you could put zener and a series polyfuse (zener after the polyfuse). The zener would also protect against reverse polarity as it would conduct in the forward direction. The polyfuse would protect the zener.

Be aware that the Polyfuse will drop some voltage so there will be bit less than 5V at your circuit.

I use a zener, resistors and an SCR. 25A for an SCR is easy to get (you can pick any comfortable size) and when it trips the over voltage is gone. I normally use fuses causing the problem to be checked out. With the poly fuse it will conduct enough current, probably greater then the SCR holding current, forcing you to power down to reset. This setup is easy to test. Nice part it is generally more cost effective then heavy duty zeners.