madbeanpedals::forum

I would like to protect a circuit that

• runs at regular 9V and
• requires around 400 mA max

against reverse polarity and over-voltage. Would this circuit work for all these purposes?

(https://i.stack.imgur.com/I4SZu.png)

The circuit can run safe with 10 Volts for some time, so component-wise, I would choose a

• Zener Diode with 10 Volts, 5 Watt and a
• Fuse with I_hold=0,5 A  and I_trip=1A.

Would the fuse sufficiently protect the zener? My understanding is, that the fuse will increase resistance when current is over 1A (e.g. in a reverse polarity situation, where current flows freely through the Zener) and "open" within 4 seconds. I am having a hard time estimating, if the Zener will survive this.

With Voltage >10 Volts, the Zener will open up in reverse direction, thus protect the circuit behind it. Fuse will also "open" within 4 seconds.

Hang it on a lab power supply and test it!  :)

This is how it's usually done: https://en.wikipedia.org/wiki/Crowbar_(circuit)

We use this for the FFX stuff: https://www.mouser.com/ProductDetail/Bourns/MF-R010?qs=sGAEpiMZZMsxR%252bBXi4wRUBaq0BDiykw29p%252brkMB9bjY%3d

Only thing we found is that the 5v supply for our relay switching had an issue when run after the PTC. So, for that we just use a 1N4001 in series with the power supply straight to a 5v regulator (the VIN connection).