I want to add a reverse-polarity diode to my design.

It has an input of 24 V (230 Vac to 24 V power supply) which is fed to a 24 to 12 V converter. I plan to draw up to 4 A from the 24 V power supply. The PCB works in a generally room temperature environment and has an SBC, mini pcie port and an RS-485 comm line.

I was planning on adding a diode with a serial fuse too, in parallel to the 24 V in order to prevent reverse-polarity connection damage.

I also want to add a surge protection to the same power terminal.

  • Can I just use a TVS (bidirectional or unidirectional) instead of the diode?
  • Do I need to have both?
  • I know that a TVS adds a substantial amount of input capacitance. Should I be worried about it?
  • Do I need to have other things in mind, when designing the circuit?

2 Answers 2


@metsik: Below is an NMOS circuit example for reverse polarity protection. You can see that this is simply a mirror of the PMOS circuit posted by Lundin.

I recommend using a 10V Zener so as to protect the FET gate. This is enough to fully saturate the FET, without getting close to the max Vgs (usually 20V).

Do you need hot-swap protection on this circuit? I would submit you only need it if you are switching this circuit to a live 24DC supply (hot swapping). Most off-the-shelf supplies can power up into a capacitive load (in a controlled manner). Also, a larger concern than the TVS will be the input capacitance you need to stabilize your flyback.

enter image description here


Without any statistical evidence for it, I think the most common solution for rugged industrial/automotive solutions is a P-MOSFET (with built-in diode) with zener + resistor for polarity protection. Then TVS after that for the sole purpose of getting rid of spikes. Example (component values may be tweaked):


simulate this circuit – Schematic created using CircuitLab

Optionally with a fuse before the MOSFET. N channel versions exist too if you are very picky with Rds(on).

For surges, a big bulk cap is usually good enough for most applications. For the more extreme scenarios with huge surges, there's specialized "surge stopper" power management IC you can use: https://www.analog.com/en/products/monitor-control-protection/surge-stopper-overvoltage-overcurrent-protection.html.

  • \$\begingroup\$ For surges, a big bulk cap is usually good enough - can you expand on this? Having a big bulk cap can actually make inrush worse. \$\endgroup\$
    – Reinderien
    Apr 28, 2021 at 13:47
  • \$\begingroup\$ @Reinderien Oh yeah it gives a huge inrush at power on. But then it typically doesn't matter because you haven't started yet. These caps help from surges when you are already up and running. It's a pain to combine them with fuses though, very delicate balance in that case. \$\endgroup\$
    – Lundin
    Apr 28, 2021 at 14:01
  • \$\begingroup\$ You can also mirror this circuit and use an N-channel mosfet on the return leg if you can tolerate separate grounds. \$\endgroup\$
    – Troutdog
    Apr 28, 2021 at 14:11
  • \$\begingroup\$ @Troutdog I plan on designing a flyback after that protection circuit, so I will have seperate ground either way. how does a protection circuit with an N-MOS look like? does that mean adding a 3rd ground? \$\endgroup\$
    – metsik
    Apr 29, 2021 at 7:06
  • \$\begingroup\$ @Lundin, the velue of the 24v zenner seems to result in a 0v voltage across R1. woldnt it put the PMOS in an ohmic-cutoff state? also, as I see it, the added TVS will result in high inrush current that I wish to avoid. is there another way to aproach it without introducing voltage spikes and high inrush current to my 24- 12v converter? \$\endgroup\$
    – metsik
    Apr 29, 2021 at 7:19

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.