I'm trying to create a power supply protection (overvoltage, undervoltage, reverse polarity) for a ±33 V, 7 mA power supply for an op-amp.

The opamp would be powered by a lab supply in theory. So the idea is to protect it in case the operator hasn't set the voltage right, the protection will correct it so it's never above +33V (and -33V for the other rail).

Second point, it would be nice to have: if the current is rising above the set limit, the protection would cap it at chosen value.

Third protection needed: if the operator plugs the cable the wrong way, the opamp is ok.

Do you have any suggestions? My system must withstand ±50 V, about 10 mA, reverse polarity (and no shut down, caused by a fuse for example, would be a massive plus).

  • \$\begingroup\$ What you have shown in your picture is not going to work at all. If you have moved away from that then don't show it because it will melt when the input voltage is negative. Show the circuit that you are working on and ditch the picture of what you are not working on. \$\endgroup\$
    – Andy aka
    Apr 6, 2021 at 11:27
  • 1
    \$\begingroup\$ Instead of focusing your question on what doesn't work and what you don't want, explain more what provides the power and what circuit you're trying to protect and from what. The solution can be as simple as a couple of regulators. Under reverse polarity there will be no operation so "no shutdown" is then no option unless you design for both polarities (use a bridge rectifier + smoothing caps). I see many beginners dive into the implementation (circuit and components they want to use) too soon before even getting clear what is needed and what type of circuits can do that. \$\endgroup\$ Apr 6, 2021 at 11:48
  • \$\begingroup\$ Why do you think you need a current limiter? Yes, once a current limiter starts operating the voltage drops in order to maintain ohms law and the laws of physics; you can't have an output voltage that remains at a constant value when a current limiter is starting to kick in. So, why do you need a current limiter? \$\endgroup\$
    – Andy aka
    Apr 6, 2021 at 11:54
  • \$\begingroup\$ @Andyaka Well if i want to protrect my Opamp power supply, shouldn't I have to protect it from as many fault as i can? In case of a short-circuit for example. \$\endgroup\$
    – Neeko
    Apr 6, 2021 at 12:25
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    \$\begingroup\$ And what might be the cause of the short circuit that would still allow the circuit to actually carry on working? \$\endgroup\$
    – Andy aka
    Apr 6, 2021 at 12:35

1 Answer 1


Use Nexperia PESD1CAN for transient over voltage event:


They are excellent and I successfully tested them with an EFT tester.

They are automotive qualified and cost nothing.

For reverse polarity use diode S1M

For under voltage use a lot of capacitors on the regulated output of your regulator


PESD1CAN TVS comes first and then the S1M diode.

If you switch them, the diode will blow at the first over voltage event.

  • 2
    \$\begingroup\$ Components for transient protection cannot protect against permanent overvoltage. If you apply +50V here the TVS will burn up. It needs to be a (resettable) fuse here, but at a 10mA rating that is no trivial case. \$\endgroup\$
    – Arcatus
    Apr 6, 2021 at 13:14
  • \$\begingroup\$ You probably mean a TL431. I do not see how using a TL431 is more convenient than using a simple pass-regulator (the TL431 is a shunt regulator). \$\endgroup\$ Apr 6, 2021 at 13:55

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