I want to make a circuit which performs these tasks:

1) Protect the load against reverse polarity.

2) Protect the load against over voltage.

Load = Raspberry pi

Reason for this circuit - Pi is not directly being powered through the USB jack. There will be a DC jack where the users are supposed to plug in a 5V 2A power supply with circular pin. Ultimately this will be given to the micro-USB jack. This is being done because Pi is inside a casing and it's not possible to expose the micro-USB port directly.

I have come up with this circuit:

protection ckt

How I think it works:

Mosfet will prevent turn ON in case of reverse polarity.

If polarity is correct, mosfet will turn ON. I have set up a voltage reference to output 5.6 VDC. A comparator will compare this to DCOUT. If DCOUT > 5.6 VDC, OPAMP will generate a HIGH turning the mosfet OFF.

Am I correct?

Possible concern - When OPAMP turns off the mosfet, the entire circuit will lose power and mosfet will turn ON again. I will get a oscillating voltage on DCOUT equal to 5.6 VDC. Am I right?

Should I put another mosfet downstream which will cut off in case of DCOUT > 5.6 VDC? If it's downstream, I can make sure that there is no oscillations. Example:

dual mosfet design

Is there a better alternative to all this?

  • 1
    \$\begingroup\$ Protect the load against over voltage There's a diode inside the MOSFET that will conduct any overvoltage straight to DCOUT. This also makes sure nothing happens when the MOSFET is turned off, the opamp will be fed through the same diode. \$\endgroup\$ – Bimpelrekkie May 7 '19 at 9:37
  • \$\begingroup\$ I totally missed that. Looks like second stage mosfet is compulsory. Can you please check the edited circuit and let me know if that will work? Thanks a lot for pointing that to me. \$\endgroup\$ – Whiskeyjack May 7 '19 at 9:40
  • \$\begingroup\$ Instead of editing this circuit to fix its flaws I would propose a completely different circuit. Let me write an answer for that. \$\endgroup\$ – Bimpelrekkie May 7 '19 at 9:42

Your circuit has issues and I would not try to "patch" these as there are simpler, more reliable solutions!

I would use a poly fuse which is a self-recovering fuse. After it blows it will recover (become "good" again) after some time by itself. Of course you could also just use an ordinary glass fuse but make sure that the user can replace it without opening the case. There are fuse holders for this purpose.

So I rely on the fuse to cut the power when the DC supplied is not OK. That is safe and reliable. Here's my circuit:


simulate this circuit – Schematic created using CircuitLab

The diode D2 conducts when DCIN < 0.6 V so current flows and the fuse will blow.

During an overvoltage the SCR is triggered through D1 which shorts the supply, causing a high current to flow through the fuse which will then blow.

Also note that my circuit is much simpler than what you propose :-)

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  • \$\begingroup\$ yes, this. Polyswitch is good, but note that they take some time to " open". Could also use a schottky deries diode, as the main thing you are protecting against is reverse polarity. Is that zener/SCR circuit proven in practice? I would not rely on the 5V1 leakage current being low enough at 5.05V without trying a bunch of zeners from different batches, personally. I could be wrong ... but I would want to test it. \$\endgroup\$ – danmcb May 7 '19 at 10:22
  • \$\begingroup\$ @Bimp - I might be asking too much, but can you please suggest some part numbers for polyfuse and SCR? I tried finding them myself but got stuck with holding current and trip current etc. \$\endgroup\$ – Whiskeyjack May 8 '19 at 6:44

Simple electronic overvoltage protection can be done this way:

Schematic and simulation from LTspice

D1 is a zener diode, let's say 4.9V. If the voltage across R3 is higher than 0.7 (VCC higher than 5.6V) it will close the Q3 and cut the Load (R7) current.


The reverse polarity part from the question is a good approach.

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