I am building a circuit to switch a radiator fan based on coolant temp. I want to protect the circuit from reverse polarity using a diode but it would take quite a diode to handle the 30ish amps the fan motor will draw, plus connecting the fan backwards shouldn't do any more damage than it spinning the wrong direction. With that said I'm wondering if putting a diode in series with only the parts of the circuit I want to protect will work? I don't see why it wouldn't but I want to make sure and see if this is the proper way of achieving what I want.

I'll attach a very simplified version of what I'm trying to say incase I'm not being clear as well as the full circuit.

Simplified Circuit Complete Circuit


1 Answer 1


That should be OK, but it looks like the analog circuitry in the schematic shown has a relay that connects 12 VDC to an external fan. The fan connector has one pin connected to ground. The relay is rated at only 1 amp, so I surmise that the high current (30 A) DC fan is something else entirely.

Please provide more details - a 360 watt DC fan is rather unusual. And if it has a BLDC motor, it could be damaged by reverse polarity.

There is a reverse protection diode on the analog board, but there is a polarized electrolytic capacitor ahead of it, directly on the 12V input pins, so that capacitor would be damaged if polarity was reversed.

A common 1N4004 or similar 1 A rectifier should be adequate as an external protection diode.

  • 1
    \$\begingroup\$ If you're referring to the "1A" in the part number it's just part of the part number to designate "1 Form A" as it's stated in the datasheet. I'm building this for a friend and don't know how many amps the fan is going to pull, figured a 30 amp relay should be more than enough. Thank you for pointing out the capacitor, I am going to move that behind the diode. \$\endgroup\$ Jul 23, 2022 at 15:56
  • \$\begingroup\$ I think I've seen 20A fuses used for automotive fan motors, so expect up-to 100A through the motor when starting. \$\endgroup\$ Jul 24, 2022 at 12:37

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