I have a bare PCB with N-Channel MOSFET in the input (12V), however when connecting the power source, no voltage comes out from the MOSFET, and when connecting with reverse polarity, the output from the MOSFET is -12V.

Tried three different MOSFETs, all give the same results. The MOSFET is - https://www.lcsc.com/product-detail/MOSFETs_HUAYI-HYG025N06LS1C2_C2827231.html

I initially tried with the original circuit attached, and after also tried to remove everything (The Diode and Capacitor)except the MOSFET - same results. Nothing comes out from the (S)ource when connecting (+12V) (maybe 500mV...), but if connecting (-12V) (reveres polarity) the (S)ource outputs (-12V).

Is this a faulty schematic or faulty MOSFET?

Here is the original circuit

  • 1
    \$\begingroup\$ Your MOSFET symbol seems to be mislabeled. It looks like the drain is labeled with S and the source is labeled with D. \$\endgroup\$ Jan 13 at 2:10
  • \$\begingroup\$ Correct, the S and D symbols are backwards. \$\endgroup\$
    – akermania
    Jan 27 at 9:22

1 Answer 1


The circuit works as intended. The source terminal of the MOSFET is the ground connection, not 12V.

Swap the output terminals around - you have them backwards.

If you want to switch the +12V line (instead of ground), you need a reverse polarity protection circuit based on a P-Channel MOSFET instead. The N-Channel version always switches ground.

  • \$\begingroup\$ Ok I see my mistake - just to confirm, using the same exact circuit but with a P-Channel MOSFET , like - datasheet.lcsc.com/lcsc/1912111437_HUAYI-HY12P03C2_C358006.pdf should work as is (?) \$\endgroup\$
    – akermania
    Jan 13 at 1:39
  • \$\begingroup\$ @akermania Yes, installing a P-Channel MOSFET with the Source, Drain, and Gate in the same locations (source towards the load) will work to swap the polarity of the circuit. The pin-out of the MOSFET you linked is identical to the previous one, so you can just solder it on and it should be okay. \$\endgroup\$ Jan 13 at 13:55
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    \$\begingroup\$ Just an update to whoever read this post in the future - The P-channel variant works perfectly \$\endgroup\$
    – akermania
    Jan 27 at 9:23

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