I am trying to add a reverse polarity protection in my circuit using a P channel FET, but I also want to have a SPST toggle switch in series with the gate for the user to turn the circuit on or off.

I just tried this circuit on a breadboard, but it is staying on regardless if the switch is closed or open, but I am not sure why. I am thinking that when the polarity is backwards, the gate receives EMF and that is what's happening, but I am not sure.

Could anyone explain this to me, and if there is another way to do what I want?

Thank youenter image description here


1 Answer 1


The Source and the Drain are swapped, so the MOSFET reverse diode is always conducting.

Also, you put a switch at the GATE, not at the base which a Mosfet doesn't have.

EDIT: For polarity protection that is the way to connect but then you cannot expect to turn the FET off since the reverse diode will always conduct. If you want both polarity protection and switching you need two back to back MOSFETs

  • 1
    \$\begingroup\$ "Swapped" is the way you want to install the PFET for reverse polarity protection. Otherwise the body diode will conduct when the supply is reversed and there's no protection. \$\endgroup\$
    – The Photon
    Feb 27, 2017 at 5:53
  • \$\begingroup\$ Base was a typo, there is a switch in series in the circuit above with the gate. \$\endgroup\$
    – klcjr89
    Feb 27, 2017 at 5:54
  • \$\begingroup\$ @ThePhoton you are right, that is the way to connect for polarity protection but then you have no way at all for controlling it. If you want both polarity protection and switch on/off I think you need back to back FETs \$\endgroup\$ Feb 27, 2017 at 5:57
  • \$\begingroup\$ Can you provide an example of back to back FETs? Are both P channel or one N and one P. \$\endgroup\$
    – klcjr89
    Feb 27, 2017 at 6:05
  • \$\begingroup\$ e2e.ti.com/cfs-file/__key/… \$\endgroup\$ Feb 27, 2017 at 6:06

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