This trick using a P channel mosfet is a common way to provide reverse polarity protection on a power supply input.  It avoids the voltage drop problem of using a diode, with just the resistance of the mosfet to consider.

Important – ensure you don't violate the mosfet max Vgs and Vds specification!

Here's an example for a 24V power input:

reverse_polarity_protect_24v

 

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