Jeff Layman wrote:

John Rumm wrote:

In fact if you are designing DC powered kit that runs from an external
supply, then sticking a bridge rectifier on the input even though you
are only ever expecting DC rather than AC, is a way to ensure your kit
will never see the input polarity reversed if used with the wrong PSU.

Surely you only need a single rectifier connected the right way round in
either the +ve or -ve line. With a bridge rectifier three of the diodes
are effectively redundant (unless one goes short-circuit), and you've
got a double voltage drop across the diodes to take into account. That
might be significant for the equipment - a bit like trying to use NiCads
instead of alkaline-manganese batteries.

If you want reverse voltage protection, while minimizing voltage drop, a
P-channel FET is better, maybe with a zener to protect the FET from a
reverse voltage that is also too high e.g.

https://components101.com/asset/sites/default/files/inline-images/PMOS-Reverse-Polarity-Protection-Circuit.png