Does my device need reverse polarity protection?
A better solution for reverse polarity protection using a P-channel MOSFET:







simulate this circuit &ndash; Schematic created using CircuitLab

This circuit has virtually no voltage drop across the MOSFET, where-as a diode will have one (even a schottky diode might have a ~0.25V drop).

Note that this circuit requires that \$V_1 &gt; V_{th}\$, which in this case would be ~2V. Also with 2AAA batteries you might have a hard time driving some LED's, especially as the batteries drain.

If you prefer, here's an equivalent circuit which uses an N-channel MOSFET. Similarly, it requires \$V_1 &gt; V_{th}\$







simulate this circuit

For a more complete explanation on why these circuits work, I would recommend watching this (it only covers the PMOS circuit, but a similar concept applies to the NMOS circuit).

Another related question: Is a Schottky diode appropriate for reverse polarity protection?