There is a good explanation of how to use a P-FET for reverse voltage protection here.
Excellent tutorial.
I can't take any credit for the tutorial, but all of the articles on that site are pretty good. His delivery style might not be to everyone's tastes, but I find it quite amusing.
There is also a good breakdown (pun intended) of reverse protection
here which gives a pretty good explanation of the pro's and cons of most of the common methods of performing this particular trick.
Most consumer grade equipment with a wall wart, generally use the cheapest solution, and that means a diode and fuse. The fuse is in line with the supply, and the diode is as per the example above.
This is not resettable, (most manufacturers of cheap consumer goods dont care if you blow them up with the wrong polarity, 'cos then you will need to buy another one). The method is cheap and relatively safe, since the currents involved with a wall wart are fairly low. I wouldn't like to see a high amperage circuit "protected" in this way, the resultant bang would send you flying.
A reverse diode and fuse wouldn't work in our case, because we are know we are going to reverse the polarity, so if we decide the schottky diode isn't going to cut the mustard, then mosfets are the way to go.
The N-channel mosfet solution is probably the best, and you may recognise it from my
NB-4L battery teardown. The
DW-01G protection circuit uses two n-channel mosfets to do its magic, one of them is configured in a similar way to that described in the first tutorial, to protect against over discharge.
