>Sometimes, but not for me. I often take a few boards and connect them
>together to make a prototype. Every board must have its fools diode,
>because I am a fool and I will occasionally make the wrong connections
>between the boards.
>
>Wouter van Ooijen

This seems like a good point to interject this, as I don't know that I've
ever seen it on the PIC tag, but it has been in EE: a couple of times,
and cheers to Russell for introducing this to me...

A reversed orientation PMOSFET is inserted, with the gate tied to the
ground net, the source to the positive to the positive of the battery,
and taking the positive power input from the drain of the MOSFET.
Usually some resistance is inserted between the gate and the ground.

MOSFETs are basically symmetrical, with the exception of the source/
substrate electrical connection which forms an implicit diode between
the drain and source.  During normal operation, that diode is reverse
biased.  The MOSFET is placed "backwards" so that diode is reverse
biased when the power supply is hooked up wrong.  In the normal
orientation, that diode would be forward biased when the battery
is backwards and the protection would fail.

Using a good MOSFET, the only voltage drop to the protection is
the current times the FET resistance, which can be < 1 ohm.

I'm not sure how good my explanation is; anyone who can do
better please do so...

Here's a page previously mentioned in EE: which has a diagram:
http://www.geofex.com/Article_Folders/mosswitch/mosswitch.htm

Mike H.

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