Take a look at the LM2931 regulator.  It's available in both adjustable and
fixed output, offers (brief) overvoltage protection up to 60V, continuous
output with inputs up to 26V, built-in reverse battery protection, built-in
mirror-image insertion protection.  Also very low Vdo.  I love it- use it in
all of my designs; at least, the ones below about 100 mA draw.  It's also
not too expensive.

>the polarity was identical. it was 26V connected to a 5V rated board. but i
>do want to also add reverse polarity protection in my next design.
>
>>See the recent thread for details.  IMHO, the best overvoltage
>>protection is a label:  12V 250 mA.  If the user violates that label, it
>>becomes his or her problem, not yours.

>this is a fair statement. but i feel, and it's a subjective feeling of
>course, that it'd be nicer to allow the user to make mistakes, at least to
>a certain extent.

Of course, especially when YOU intend for yourself to be the primary
user.  That way you don't have to get mad at yourself for damaging
it later on, when you forget what the input was to be.  But as Jinx
pointed out, my view is a fairly libertarian one- that is, if you don't
follow the rules, then don't cry to me when the device breaks.  Try
a warranty claim on an item you violated the sticker guidelines on and
see how far the company lets you get...

>if i understand the above correctly, then using the zener's to shunt on
>overvoltage or voltage reversal sounds like it may damage the power supply.
>is that right? if so, what would be a good mechanism to protect the supply?

If you can afford the voltage drop, you could use a bridge rectifier
type design, which puts the positive input to the positive rail and the
negative to the negative rail regardless.  For a one-off hobby design,
I would ALWAYS use at least a 7805 voltage regulator, if not the
LM2931 I mentioned above.  They are just too cheap to make the
hassles assorted with zener regulation worthwhile.  Just tack 25 onto
your next order, and you'll be set.

Mike H.

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