>i had a small board with a pic and various other parts on it. i was in a
>hurry to demonstrate something and plugged a laptop power supply to it
>instead of a 5v power supply. the jacks were identical. i ended up burning
>up the board.

But was the polarity identical?  Did you end with +18V connected to
the ground net and GND to +5V?  Or with 30V connected to something
rated for 12V?  Reverse polarity protection might be more appropriate.
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.

>2. zeners. i'd put two opposing zeners in parallel with the +ve input. i
>think this would be even simpler than option 1. will this one be more
>expensive? i heard zeners are faster than varistors.

Like this?
----|>z-----
----z<|-----

If you connect them like that, you'll end up with one forward
conducting, with a voltage drop of ~.7V, and the other wanting
to reach its zener voltage but not able to.

>one thing i'd like to do is avoid damaging the input power supply. meaning
>if someone took a laptop power supply and connected it to my board, i'd
>like
>to protect both my board and their power supply. would shunting their power
>to ground be a bad thing?

That depends.  Some power supplies (probably most) would have "foldback
current limiting", where the current will increase to a certain point, and
then
the voltage and current fall off to prevent damage to the power supply.
Whether a given supply has that feature is not up to you, so I wouldn't
depend on it.

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