hi,

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.

i think i should add overvoltage protection to this board. while i'm at it,
i also want to add protection against reverse voltage input as well. i'm
considering the following solutions and thinking about minimizing cost,
maximizing forward/reverse voltage protection, maximizing speed and trying
to avoid any need for a fuse or component replacement if an overvoltage
condition occurs. i would like advice on what kind of results other people
have seen with the various solutions.

1. varistor combined with an n-channel FET. this would be a simple circuit.
i'd pick a 5V VDR in parallel with the +ve input. i'd pick an n-channel FET
with the +ve input to gate and the source of the FET as the ground output. i
haven't figured out how much reverse voltage this would be able to protect
against and also how fast this circuit would be. i guess the varistor might
be too slow to protect against supply spikes?
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.
3. semiconductor based. i haven't looked at this much. maybe a maxim 1455
signal conditioning type circuit. what kind of protection range would i get
here?

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?

thanks,
hirose

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