Powering a board with 5 volts (no on board regulator) invites all kinds of
problems including noise, glitches, fluctuations in 5 volts that will make your
circuit unstable, besides not providing any protection. An on board regulator
will eliminate this. The regulator will forgive most power supplies most of the
time.

I don't think I've ever designed a board and fed it with raw 5 volts.

If you have to, the best way is a 5.1 volt, 5 watt shunt zener and fuse. This
should protect you from  most over volt applications and reverse voltage.
Rick

Hari Seldon wrote:

> 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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