[clug] More info, please - was Re: Tridge's coffee contraption - photos

Paul Mackerras paulus at samba.org
Tue Dec 1 15:52:17 MST 2009

Alastair D'Silva writes:

> Hmm, going back to the application note above, they suggest that the
> internal diodes are suitable for clamping an input up to 1000V, but require
> external surge suppression for greater voltages. Assuming the PICs have
> similar clamping (do they?), what would it take to add surge suppression to
> their circuit? Would a MOV to earth do the trick? Is it a suitable
> substitute for isolation?

My main worry would be whether it would all still work and be safe if
someone plugged it into an outlet (or used an extension lead) where
active and neutral were swapped.

Isolation does also have the psychological advantage that you can be
sure there's no way for mains voltage to get over to the logic side.
Probably a MOV on the mains side and then 1Mohm resistors to the logic
would be safe enough, but it doesn't feel quite as safe.

I believe the PIC inputs do have internal diodes to Vss and Vdd.


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