Hi - I'm new here and I may be about to talk absolute rubbish, but I may as well start as I mean to go on! :-) On Sat, 24 Aug 2002 11:23:30 +1200, Jinx wrote: > In this case it was $30 for the LCD and $12 for the PIC, and they are > the bulk of the circuit cost. Ouch! > I think it's blown the crystal too. Err - would this happen? I've never tried putting 14VDC across a crystal, but I'd be surprised if it was fatal, unless it physically overstressed the crystal itself and broke it. > Bottom lineis that for a dollar or two I'd like to show the customer that I'm taking it > seriously (even if it never happens again, which it probably won't) I'm with you, in that a solution to a problem that *has* happened (however rare) does much more for customer relations than a shrug and "well that's never happened before - so it probably won't happen again"! Since the failure in shorted mode is so rare, my detective instincts lead me to suspect something else - disconnecting the common connection to a 78xx will work, of course, but are you sure it wasn't an incoming spike of some sort? If it's mains powered, could it have been interference from something noisy? If it's automotive, has it had any arc- or spot-welding done? What about thunderstorms, or nuclear-weapons testing nearby? (Any suspicious French fleets thereabouts? :-) In terms of circuitry, I'd go for overkill: A fuse on input, followed by a "dropper" resistor that will result in about 8V being presented to the 7805, and one of the crowbars already suggested downstream of the 7805. This means that you are giving the 7805 an easier life, the crowbar will prevent damage if it does fail or have its common disconnected, and the fuse will close things down before the fire! Cheers, Howard Winter St.Albans, England -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads