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Reply-To: "fred bartoli"
From: "fred bartoli"
Subject: Re: Short Circuit Detector
Date: Fri, 11 Oct 2002 18:17:07 +0200
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Organization: Guest of ProXad - France
NNTP-Posting-Date: 11 Oct 2002 19:18:10 MEST
red rover a écrit dans le message :
> I am currently looking at a Vcc short. I would
> guessed that you would be better off measuring
> from between the positive end of one
> decoupling capacitor and the positive end of another?
> Wouldn't that make the measurements a little easier
> because they are relative?
For copper planes, sheet resistance is about 0.5mohm/square for 35um and
1mohm/square for 17um thickness.
So you'd better have a sensitive meter, (maybe add a gain stage).
Then set to say 100/500mA the current, and protect your circuit with a
schottky between vcc-gnd : it's easy to have the psu voltage button
accidentally turned in case it happens the short opens... by himself. (I had
that and once corrected for the short I just had to drop the fried board :-(
> "Nico Coesel" wrote in message
> > "red rover" wrote:
> > >Does anyone have link to a design for
> > >short circuit detector? I recall that Bob
> > >Pease had a design for one in his Analog
> > >Troubleshooting book. I believe it was
> > >basically a micro ohmeter with an audio
> > >output and you worked your way around
> > >the pcb homing in on the fault.
> > >
> > >Anybody here made one? Any good success?
> > I've found that most short circuits on a PCB are in power
> > distribution. In case of such a short circuit I simply connect a PSU
> > set at a -safe- low voltage (current limit set to 1A; depending on
> > minimum trace width) to the board. By measuring the voltage accross
> > the decoupling capacitors (to start with) I can determine the area
> > where the short should be. The closer you get to the short circuit,
> > the less voltage you measure.
> > --
> > Reply to nico@nctdevpuntnl (punt=.)
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