Oops !

I was once debugging a board that had such a short. I wondered why I could see a light shining through the board even though there were no LEDs on it.

Unless you get a thermal imager and use that to look for hot spots.

With the four wire ohm ohm meter, I put the "current" connections on the inputs to the board, and left them there, and probed with the "voltage" connections.

A good four-wire ohm meter should use AC (or at least reversing DC) excitation to get rid of any thermal emf. This is the real advantage that proper ohm meters offer over driving a few hundred mA through the board with a current limited power supply - with AC excitation you can reliably measure much lower voltage drops (microvolts rather than millivolts).

-------- Bill Sloman, Nijmegen

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I agree that it's likely not a shorted cap - we must've placed at least 1/2 a million decoupling caps of various sizes, types, and brands in the last year, and have yet to have a significant problem with any of them, at least electrically. We have, however, seen shorted boards, which were supposedly electrically tested, but the quality of the boards was not the greatest.

Most likely the problem is a solder bridge somewhere. They can sometimes be hard to see, even with a well trained eye looking directly at the short (ex.

I made a nice KABOOOOOOOOOOOOOM 8" away from my face once when changing a breaker in a 200A 120/208V 3 Ph panel. I was getting ready to screw in a new breaker, and while trying to line up the screw to the hole that fastens the breakers tab to the bussbar, the bakelite housing between the breakers fell away (significant, but unknown not so visible damage to that area of the panel), alowing the breaker to drop, which caused the breakers tab to short out between 2 phases. I don't remember the bang being extremely loud, but other people said it was defining. I couldn't see for about 10 to 20 seconds, since the arc was so bright. My hair and eyebrows were a singed, and I had a bit of a sun burn from it. After brushing away all the singed hair and getting my eyes checked at the hospital (potential welders flash - my contacts may have prevented it), everything was fine. Looking at the damage afterwards, the bussbar had serious arc nicks in it, the breaker tab was partially missing, the screw no ware to be found, black and gray smoke stains all over the place, on the bussbar, bakelite, breakers, shielding plastic, the screwdriver tip and insulated shaft, etc.

What you say about AC measurements is true, but do you know of any specific bench multi-meter instruments that use ac excitation?

Got a call to bring some fuses to a remote CO once. When I got there a

Charlie

I read in sci.electronics.design that Winfield Hill wrote (in ) about 'Oops ! - Thanks !', on Wed, 28 Sep 2005:

Not a multimeter but I have a very old (toob!)4-wire milliohmmeter that uses a 50 Hz constant-current source. 1.2 mohm to 120 ohm FSD. Still as accurate as the analogue meter allows.

Polar Instruments

Regards,

Gerry