5 Feb
2011
5 Feb
'11
6:39 a.m.
From: robert.jarratt at ntlworld.com
---SNIP---
I was checking the capacitors in-circuit and understand that this is not
correct.
However, I would have thought that a 0 ohm measurement in-circuit would
indicate a shorted capacitor because there would surely not be any point
designing a circuit where both ends of the capacitor are connected together?
That capacitor also measured differently on a working board so it was
clearly suspect. However when I de-soldered it, it turned out to be open
circuit. The resistance in-circuit was not absolutely 0 though, although it
had seemed negligibly close, so it must have been the surrounding load as
has been suggested to me.
Is there a simple way to find shorted caps in-circuit? Would an absolutely 0
ohm resistance while in-circuit indicate a shorted cap for the reason I
suggest above?
Thanks
Rob
Hi Rob
The 0 ohms indicates a short but doesn't tell you what is shorted.
It could be the cap you are measuring at or one half way
across the board. It may not even be a cap since there are
many ICs also across the power lines.
I have a method that works quite well at finding such shorts
but requires a uV meter or a good 5 digit meter and a bench
power supply that can be set on current limit.
Alternate methods use a bench supply set on current limit
and hunt around with ones funger or take a picture in
the dark without the flash.
Also, there is that thermal paper.
The hunt and peck method of lifting one lead of each
part until you find the shorted one.
Some have success with a two lead ohm meter but
I've not seen this work well. A 4 lead has a better
chance.
Dwight