Finding shorts in boards

From: Pete Turnbull <>
Date: Wed Dec 1 13:21:23 2004

On Dec 1 2004, 13:21, Jules Richardson wrote:
> I've got a board here with a short between ground and the +5V rail
> (actually not quite dead - I'm getting around 10 ohms between the
> Any useful tips for finding the fault? It's a large board,
> lots of silicon on it unfortunately :-(

That's only about 500mA at 5V. Sure it's a short? Of course, it may
draw lots more than that under power, if your meter is not using enough
voltage to turn on junctions.

> Are there any particular components that are likely to fail in this
> that might be found across PSU rails? (decoupling caps, certain ICs,
> crystal modules etc.?)

Decoupling caps, TTL, and some MOS memory chips all can draw excess
current when faulty. I usually put them on a hefty 5V supply and feel
for what's hot (you can sometimes burn out the short in a decoupling
cap that way -- they often go bang). Point of reference: a BBC B board
normally draws about 2A at 5V, but the last one I had with a
particularly nasty set of faults drew over 10A at 5V -- and the faulty
TTL chips got hot enough to feel without actually touching them.

> Given a suitably sensitive meter is it sensible to assume I can try
> home in on the short location a little? (I've found readings between
> GND/supply on various LS chips of anything between 9.5 and 12 ohms so
> far)

You really want a current tracer. HP made a really nice one, and I'm
still annoyed I didn't bid high enough on the last one I saw :-(

Pete						Peter Turnbull
						Network Manager
						University of York
Received on Wed Dec 01 2004 - 13:21:23 GMT

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