Locating short circuits (was Memorex 102 20MB Hard disk)

[Chuck Guzis]

On 14 Apr 2010 at 12:27, Dan Roganti wrote:

> But you really want a Milli-Ohm Meter to find shorts with more
> precision when it comes to connections with several components on
> there, about 0.1 milli-ohm resolution. That lets you pinpoint the
> short directly to the sport on the pcb, even between the next pin on a
> chip. I just found a place online the other week that has one for $150
> from Ruby Electronics, other costs 3 times as much,  but I still have
> to wait to get one myself--other bills still have priority :)
> http://tinyurl.com/y4d7op5

As long as you don't require calibration, isn't a milliohmmeter for 
this application pretty much a matter of wiring up an inexpensive op-
amp with a small current source?

http://baec.tripod.com/DEC90/ohmmeter.htm

Here's a neat little circuit that uses an LM317 regulator.  You could 
build it out of components in your hellbox in a matter of minutes:

http://www.edn.com/article/CA408390.html

The basic idea is that you don't actually need an accurate 
quantitative measurement, but rather a relative one as you probe down 
the PCB.

Alternatively, one could use a wheatstone or kelvin bridge with a 
microammeter.

While a lab milliohmmeter would be a cool thing to have, it's not 
necessary.

Cheers,
Chuck