milli ohm meter query

[Magnus Danielson]

From: Paul Perry <pfperry at melbpc.org.au>
Subject: milli ohm meter query
Date: Fri, 18 Feb 2000 23:56:45 +1100

> I'm doing a lot of hunting for shorts on PCBs at the moment.
> Naturally i can't afford a 'proper' millohm meter (and I only
> need somethng going up to say 100 millohms full scale.
> 
> Is there any way I can adapt my cheap digital multimeter for this,
> with some kind of little external add-on ckt?
> Accuracy isn't important.
> It  has a 4 ohm scale.

Why not built a small 4-point resistance rig?

You create a current source which have foldback at some suitable voltage and
you have a diffrential input amp.

The reason that the current source should have foldback is that if you
connect or disconnect something to either of the current terminals (I+ and I-)
then you do not want any interestingly high voltages ;)

Also, one has to consider how much power one dissapated into the measured
curcuit. Also, excess current could be an issue.

A simple solution would provide a fixed current (say 100 mA) up to some
breakpoint voltage (say 10V). Then you would have the diff-amp stage followed
by a scale gain stage (that would be two op-amps and a binor bunch of
resistors). You would have 4 terminals for the resistance measurement end and
2 terminals for you DMM. Hook in some power and you are set. It should be a
simple DIY thing.

Does this sound in parity with what you are looking for?

Cheers,
Magnus