milli ohm meter query

[patchell]

Paul Perry wrote:

> 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.
>
> paul perry melbourne australia

    Paul:

    I have used a Fluke 8050A for this purpose (4-1/2 digit DVM).  The
feature it has that makes it nice for tracking down shorts is a little
button on the front called "relative".  I short the probes together,
push that little button, and it knocks out the probe resistance.  This
is not quite as good as a four wire probe, but good enough for tracking
down the short.  The lowest scale is 200 ohms full scale (same as most
of the 3-1/2 digit meters), but that extra digit means .01 ohm
resolution.  This I find good enough for going down the trace looking
for the lowest resistance.  You still have to be patient, but I have had
a 100% success rate doing this (just did this only yesterday).

    Another tool I used once put a very low current pulse on the trace.
You connected the two circuits that were shorted to this thing, and used
some sort of probe that would tell you which way the current was
flowing.  Kind of neat, but in reality, I have had much better luck with
the 8050a.

    Just my opinions.

    -Jim



-----------------------------------------------------------------
Visit:http://www.silcom.com/~patchell/
-----------------------------------------------------------------