milli ohm meter query

[Harry Bissell]

There is another way... if you are sure the short is not "real" (ie its just a
whisker or contaminent on the board... then do what I do (if you dare)....

Attach wites to the shorted runs....

attach a switch (heavy duty!!!) in series with a large (5000-10,000 uF) cap
charged to the highest safe voltage the circuit can withstand....

Turn on switch...

note this is a somewhat violent method... and if you are not SURE that it is a
nuisance short

DON'T DO IT.... but for those "subterrainian" shorts that sometimes show up on
PTH
PC... like a whisker in the laminate itself... this will clear it... and maybe
even leave a little
crater  for you to see... OH that's where it was....

Also be sure you securely attach the wires to the shorted runs... it you
charge the cap ant then tough the board you will arc the run right off where
the probe hit.

Don't do this on "vintage" boards... OK. If you do don't come crying to me !!!

H^)


Grant Richter wrote:

> One of the desktop Flukes, a 3 1/2 digit has
> a 2 ohm and 20 ohm range for this purpose.
>
> ----------
> > From: patchell <patchell at teletrac.com>
> > To: Paul Perry <pfperry at melbpc.org.au>
> > Cc: synth-diy at mailhost.bpa.nl
> > Subject: Re: milli ohm meter query
> > Date: Saturday, February 19, 2000 10:21 PM
> >
> >
> >
> > 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/
> > -----------------------------------------------------------------
> >