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Subject: Re: Short Circuit Detector
Date: Sat, 12 Oct 2002 19:44:06 -0700
X-Mailer: Mozilla 4.78 [en] (Win98; U)
I've never had a problem with a digital meter.
Some of the vintage VOMs have 22.5V batteries on the high ohms ranges
and push a lot of current on the low ohms ranges. I'd avoid those.
I have a Hipotronics Digital Milliohmeter with 10 MicroOhm resolution.
Pumps 10A from a 3V battery. Don't think I'd use that on a populated
board ;-) Shorts tend to vanish in the manner that produces the most
The key is to know the specs of your measurement instrument.
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> If Tom decides to publish I hope you'll let the group know?
> I'd keep an eye out for it. Sounds useful :)
> While I'm at it has anyone ever heard of a regular
> DMM measuring ohms being a dangerous thing to
> use find shorts on an un-powered board? (damaging
> parts etc.)
> "Tom Bruhns" wrote in message
> > I have one I've used for years (and happen to be in the process of
> > re-vamping to use more readily available components). It's very
> > simple. In the configuration I _normally_ use, it injects a current
> > from a 1.5v cell through a 1k resistor and a diode, so the
> > short-circuit current is about 1mA. The voltage across the "short"
> > converts to a current to drive an oscillator, so the higher the pitch
> > the shorter the short. With it I can hear the difference between 1
> > ohm and a dead short, about 1mV delta. BUT...if you inject a current
> > from an external source, you can use it to find where in the circuit
> > the drop is the lowest: the point of the short. So with an amp
> > excitation, you can hear milliohm changes. The circuit is currently
> > an A77 watch cell (or similar 1.5V), 2 pnp transistors, 2 npn's, a
> > tiny audio transformer (Mouser TC001), and as tiny an 8-ohm speaker as
> > you can find, and a very few R's and a couple C's. When not
> > connected, the current drain is less than a microamp, so no on-off
> > switch. The one I've had a long time draws a lot more "off" current,
> > and even so the AAA cell lasts over five years.
> > I'm planning to make the circuit available through the Wenzel web site
> > (where there are a few other neat little test circuits) eventually,
> > but could perhaps give it limited distribution before that.
> > Cheers,
> > Tom
> > "red rover" wrote in message
> > > Does anyone have link to a design for
> > > short circuit detector? I recall that Bob
> > > Pease had a design for one in his Analog
> > > Troubleshooting book. I believe it was
> > > basically a micro ohmeter with an audio
> > > output and you worked your way around
> > > the pcb homing in on the fault.
> > >
> > > Anybody here made one? Any good success?
> > >
> > >
> > > Steve
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