Subject: Re: Short Circuit Detector
Date: Sat, 12 Oct 2002 21:55:09 -0400
Organization: CenturyTel Internet Services
NNTP-Posting-Date: Sun, 13 Oct 2002 01:59:16 +0000 (UTC)
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
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
"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.
> "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