Short Circuit Tracer

This is a simple circuit that can track down a short circuit on a PCB or elsewhere to within a few mm or so. The accuracy depends on the parts you pull out of your bits box. The device reads lowest when it is close the offending short circuit.

The transformers are the most critical part; they can be almost anything :). I used a couple of miniature audio transformers. You need to be able to slip an extra turn of wire around the core. I happened to have some 0.6mm enameled wire but you can just strip off a bit of hook-up wire and use that if the winding doesn’t short out.

The oscillator is anything that works. This just happened to be most convenient for me. This one runs at about 5kHz but that really doesn’t matter, you can run it at 50Hz if you want, 5kHz is as high as you want to go.

The probes are connected as a Kelvin probe. You can also get Kelvin clips; these are like crocodile clips, but each jaw is electrically separate. The voltage on the probes is very low, a few micro volts, definitely not a shock hazard. The wires from the secondary of T2 and the primary of T1 are connected directly to the probes; you can’t just wire across between the transformers, it doesn’t work. You’ll get a reading on the meter ok, but the sensitivity is rubbish. The connections from the oscillator are the force and those to the meter the sense. It is a sensible alternative to put the force on crocodile clips and the sense on probes. The force is then connected at either end of the shorted circuit and the sense probes moved along ’til you get the lowest reading.

The meter can be a suitable AC voltage range of a cheap multimeter. I do find that the cheaper meters don’t handle the higher frequencies very well though.

for the power supply I used a couple of Li-ion cells I have lying around. You use what you like depending on the circuit you use. The circuit only takes a few mA so my 2400mAh batteries should last forever if I switch off when not in use.

The finished project

The Tracer reads about 6V when open circuit. If you short the probes together then it drops to about 2.5V and a few inches of hook-up wire reads about 3V. What yours reads depends on the transformers.

You can just see the added turns on the transformers. These are connected to the terminals on the left. The two white wires lead to the meter.

I wish I had used some lower impedance transformers; these were 10k centre tapped on all the windings, 600ohms would have been better. I also think running at a lower frequency would give a cheap meter a better chance. It’s not a perfect device but it does the job. When I first came up with a design very similar to this about 40 years ago it saved several hours of work for two men every day.

This design is suited to almost any decent bits box. it took about half an hour to make up and is very forgiving of components. It’s suprising what rubbish parts you can use to make it up.

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