Tracing short circuit on pcb

If you have something that can measure very low resistances, you can use this to track down a short. The closer you get to it, the lower the resistance will be.

You probably need to be able to resolve milliohms.

 

A multimeter is probably not up to the job - unless it is a very precise one.

First: have you done a visual inspection? Sometimes solder whiskers can be seen with the aid of a moderate magnifying glass.

Second: In order to locate a short you need to measure the resistance, right, but you'll need either a very sensitive instrument (wich I doubt your "old" multimeter is) or you need a lot of current. If you can apply a low voltage source (say 1 V ... 2 V), referably adjustable, with a good current output (say 1 A) you could connect this source to the suspect traces. connect a voltmeter (multimeter) with (-) to (-) of the power supply. Use the (+) tip of the voltmeter as a probe. Touch the probe to the (+) connection of power source and trace. Ramp up the voltage until the voltmeter shows a measurable voltage. If your measuring range is e.g. 200 mV (a typical range), you will have a reading of 10 mV for each 10 mOhm at 1A. Now you can move the (+) tip of the probe along the trace until you find the spot where the voltage is lowest. The short circuit should be near this point.

If you are lucky, the high current will burn ot the short circuit (that's why your power supply should be low voltage, to minimize risk to the rest of the circuit). If you see the burnt spot, you can then try to find the reason for the short circuit. If not, it is possibly within a component. This is a difficult situation becaus this component is 99.9% defect, but difficult to locate.

Regards,
Harald

P.S.: If you suspect more of a component failure, not a short circuit by added material (e.g. whiskers) on the PCB, you may want to have a closer look at any electrolytic capacitors found on the PCB. Those are a rather common cause of problems.

 

Check this out. Something built for exactly this purpose.'

Uses an 8V constant current source (I would expect the current is reasonably low, owing to the 150 ohm maximum resistance. (suggests 50mA or so) but I guess that could change with observed resistance.

The videos are well worth watching if just for the curiosity factor in seeing it easily identify a difference in resistance of 6mm of PCB track.

edit: Here's a really simple alternative that is less sensitive, but probably all you require if the short is a low resistance. (I should probably say "failure" rather than short)

 

True, an ESR meter can be useful.

Here are two other devices (both of which have videos showing what they do).

Short Sniffer and Leak Seeker