Tracing short circuit on pcb

[Dan_68]

Hi all,

I am an engineer so i have done a bit of electronics, i have read all the different methods of testings different components, resistors, capacitors, diodes, transistors etc. However i am having great difficulty locating methods to find problems on boards. I have checked voltages and found i have a short but not sure where. Any one have any tips on how to find this. Repair work is very difficult when just starting out. i need to fix something get my confidence and enthusiasm back up.
thanks

 

[(*steve*)]

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.

 

[Dan_68]

I have an old analog multimeter of good quality ill see what resolution it has and give it a try. Any more tips or links to help would be much appreciated so frustrating working for hours and finding nothing

 

[Harald Kapp]

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.

 

[Dan_68]

Thats sounds like a very good method i will try and get some sort of a power supply to try that. I have already replaced majority of the caps of the secondary side and that hasnt fixed it yet, i suspect its a short in a component but i will give it a more thorough inspection with a 40x magnify glass.
Any more tips would be much appreciated even for tracing other problems the more knoledge the better.

 

[JMW]

Harald beat me to it. It is really quite simple. You need variable power supply voltage and current and a can of freeze spray. Set the power supply at the correct voltage, but the current at 25 to 50% of the fuse that is clearing. Connect the negative lead first then the positive lead. The "shorted" components will heat up, you maybe able to use your finger as a thermometer, saving your freeze spray. If the PCB requires multiple voltages, remove the fuse or cut the supply pin of the voltage with a problem and insert your power supply.
Let us know how you made out.

 

[(*steve*)]

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)

 

[Dan_68]

Ohk thanks for all the information, That device is awsome "makes it look" so easy. Il do a bit more research on this method and give it a go. I dont have a variable power supply yet so il try work something out from what i have.

Thanks all, anyone got other methods for tracing any other faults. Much appreciated valuable hand on experience only gets these answers

 

[shiekh]

And ESR meter can be good at locating shorts.

 

[(*steve*)]

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