Finding shorts: The 4 wire Kelvin way.

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Len
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Joined: Sat Apr 28, 2018 6:49 pm

Finding shorts: The 4 wire Kelvin way.

Post by Len » Thu Jan 12, 2012 11:41 pm

The best way to locate PCB or component short(s) is to use the ancient 4 wire Kelvin measurement technique.  Use say a 3.3V or 5V DC bench supply  but set it to be 1A current limited. Or put a 5R 5W resistor in series.Connect the supply onto the faulty board at its normal DC input or after any regulators in the problem area.  Use the DVM’s 300mV range or a  microvolt DVM. With a 1mV resolution you can measure down to 1millli Ohm. With 1 uV resolution you can measure down to 1 micro Ohm.  With the supply connected and drawing current: walk the 2 DVM probes across each of the components to find the point with the LOWEST voltage drop.The fault is close to or at this LOWEST voltage drop point.  You can also measure each the  copper tracks and parts hanging off it. Measure the tracks end to end or hole to hole  and find the track with the HIGHEST voltage drop.The short is somewhere along that path.  In total desperation with multi layer boards with inner shorts or holes to power rail faults I resort to a  brute force technique to blow out the faulty track or part or holeI to use a 50A 5V supply and wait for the smoke to clear. (grin).  With sensitive (RF) components still on board this can/will result in their death but you will find the problem area.RegardsLen Spyker

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