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Having 3.3V short to GND in complex board.Looking for method to solve quickly
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muthukural001:
Hello All,

I am working on a board which is having 3.3V short to ground and the impedance between them is 5 ohms. Since this 3.3V is going to many places, removing components one by one is time consuming.My colleague told me an idea that give 100mv @ 0.5A  on the 3.3V path and look where the voltage is low to find the short. I have done the above method but the voltage is 70mV everywhere on the 3.3V path. Plus, I don't understand the logic of the above method that different voltage drop on the same path. Please let me know if the above method makes sense to anyone.

Also, if there is any other ways that I can try, please suggest.

Thanks and Regards,
Muthu
langwadt:
the Louis Rossmann way is to power the board and pour some alcohol on to see where it gets hot and evaporates
KrudyZ:
5 Ohm isn't exactly a short.
At 3.3 V you will have 660 mA going through your rail and dissipate over 2W of power. Something is bound to get warm.
You can use a thermal camera to see what gets hot. Lacking that, you can use your finger to feel it or your nose to smell it.
Never tried the alcohol thing, but in theory it should work.
An alternative is to measure the voltage drop on the rail itself using a micro volt meter.
You would be able to track the direction of the current in the 3.3V rail by measuring the voltage drop over its own resistance and zero in on the location of your "short".
nali:
Something doesn't add up in your figures: If you apply 100mV then 5 ohms will draw a whopping 20mA which shouldn't give any appreciable voltage drop on a power bus. Or, if you're using a supply operating in constant current mode at 0.5A you should get about 2.5V if you do in fact have 5 ohms across the bus.

Are you sure the 5 ohms measurement isn't an artifact of whatever else is on the bus?

If you *do* have a partial short as others have said apply 3.3v and see what gets warm. If you don't have a thermal camera try the alcohol method or maybe freezer spray (or air duster turned upside down) and see what defrosts first.
bob91343:
A 4-wire Ohmmeter moved around will usually pinpoint the trouble spot.  Make careful resistance measurements that can resolve milliohms and you can find the lowest resistance place.  Use a stylus to make consistent contact, or whatever works best.  All this with power disconnected.

Another way is to pump a small current in and measure potentials around the board.  The lowest potential is closest to the short.  This in millivolts or lower.

The brute force method of putting in lots of current and looking for smoke or, at least, heat, will work but might destroy good parts.
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