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Having 3.3V short to GND in complex board.Looking for method to solve quickly
xavier60:
Also, with about 0.5A of bias current, a shorted MLCC can be confirmed by measuring between the solder fillet and the top of the termination cap.
The DMM needs to have 10µV or better resolution.
muthukural001:
Hi All,
I checked the impedance everywhere on the 3.3V path, it is 4.7 ohms but actually it should be around 3.7K. @nali,I am not getting your point that it should drop 2.5V if my supply can give 0.5A. I set 100mV and max current limit is 0.5A in my power supply. The 100mV is not going down at power supply terminal and the power supply current display tells the load draws only 20mA when I connect my board to the power supply.
Thanks,
Muthu
nali:
Hi Muthu
Sorry, I wasn't sure if you meant you were either supplying a constant 100mV to the board, or using a PSU in constant current mode to force 0.5A to look for something getting hot.
As you're supplying 100mV and the board is taking 20mA - are you saying you're measuring 100mV at the PSU yet only 70mV on the board? That's quite a voltage drop for such a low current, although it's not really relevant to your problem.
It may help if you let us know what test equipment you have access to e.g. if you have a high resolution DMM you can look for voltages or voltage drops, or if you only have basic equipment then the "get warm" method is probably your only approach.
Personally, as it's not a direct short I'd just apply 3.3V after using an air duster can held upside down to cover the board in frost then the offending area should warm up pretty quick. A 5 ohm load will only generate a couple of watts heat enough to create a hotspot but not high enough to cause damage (maybe limit your supply to 500mA in case the bad part gets worse)
james_s:
Set the power supply to 3.3V with .5A current limit, 100mV is much too low to push enough current through the partial short. Using 3.3V will cause something to get warm and should make it easy to locate.
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