How to check electrolytic capacitors on PCB(not desoldered)


Joined Sep 9, 2010
The most prominent failure mode of an electrolytic capacitor is increased ESR. This can disable a circuit with no visible sign of damage to the cap, and it may even still have its rated capacity. So if you want to check an electrolytic cap, anything less than ESR testing is a waste of time and effort.

There are ESR testers you can buy or build. I believe they use a low (<0.6V) voltage test signal to avoid triggering any attached diode or BJT and so they're meant to allow on-board testing. But any resistive load in parallel with the cap will affect the reading.

You're often ahead on time and effort if you simply replace every electrolytic cap without bothering to test. This may seem a waste but it often works and has the added benefit of renewing all the caps and thereby lowering the odds of a future cap failure after your repair.

Thread Starter


Joined Jun 11, 2020
i have replaced one diode transistor and fuse from computer power supply and today have tested with serial connected light bulb and it lit up so there`s still shorted element on the board