I was testing my SVPWM power board with a light bulb load instead of the AC servo motor. It worked a few times while testing. The bulb would turn on and my PWM was verified.

The last time I turned it on I heard a spark and the bulbs would not turn on. Upon inspection I see the the DC link capacitors (5 parallel in total) are showing short on DMM. The capacitors are connected to the AC supply after a bridge rectifier to get a clean DC line. I need to replace the capacitor that is faulty.

First, I would like to know why this fault possibly occurred so that I can avoid it in the future.

Second, is there any way to find out which one is the faulty capacitor. Seems like a tedious task to remove each capacitor from my board to check which one is actually short.

Update: I removed all the DC capacitors and there is still a short on the DC+ DC- supply. I am not sure what the problem is here.

  • $\begingroup$ Why remove completely? just disconnect 1 leg. $\endgroup$
    – Solar Mike
    Sep 1, 2023 at 8:58
  • $\begingroup$ Thankyou for the suggestion. I thought of that too but they are soldered pretty neatly. The capacitors are pressed completely onto the board. It will be impossible remove a leg. Ill have to completely remove it from the board in order to disconnect $\endgroup$ Sep 1, 2023 at 10:15
  • $\begingroup$ It's probably somewhere near your rectifier. Cap getting an internal short is pretty rare without external signs or getting hotter than the rest first. $\endgroup$
    – Abel
    Sep 1, 2023 at 17:32

1 Answer 1


It is common for the filter capacitors to fail because the diode bridge failed first. Replacing the caps will not fix the problem. You will need to remove the rectifier diodes and check them as well.

It is good design practice to use caps and diodes that have higher voltage ratings than the stock parts did. If I am replacing a cap rated for 300VDC, I'll put in the same value of cap but with a 400VDC rating instead. Similarly, I'll replace diodes rated for 500VDC with 1000VDC parts.

Finally I recommend you install a fast-acting fuse between the transformer secondary and the diodes to protect the rest of the circuit, if your board does not already have one.

  • $\begingroup$ Thankyou for the suggestion. I will be sure to add such protections and safeties in my next version. $\endgroup$ Sep 2, 2023 at 10:52

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