I was replacing a 400 V 120 µF capacitor in a power supply (AC 120 V - 19 V DC 4.62 A).

I tested the dc out end while it was still plugged in and got 0.07 V.

As far as I can tell although being plugged in for 2 minutes (approx) it did not get boiling hot (was barely warm when I reopened it).

Upon visual inspection there does not appear to be any bulging or cracks.

I don't just want to turn it around have it explode, is there a way to check and see its still safe? Is it even likely to be? From what I've read people have mixed opinions but the general consensus is how long vs how many volts, is there a way to be certain?

  • 3
    \$\begingroup\$ is there a way to be certain? ... yes, there is, throw the capacitor away \$\endgroup\$
    – jsotola
    Commented Dec 12, 2018 at 4:31
  • \$\begingroup\$ It's not worth the time to test if it's still functional, really. There are failure modes that don't involve the electrolyte boiling, so the fact that it's not hot doesn't necessarily mean it's fine. \$\endgroup\$
    – Hearth
    Commented Dec 12, 2018 at 4:54
  • \$\begingroup\$ Normally I would agree however in this case I really just need a crappy but working power supply to test some old laptops. I don't really care if it fails a year or even a month from now... I just don't want to cause an explosion in my apartment lol \$\endgroup\$ Commented Dec 12, 2018 at 6:31
  • 1
    \$\begingroup\$ Modern elco caps don't really explode, they just vent. If you're ok with a possible PSU failure, and it's no problem to air the room out, just keep the cap. \$\endgroup\$ Commented Dec 12, 2018 at 8:57
  • \$\begingroup\$ Where was the capacitor in the circuit when it was connected backwards? Was it on the primary side (120VAC -> 170VDC), or on the secondary (19VDC)? \$\endgroup\$
    – marcelm
    Commented Dec 12, 2018 at 11:50

2 Answers 2


You can try a tester like this:


I'm not sure what properties it checks, but it probably gives you some clues about the component's properties.

enter image description here


if you have a reference compare charge time in miliseconds with a tester in ohms mode. maybe 2k to 20k ohms will be okay and it could charge to Rmax in about 1 to 3 seconds. check backwards after depleting the charge it would take 1/3 to 2/5 more time to charge in reverse. that would be fine if both reference and DUT are quite similar which may depend how you interpret miliseconds and its internal resistance.


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