I've found two 20 000 uF 50 V capacitors on a shelf, they've been sitting there for some 8 years. How can I find out if they're still good or not? I need them for a rectifier for my audio power amplifier. Additionally, I would like to check the capacitors that are currently installed in the amp to find out whether they all need replacement or if some are still OK. They have slightly varying capacities and voltages - 10000 to 20000 uF, 35 V to 63 V.

I don't have sophisticated lab equipment, all I've got is a DMM and a DC power supply with voltage source and current source modes (both V and I can be limited at the specified values).


Put your power supply on it with a low current limit, limit the volts to 50V and watch the voltage. The volts should start at a low value and then increase to a high value. If you can set the current limit at a known low value you could even time the voltage rise to give you capacitance V=I.t/C. Then check for leakage put 45V on it and check that the current drops to a very low value.

Checking the ones in the amp without removing them will be much more difficult as they will have components around them and you could damage other things if you try this. Check that non of them are swollen or leaking or have split tops then power it up and check functionality.

  • \$\begingroup\$ I've had a similar idea, but how do I come up with specific threshold values? What's the value of 𝛕 in this case? \$\endgroup\$ Jan 3 '17 at 10:06
  • \$\begingroup\$ I realize I'll have to remove the caps from the amp to check them. It's inconvenient, but not terribly so. I know at least some of them are bad because the amp leaks hum into the output. None are swollen or leaking, so no easy way out of this for me :) \$\endgroup\$ Jan 3 '17 at 10:07
  • \$\begingroup\$ Is there a way to also measure the capacitor's internal resistance in addition to capacity? \$\endgroup\$ Jan 3 '17 at 14:07
  • \$\begingroup\$ This is addressed in the answer as to how to check for leakage. \$\endgroup\$
    – RoyC
    Jan 3 '17 at 19:07
  • \$\begingroup\$ You are implying that the internal resistance is connected parallel to the capacitance, not in series? \$\endgroup\$ Jan 3 '17 at 19:25

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