How to test a 440vac capacitor?

Thread Starter


Joined Jun 2, 2017
I have a small micro-controller board that has analog output and can operate external low-voltage devices. I am looking to have a replacement harness of some type that will connect to existing/installed capacitor (start capacitors, 370vac+), and allow my board to perform tests.

it can be presumed that the board can detect if the entire system is operational, and can prevent the entire system from being turned on during this test procedure.

I want to be able to:

  1. isolate the capacitor from rest of system - via the harness. This prevents any bleed to other components.
  2. discharge this capacitor - i.e. shorting the poles with a screwdriver.
  3. perform a mfd test - as done with a multi-meter - to determine if capacitor may be bad.
  4. undo operation performed in 1. allow the capacitor to re-charge.
These are high-voltage start capacitors on large motors, compressors etc..

Can this be done?

Many thanks!


Joined Aug 21, 2008
Yes, this can be done.

Even with AC high voltage capacitors assume that the voltage across the plates can build up after the short is removed. Be careful!


Joined May 14, 2017
DickCappels is correct on this one. I have seen this tendency on electrolytic bulk capacitors. A better approach would be to discharge the cap using a suitably sized resistor.
Keep all the following parameters of the discharge resistor in mind:
1.) Voltage rating
2.) Wattage
3.) Resistance

I have made a custom discharge *tool* by soldering alligator clips onto a high wattage resistor and then heat shrink the lot.

To test the capacitor it is important to not only test the capacitance, but also the ESR.