Capacitor Discharge Formula

Discussion in 'General Electronics Chat' started by Dibubba, Aug 22, 2015.

  1. Dibubba

    Thread Starter New Member

    May 15, 2015
    Posit: a device containing capacitors is running perfectly, and the turned-off. It then sits for a period of time on a shelf in the garage, and some months later is opened for dissection. And/or, I charge big honkin' caps on the bench, get distracted, go on a business trip, and return a few weeks later. In that case, r=infinity.

    What formula can be applied to determine what charge still remains?

    Waaaay back in 2011, #12 was kind enough to post:
    The formula written on my wall says, Vo=dV (e^-t/rc)

    so Vcap = original Vcap times e to the negative time over (resistance x capacitance)

    Would the above apply? I'm not concerned with zapping myself - I know how to sweetly and gently discharge a cap! I'm focused on "how long will a charge (and "what charge?") will remain?" on those bigger ones.
  2. OBW0549

    Well-Known Member

    Mar 2, 2015
    Yes, the formula still applies, as always; except the "R" in this case is the internal insulation resistance of the capacitor; and without any practical means of measuring it directly, you'd have to infer the resistance from the measured self-discharge time of the cap.

    A good quality electrolytic capacitor might have a self-discharge time of many hours or even a couple of days; a good polyester film capacitor, weeks or even months if kept in a low humidity environment.
  3. #12


    Nov 30, 2010
    When R is infinity, that formula is worthless.
    The ways to discharge a capacitor by ignoring it include internal leakage, circuits that happen to be connected even though you aren't thinking about them, and dirt. The opposite effect is caused by dielectric absorption. A disconnected capacitor can actually gain voltage because the dielectric material can settle back towards the condition it was in before any voltage was applied (when it was born). The perfect example is the CRT (picture tube). Those suckers can knock you silly a month after you, "discharged" them.

    I'm running out of words. Ask another question if you want something more specific.

    ps, whatever you heard about me in 2011, it was all lies! :D
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
    Anybody that designs a product containing large storage or filter capacitors that doesn't provide "bleeder" resistor(s) to discharge the capacitors when the device is turned off is an idiot, and should be sued for gross negligence....
  5. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    Seems a bit harsh to me.