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When an empty (discharged) capacitor is connected to a battery, it slowly charges up as one plate fills up with electrons, while the other plate has electrons drawn away from it towards the positive terminal of the battery, resulting in one plate having a positive charge and the other having a negative charge.

However, what happens when a capacitor discharges itself? Do I simply assume that the negatively charged plate returns its electrons to the negative terminal of the battery while the positively terminal returns electrons to the positively charged plate, resulting in an end state where both plates of a capacitor are no longer charged (neutral)?

If so, how am I able to power things like an LED with a capacitor, since a LED or any diode only lets electricity flow through it in one direction?

E.g. If I connect a diode to a capacitor (in series), depending on how I place/orient the diode, the capacitor would not be able to charge/discharge?

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    \$\begingroup\$ When a capacitor discharges its current flow direction is not alternating so it is a perfect match for the LED's requirement for DC current. Off course if you hook up the LED the wrong way the cap won't be able to discharge quickly enough to cause the LED to glow or will end up burning the LED if the capacitor voltage is high enough. \$\endgroup\$
    – user34920
    Jun 25, 2014 at 20:51
  • \$\begingroup\$ If you have an LED connected to a capacitor, then current can only flow through the LED one way. It doesn't stop current going both ways through the capacitor if there's something else connected to the capacitor. Diodes only block current flow in one direction through themselves. \$\endgroup\$
    – user253751
    Jul 19, 2014 at 5:32

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You have the right idea. If you add a diode in series, you can only charge the capacitor, and wait for self discharge to occur. Or if the diode is reversed, you can only discharge it (or charge it with reverse polarity), and once again wait for self discharge.

An exception to this would be to have some other connection between the diode and the capacitor.

So long as the battery is connected, the capacitor will just remain charged. Once the battery is removed, if there's some closed loop path between the plates of the capacitor, then the excess charge on one side of the capacitor will use the closed loop to balance out the charge. Excess electrons from the negative terminal of the capacitor will move towards the positive plate of the capacitor to allow charge balance to occur.

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  • \$\begingroup\$ A quick search of self discharge on google brought up capacitor leakage. Can i thus assume that capacitors are unable to hold a charge indefinitely and that leakage = self discharge? Also how exactly do i charge a capacitor with reverse polarity? If i place a polarised capacitor in reverse, it would damage and/or blow it up. Wouldn't charging it in reverse( positive charge in negative end & vice versa) also damage and/or blow the capacitor up? \$\endgroup\$
    – Kenneth .J
    Jun 25, 2014 at 20:44
  • \$\begingroup\$ @Kenneth.J Yes definitely don't reverse charge a polarized cap. I was referring generally to non-polarized caps. And yes, self-discharge = capacitor leakage. \$\endgroup\$
    – horta
    Jun 25, 2014 at 20:47
  • \$\begingroup\$ The GIF at sparkfun.com (cdn.sparkfun.com/assets/d/2/d/5/1/519a737ece395fe042000002.gif) seems to show electrons flowing from the negative end of the battery into the capacitor plate, creating a negative charge on that side and a positive one on the other. Then when the connection to the battery is cut, electrons seem to move from the positive plate to the diode (LED) and back to the negative plate. Is the GIF completely wrong? \$\endgroup\$
    – user3932000
    Jul 9, 2016 at 3:41
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    \$\begingroup\$ @user3932000 The gif is oversimplified and the yellow flow direction is wrong. Remember that electrons are negative (-). So the electrons flow from the bottom plate of the cap through the diode and back to the top plate. If we define current flow as positive to negative (opposite electron flow), then the yellow line direction is wrong to begin with, but the gif gets it correct on the second half of the animation. Let me know if you need further clarification. \$\endgroup\$
    – horta
    Jul 9, 2016 at 17:40
  • \$\begingroup\$ @horta A more interesting question is what happens if you charge up the capacitor then disconnect the battery: Where does the charge 'go' then? \$\endgroup\$
    – BLAZE
    Aug 22, 2021 at 1:06

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