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S. P. Prasad

New Member
I would like to know what is a Capacitor ? Can it be utilised in Both AC and DC circuits ? If not why ? What is the total time taken to charge and discharge a Capacitor?


A capacitor is a device which stores energy, in the form of an electric charge. It is made of two sets of metal plates, separated by a thin insulator. It does have applications in both AC and DC circuits.

When a capacitor charges, one plate becomes more positive, and the other, more negative. It can retain this charge, even if removed from a circuit. For this reason, you should always discharge a capacitor before handling it, or you can get a nasty shock! (Although a resistor is safer for discharging, most electricians prefer a screwdriver :lol: )

The golden rule of understanding what a capacitor does, is that it OPPOSES a change in VOLTAGE. It does this by sinking or sourcing CURRENT. If you want to get mathematical, the current into/out of the capacitor is proportional to the rate of change in voltage, across the capacitor:

Ic = d(Vc)/dT (simplified, without constant multipliers)

In DC circuits, it is typically used for filtering, to maintain a (fairly) constant load voltage. Also used for timing, as the rate of charge/discharge is predictable (within tolerances).

In AC circuits, a capacitor is a load, but the current through the capacitor has a 90 degree phase shift, with respect to the voltage, ie, if:
Vc = sin t then Ic = cos t (where t is time)

This correlates with the first equation, ie:
d(sin t)/dt = cos t

In large electrical installations (factories etc) capacitors are used for power factor correction (if you want more info on power factor, let us know)

How long does it take to charge a capacitor? This is depends on a few variables:
1) The resistance in series with the capacitor
2) The charging voltage
3) The value of capacitance
4) The voltage you actually want to charge to! :wink:
I assume that you mean the time in a RC series circuit, with constant voltage. See this post: http://www.electro-tech-online.com/threads/flashing-leds-resistor-or-not-resistor.856/

Hope this helps a bit.
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