# Charging a capacitor with AC voltage

Discussion in 'General Electronics Chat' started by p75213, May 27, 2012.

1. ### p75213 Thread Starter Active Member

May 24, 2011
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Can somebody explain how it is possible to charge a capacitor using ac voltage? The way I see it is the capacitor wouldn't have time to charge as the voltage is constantly switching. In the case of 60Hz - 120 times a second.
The attachment shows a tesla coil where the capacitor is in series with a primary coil and the source is a transformer. A spark gap is in parallel with the source and capacitor/transformer. I can't see how the capacitor will charge.

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2. ### #12 Expert

Nov 30, 2010
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That capacitor has no intention of charging to a DC voltage. It is designed to work in conjunction with the coil at an AC frequency.

3. ### crutschow Expert

Mar 14, 2008
21,757
6,271
A capacitor can readily charge and discharge to the AC voltage level. High frequency type capacitors with low parasitic inductance can charge and discharge at up to GHz frequencies.

A capacitor can theoretically charge and discharge instantaneously. It's the resistive and inductive stray values in series with the cap that limit its frequency response.

4. ### p75213 Thread Starter Active Member

May 24, 2011
70
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Thanks for that crutschow.
I never realized they could charge and discharge so quickly. Given that i=Cdv/dt the voltage change would have to be less than instantaneous otherwise you would end up with infinite current.

5. ### crutschow Expert

Mar 14, 2008
21,757
6,271
Of course an instantaneous change of voltage on a capacitor would require an infinite current. But with a low impedance in series with the cap, the charge and discharge can be very fast.