# Understanding capacitor charging

**URL:** <https://boards.straightdope.com/t/understanding-capacitor-charging/169261>\
**Category:** Factual Questions\
**Created:** [April 17, 2003, 6:18am UTC](https://boards.straightdope.com/t/understanding-capacitor-charging/169261 "2003-04-17T06:18:44Z")\
**Posts on this page:** 5\
**Page:** 2

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**Author:** ![Trigonal\_Planar](https://avatars.discourse-cdn.com/v4/letter/t/0ea827/32.png) [@Trigonal\_Planar](https://boards.straightdope.com/u/Trigonal_Planar)\
**Post date:** [April 18, 2003, 5:53am UTC](https://boards.straightdope.com/t/understanding-capacitor-charging/169261/21 "2003-04-18T05:53:58Z")

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Well, they can be charged with AC, as that is how a tesla coil works. The key is positioning the spark gap distance such that it will break down when the maximum voltage of the AC cycle is achieved across the caps.

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**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\
**Post date:** [April 18, 2003, 10:00am UTC](https://boards.straightdope.com/t/understanding-capacitor-charging/169261/22 "2003-04-18T10:00:35Z")

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> [@](#):
>
> \*Originally posted by Trigonal Planar \*  
> **Well, they can be charged with AC, as that is how a tesla coil works.**

[Being Facetious] If your capacitors can store an AC voltage, then you better file for a patent [/Being Facetious].

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**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\
**Post date:** [April 18, 2003, 11:19am UTC](https://boards.straightdope.com/t/understanding-capacitor-charging/169261/23 "2003-04-18T11:19:12Z")

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**Crafter\_Man** , **Trigonal\_Planar** hasn’t described the circuit of the Tesla coil properly.

It’s a resonant circuit, with the Tesla coil’s primary in series with the capacitor. The capacitor charges within a single half-cycle of the 60 Hz driving voltage. Once the breakdown voltage of the spark-gap is reached, a much higher frequency oscillation begins.

See schematic [here.](http://www.kronjaeger.com/hv/hv/src/tesla/)

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**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\
**Post date:** [April 18, 2003, 12:22pm UTC](https://boards.straightdope.com/t/understanding-capacitor-charging/169261/24 "2003-04-18T12:22:16Z")

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Thanks Desmostylus. Trigonal seemed to be implying that a capacitor can “store an AC voltage,” which is obviously not true. But as you noted, a capacitor _can_ store instantaneous energy at instantaneous voltages in a sine wave…

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**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\
**Post date:** [April 18, 2003, 12:39pm UTC](https://boards.straightdope.com/t/understanding-capacitor-charging/169261/25 "2003-04-18T12:39:27Z")

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You could have problems, **Trigonal\_Planar**.

Some caps are rated for a.c. voltages, some for d.c.

If you’ve got a 22 kV a.c. rating, you’re probably O.K.

If you’ve got a 22 kV d.c. rating, you’re pushing it. The 15 kV rms rated transformer has a peak voltage of 15 kV x √2 = 21 kV. The transformer output voltage could well have a tolerance of ±20%, meaning an actual peak voltage of up to 25 kV.

On preview:

No worries, **Crafter\_Man**.

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