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Re: Caps DC to AC rating



Original poster: "Terry Fritz" <twftesla-at-uswest-dot-net>

Hi Kevin,

At 08:58 PM 1/17/2001 +0000, you wrote:
>In Terry’s MMC calculator, I am asked for a caps AC voltage rating. Given 
>the caps DC rating, how do I get the AC rating? 

The program askes for the DC voltage of each individual capacitor.  This
value is used as the maximum peak voltage on each cap.

If one has say 10000 VAC the peak is 1.414 times that or 14140 volts.

Vpeak = SQRT(2) x Vac     <--- Basic important equation

So if you have a 15kV transformer and 1600 volt DC caps, the peak voltage is:

15000 x 1.414 = 21213 volts

The number of caps needed is 21213 / 1600 = 13.26 caps.

Note that many people run the caps at a bit higher voltage to save money
since the caps don't seem to mind.

The caps are actually rated around 630 volts AC for standard poly caps but
that rating is for full time AC use which really does not apply to us.  We
just worry about the peak voltage where they will start to break down.

I hope that is what you were asking.

Cheers,

	Terry

>I found a few references to 
>this question in the archive. From what I could understand the AC rating 
>will be somewhat lower than the DC rating. And the caps actual AC voltage 
>rating is inversely proportional to frequency due to partial discharge and 
>properties intrinsic of the dielectric. But I couldn’t find a definite 
>equation. If anyone has an equation or at least a general guide, please post 
>it. Thanks, Kevin, thebiggiantkevin-at-hotmail-dot-com
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