# Re: Cap AC voltage ratings

On Sun, 14 Mar 1999 09:14:06 -0700, you wrote:

>Original Poster: Gary Lau  13-Mar-1999 2038 <lau-at-hdecad.ENET.dec-dot-com>
>
>I have begun making a new cap using strings of 16 Wima FKP-1 .015uF/1600VDC
>polypropylene caps.  The thing that concerns me is voltage ratings.
>While the DC rating seems to be no problem (16 x 1600 = 25.6KV), I'm
>concerned with how much the mfgr derates the DC rating for AC.  My caps
>are AC rated for only 500VAC each (16 x 500 = 8KV).  I believe the two
>reasons for the derating are 1) to allow for the 1.414X RMS to peak
>conversion, and 2) to limit the temperature rise due to ESR dielectric
>heating.  The graphs I have seen show a derating curve, being a function
>of applied frequency, and I would expect this.  The unexpected thing
>however is that the curves do not converge to a simple derating of 1.4X
>at a sufficiently low frequency, but instead, never allow over the rated
>AC voltage, regardless of frequency.
>
>While the DC rating of the caps may not be in danger, the AC rating
>appears to be a problem.  Or is the AC rating that low only if given a
>continuous duty cycle?  While the tank oscillations in TC use occur at
>very low duty cycles, the 60Hz component is continuous.  But then, we
>typically don't run our coils for more than a few minutes at a time.
>
>So, does anyone have a comforting explanation that will make be believe I
>won't be blowing by caps due to violating their AC ratings?
>
>Gary Lau
>Waltham, MA USA

I have a similar question - I just picked up a very cheap (ukp 5) box
of a thousand Philips polyprop caps, 47nF at 250VAC, again the
question is how might the mains frequency rating relate to TC use ? I
was thinking that series strings of 47 caps would give a nice round
1nf per string at 11.75KV - maybe I should try testing some to
destruction...!