[TCML] My last question on caps. I swear. (maybe)
Gary.Lau at hp.com
Tue Jul 8 19:07:09 MDT 2008
The question of AC vs. DC voltage ratings for caps always generates confusion and controversy. You are correct that the caps have an "official" AC voltage rating of typically 300-500VAC. This voltage is called the "corona inception voltage" by the manufacturer. If an AC voltage exceeding that value is applied to a single cap, some corona may be generated within the cap, and this will slowly degrade the dielectric until a failure occurs. Now the manufacturer is aware that industrial users will want to use their caps 24/7 at the worst possible temperature, humidity, and environmental conditions. So the manufacturer is saying that one may run a cap continuously at 500VAC and it will last some very long time that commercial users find acceptable. If a higher AC voltage is applied, then the expected life goes down.
As hobbyists, our needs are much less demanding. We typically use our coils for no more than a few minutes at a time, probably less than a couple of hours total per year. So we typically (and arbitrarily) choose a cap's rated DC voltage to match or exceed the peak AC voltage of our transformer (9000x1.414=12.7KV in your case), and over-voltage capacitor failures in MMC's among hobbyists are practically unheard of. If a coil was intended to be used extensively, say in a museum, then a more conservative rating would be justified. But the cost of an MMC rises exponentially as the voltage rating rises, so the folks who buy and build MMC's will typically believe in using a lower voltage rating than those that sell the caps.
Terry Fritz performed an excellent experiment to determine the approximate expected lifetime of CDE caps exposed to continuous 2000V peak AC voltage. The answer came out to about 75 hours. Full details at http://www.pupman.com/listarchives/2005/Sep/msg00917.html
Regards, Gary Lau
> -----Original Message-----
> From: tesla-bounces at pupman.com [mailto:tesla-bounces at pupman.com] On
> Behalf Of Nicholas J. Goble
> Sent: Tuesday, July 08, 2008 4:04 PM
> To: tesla at pupman.com
> Subject: [TCML] My last question on caps. I swear. (maybe)
> With a 9/30 NST I found my LTR capacitance to be .0124uF. (right?). I
> placed an order for 13 (1 more just in case I need an extra)
> 942C20P15K-F caps. They're rated at 2000VDC and .15uF. If I string
> them together in series, I'll get an MMC at 24000 Volts and .0125uF.
> Two things worry me.
> 1). The website says that it's rated for 2000VDC. I'm not using
> direct current. There's a link to a data sheet for the caps and it
> says that the voltage range is 600-2000 VDC or 300-500 VAC. If I
> series 12 of these together, will I get an MMC rated at 24000 VDC and
> only 6000VAC? Does this mean that I've got to string 48 caps to get an
> MMC that will work at 24000VAC? Or will 12 2000VDC caps work just fine?
> 2). The website shows a picture of the caps. On the side of the caps,
> it says it's rated .15MFD. Why would it say that? I thought
> 942C20P15K-F caps were rated at .15uF. This isn't a huge concern of
> mine, but it's making me worry about my purchase.
> Thanks for putting up with me.
> Nicholas Goble
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