Re: Cap AC voltage ratings

>Original Poster: "Reinhard Walter Buchner" <rw.buchner-at-verbund-dot-net>

>>I'm not sure that if a capacitor survives a one minute test at a
>>particular voltage, that it is safe to assume it will endure that stress
>>for any other length of time.  The vendor data that I've seen
>>described the AC voltage rating as being something that will not allow
>>ionization to occur.  I suspect that if that did happen, it would not
>>result in instant failure but rather a slow degradation of the
>>dielectric.  So, I fear that while a string may survive the initial test,
>>it may slowly degrade to an eventual dielectric failure. - Gary Lau

>Sure, ANY corona (partial discharge mail) will degrade the dielectric
>and should be avoided as much as possible. However, a degradation
>of the dielectric should (or am I making a mistake, here?) show up as
>a loss of capacitance. This is why I am keeping tabs on my
>capacitance (each cap, string and total) during the testing phase.
>Up to now (total accumulated run time ~1hr.), I haven=B4t noticed any
>change greater than +/- a few (<20) pico Farads. Of course, in the
>end, I will only be able to keep tabs on the total capacitance.
>Seperating 26 strings (=E1 15 caps each) would be too much work.

I'm not sure that one can predict an impending dielectric failure or
degradation by monitoring the capacitance.  With my failed rolled poly
cap, the only area affected by the dielectric etching was at the
perimeter of the foil plates, an area neglegible in comparison to the
total dielectric area.  And when it did fail, there was no reduction in
performance immediately prior to the failure to suggest that the
capacitance changed and the tank went out of tune.

>>Further, the message in the Wima data sheet was clear - dielectric
>>strength is reduced from it's DC-rating as a direct function of operating
>>frequency.  I fear that the success we've had is just due to the generous
>>overdesign factors that responsible vendors use.  Plus using these
>>devices for much shorter durations and total operating hours than
>>conventional applications require.

>The DC --> AC derating factor is for EQUAL lifetime.  Keeping run times
>short should (I hope) keep the degradation of the dielectric down to a
>minimum. We will have to live with the fact that a pulse cap will ALWAYS
>have a limited lifetime. On the other hand, I paid about $100 for all my
>capacitors and this will allow me to build a 120nF cap. I don=B4t think
>you can find a commercial pulse cap (as a single unit) that cheap. A
>rolled PE cap isn=B4t much cheaper to build, either. Plus the MMC has the
>advantage, that even if you blow 50% of the caps, you still have 50%
>left over that you can re-use. Once a rolled PE cap has gone south, the
>only thing you can do is dump it in the trash bin :o((.

Agreed in full, though I'm not sure what you mean by:
"The DC --> AC derating factor is for EQUAL lifetime."
And it sounds like you found a terrific deal on your caps!  I spent well
over $100 for my caps, ending up with just 10nF -at-25.6KVDC.

>> I've also been feverishly working to complete my own
>>MMC.  Last night was it's first test run for a total of about 4 minutes.
>>So far, so good.  I want to log some more time on it before I declare
>>success, and then document it on my web site.  After the 4 minute run
>>it was just slightly warm, but only just slightly, much less than my
>>rolled poly or Fair Radio Sales caps.

>Well, that is good news. I hope your results stay positive! What would
>be of interest to me is a comparison of spark lengths. MMC vs. Fair vs.
>rolled poly. I found that the MMC gives me longer and hotter discharges
>(in comparison to a rolled PE cap). As a matter of fact adding (and
>retuning) my PE cap to the setup actually decreases the spark length.

I've not seen any difference in performance between my Fair Radio Sales
commercial cap, my (failed) extended-foil rolled poly cap, and my new MMC
PP cap.  They are all the same capacitance value and the only difference
appears to be how warm they get.  

Could it be that adding your PE cap, while maintaining your tank
resonance, has altered the mains-resonance point of your power supply
transformer to a less-than-optimal point, and you are just not pulling
as much power out of it?

Regards, Gary Lau
Waltham, MA USA