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Re: Cap AC voltage ratings
Hi Gary,
>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.
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 havenīt 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 (á 15 caps each) would be too much work.
>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 donīt think
you can find a commercial pulse cap (as a single unit) that cheap. A
rolled PE cap isnīt 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((.
> 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.
All the best for you, your coil, and your newborn MMC.
As soon as I get around to fiddling with my coil again, I will continue
to post results (good or bad) as they turn up.
Coiler greets from germany,
Reinhard