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Re: Cap AC voltage ratings

Hi Reinhard:

It's encouraging that you were able to AC-operate your PP caps at their
DC voltage rating.  But I'm not so sure though if I agree with your
technique to determine their breaking point.

While a cap's capacitance value is a tightly controlled parameter, it's
breakdown voltage is not.  {numbers that follow are speculative:}
If a manufacturer sells a cap marked 1000V, I suspect that if you bought
a hundred of them and destructively tested them to the point of
breakdown, you might find values from 1500V to 10,000V.  The 1000V is a
guaranteed minimum.  There is no maximum.  If the test string that I used
happened to be built with some of the higher-valued breakdown voltage
devices, I will get a very optimistic "max" voltage rating.  But then
when I build the big array of 200 of these, I will have many of the lower
voltage parts mixed in too, and when these break down, the voltage across
the remaining parts goes up, then they fail too.

I think only time and experience will tell if we really can push these PP
caps to operate so far beyond their published ratings.  If we can, and
the testimonials I've seen so far seem to indicate that we can, then we
should be cautious and realize that that success is only valid for that
particular cap vendor and model, and maybe even date code.  I should be
able to add to the experience base for this in a few days...

Regards, Gary Lau
Waltham, MA USA

>Original Poster: "Reinhard Walter Buchner" <rw.buchner-at-verbund-dot-net>=20
>
>What you shouldn=B4t forget is the fact that a cap manufacturer will
>NEVER publish the max values a cap can take, because he wants to
>keep his (good) reputation. Secondly, the frequency derating is for
>EQUAL cap lifetime (at 50/60Hz). Now you have to make a
>compromise. But on the other hand, will you be running your TC for
>10^5hrs (typical cap lifetime cycle)? :o). I run my MKP caps at DC
>=3D peak AC voltage rating and they survive. There is NO change in
>temperature during a 5-8 min run. I don=B4t use a safety gap or
>equalizing resistors across the caps (at the moment). During tuning
>I had the xformer safety gap ignite many a time, but the caps
>didn=B4t seem to care. I will be adding a 30% safety margin for my
>high power setup. A friend of mine (Stefan Kluge) made an XLS
>spreadsheet for cap derating using manufacturer specs. Real cap
>killers are ultra high bps (>1000bps). His factory (!!) derating
>factor for MKP was 4-5. (For MKT it is ~30!!) However, I found
>this isn=B4t necessary (for MKP caps). Another advantage these
>commercial caps have is their self healing ability.
>
>
>Your best bet is to start out with a string (just add it to your
>existing setup and retune) that has enough safety factor (4-5) built
>in. Run your coil and start shorting out the caps one by one. As soon
>as you reach peak AC =3D DC rating, quit (or they might fail). If they
>fail before that, you now know the max limit your caps can take.
>This will allow you to design a MMC that fits your needs without
>blowing too many. A 30-40% safety margin should be well suited
>for TC use (kickbacks, strikes, etc). You will need to retune the
>coil for every shorted cap of course. Check and note the
>capacitance of each cap before a run. After the run, but before
>going to the next step (shorting an additional cap), re-measure
>the capacitance. If the capacitance starts to change more than
>0.5nF I would start being careful about shorting out more caps.
>(This could save a string from destruction). After you find the
>optimum construction for your MMC measure the total
>capacitance and write it down (include the date). Every now
>and then re-measure your capacitance, to see if it starts
>changing big time. This will let you keep tabs on cap
>deterioration.