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Re: Geek .15uF caps Was: "plate" capacitors
Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" <uncadoc-at-juno-dot-com>
Hi David, Terry; All, So in other words, you are saying the MMC's will
NOT handle long duration current usage in a Tesla unless you seriously
under-rate them for current? They start to fail, first just one or two of
the caps, then several more; then a whole bank breaks down after five
minutes Therefore, you must double, triple, or quadruple strings of the
MMC's just to borderline handle a long sustained run of a few minutes or
more? Gee, sounds awful expensive to have a MMC bank that will give more
than a few minutes action. And then without swelling and self healing(how
long can these things self heal without permanant damage?) , lets be
realistic; WORD IS THAT THE MMC's can handle hundreds, if not thousands
of over voltage punches. Well, do you folks realize that you are getting
hundreds of multiple punctures in those MMC caps every time you fire your
coil? Sure now, they self heal, all well and good; but how many times
can the skinny little cap string do this? And for how long of a period?
Well, I am waiting for answers from the one minute run or less MMC
crowd! And please do not tell me that nobody needs to run their coil
for more than a minute or two for maximum scare of the multitudes. We
are not trying to scare the masses. We simply want to develop a better
system of power for all the people. Do you get it now???? Al.
On Fri, 20 Apr 2001 12:28:39 -0600 "Tesla list" <tesla-at-pupman-dot-com>
writes:
> Original poster: "Terry Fritz" <twftesla-at-uswest-dot-net>
>
> Hi David,
>
> Just for the record I'll give the long answer to you short
> question :-)
>
> Metal foil/film caps can self heal after a short over voltage spike
> which
> is a tremendous benefit to us. They can absorb thousands and
> probably
> hundreds of thousands of sort over voltage punches. It appears the
> only
> way to kill them with voltage is to get the voltage so high that
> they
> constantly arc internally and burn up. That is at about 3X their DC
> voltage rating. Hard to do in our case unless you really try. The
> transformer or something else will blow up first.
>
> Current is our main worry and it comes in two flavors dV/dT and RMS.
>
> The first concern is that the cap must handle short ~1% duty cycle
> bursts
> at hundreds of amps at say 120BPS. Even though there is not much
> heating,
> those giant spikes will easily blow poor connections inside the cap.
> That
> is why we want caps that have a high dV/dT rating. The problem area
> is
> were the leads attach to the end plates (easy to fix) and where the
> plates
> attached to the end plates (not so easy). Cheaper film caps use
> evaporated
> film for the electrode plates which have a very poor attach that
> will fail
> under very high current. The few millionths inch thick metal film
> on
> plastic is just hard to connect to. The foil/film caps have solid
> metal
> foil for the end electrodes that is easy to connect to and will not
> easily
> burn up. That is why we always want "metal/film" high dV/dT types.
>
> The RMS current is the "real" limit. All caps have some loss which
> is in
> effect like an internal resistor. Polypropylene has the lowest loss
> for
> common capacitor materials at about 5X better than anything else at
> TC
> frequencies. Thus, they run 5X cooler. In Tesla coils, the RMS
> currents
> can be very high and we push it to the limit since caps are
> expensive and
> we want to run as much current as we safely can through the minimum
> number
> of cap strings. The power lost to heating the cap is simply
> proportional
> to the RMS current squared as in a simple resistor.
>
> Polypropylene is a soft low temperature plastic that does not like
> heat at
> all. 85C is the limit although they play a few games to get that as
> high
> as 105C. One should not run them with the outside temperature
> getting much
> over 10C above the air temperature. Even thought the cap is say 40C
> on the
> outside, the poly film is a very good insulator so the heat
> generated in
> the center gets traps and the middle may be cooking at 110C where it
> melts
> and fails. So the thermal design of the things could stand a ton of
> improvement to even out the temperature. The metal foil plates help
> draw
> heat out a little but it could be far far better. "i" would make
> them
> hollow so the cap is more of a tube. Then the heat would not build
> up in
> the center and I think you could get far higher currents through
> them
> without overheating. Most of the capacitance is in the outer layers
> anyway
> so making them hollow would not reduce the capacitance/volume that
> much. I
> would not be surprised if you could get 4X the current through one.
> Maybe
> Chris could mention this to his pals at CD ;-)) You need fancy
> stuff to
> make poly film/foil caps so it is not a home project...
>
> Unlike tcp/ip DNS server network stuff, I do know a bunch about poly
> caps
> :-)) The hollow cap idea seems to me like a really promising thing
> to try.
> Maybe it could be patented buy some big cap manufacturer and make
> them
> rich. Of course, we could use them too. If it were a big deal
> idea, I
> would not care about the patents and all that crap. I just want the
> caps
> for my coil :-))
>
> Cheers,
>
> Terry
>
>
> At 08:59 AM 4/20/2001 -0500, you wrote:
> >> Original poster: "Terry Fritz" <twftesla-at-uswest-dot-net>
> >> Personally, I would go by RMS current vs. center temperature
> rise. A
> >> hollow capacitor, sort of like a tube, should have much higher
> dissipation
> >> since the highly thermally insulated center is removed
> >
> >Is the power handling primarily thermally limited?
> >
> >DK
> >
>
>
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