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Re: [TCML] homemade pulse cap Now MMC cap
Hi Bert,
Sorry for my greatly delayed response to this, been busy out of my
mind... Anyway for my next big project I did order MMC caps tonight,
cost more than a pretty penny (40kV 50nF worth.) I bought the CD940
series, the 2kV .1uf kind. I know these are on the naughty list, due to
being just film, as opposed to foil/film, but considering the cost
difference to the CD942 series I think it should be good because it was,
I think, very over engineered. I guess my question is this, as long as
I am well below the pulse and RMS current ratings (which I very much am)
per string I should be Ok yes? I plugged my values into Java TC and got
"good" on standoff voltage (using a 14400 piggie.) In the eyes of the
MMC people, do these values look Ok?
11 strings of 21 caps, 231 caps in total
per cap
V=2kV
per cap C=.1uF
DV/DT=1712
Peak amps=171
RMS Amps=8.3
So the whole array
V=42kV
DV/DT=I have no idea, as I'm not sure what this is or why it is important...
Peak amps=1881
RMS Amps=91.3
Now knowing this and using a properly sized cap, my expected transformer
and break rate specs, and approximate values for primary inductances I
get the following requirements for my cap bank...
V 14400/.707=a shade over 20kV (so I have just over double what I need)
Peak amps 394 (so I have 4.7 times what I need)
RMS current 41.64 amps (so double on this one.)
It looks to me like it should be good, the only thing I worry about is I
don't know is the specs account for the high frequency at which these
will be used (about 60 kHz)... Anyway just want to know if this looks
Ok, or if I should expect flame-age... Thanks!
Scott Bogard.
On 7/23/2012 11:19 PM, Bert Hickman wrote:
Hi Scott,
I didn't see any replies to your question, so I'll take a stab at it...
The strategy of using two (or more) thinner layers of dielectric
between plates has historically been used for decades by commercial
and homemade capacitor builders for many of the reasons you mentioned.
Multiple dielectric layers are not inherently self-healing. A defect
in one dielectric layer is protected by the remaining good layer(s),
making the cap considerably more robust than one made using a single
layer. However, if the remaining layer(s) are punctured, the entire
capacitor catastrophically fails.
Problems of corona in voids between dielectric layers, or between
metal and dielectric layers, were initially resolved by inserting a
thin Kraft paper layer in film-foil capacitors. The Kraft paper layer
was saturated with a dielectric fluid having a comparatively high
dielectric constant (~3 - 7). Capacitors using this approach were
sometimes called "soggy film" caps.
Most commercial capacitor manufacturers eventually migrated to a "hazy
film" dielectric where one or both surfaces were mechanically
roughened. The roughened surfaces were more easily wetted by
dielectric fluid, and hazy film eventually eliminated the need for
Kraft paper in all but the toughest energy-discharge applications.
Gaps between the foil plates and dielectric were eliminated by
evaporating a thin layer of metal directly onto one or both surfaces
of the dielectric. And, by carefully controlling metallization
thickness (and sometimes adding thin-film fusible isolating links),
dielectric failures could be isolated hundreds, or even thousands of
times, resulting in graceful degradation rather than sudden death.
"Self-healing" technology also eliminated the need for multiple
dielectric layers per capacitor element, and permitted manufacturers
to more highly stress the dielectric system. This revolutionized HV
film capacitor technology, allowing manufacturers to build caps with
much higher energy-density.
It turns out that your idea of stacking a series of "floating"
conductive and dielectric layers is actually being used (albeit in a
slightly different configuration) in the very snubber caps that most
folks use for MMC's. CDC effectively stacks two (or more) identical
single-layer self-healing capacitor elements in series (using a clever
metallization pattern and proprietary construction techniques) to
create reliable "dry" film HV capacitors.
Conceptually, your idea could work. However, almost nothing "sticks"
very well to polyethylene (PE) or polypropylene (PP) film... unless
the film has previously been corona-treated. Simply painting an
adherent conductive layer onto untreated PP or PE film will not work.
Your approach might work if you can locate some metalized PP capacitor
film for a reasonable price. Metalized PP film does appear every now
and then on the surplus market, but it's usually not cheap. You would
also need to make an appropriate multilayer winder and may also need
to construct a filtered clean air work environment. End termination
could also be a problem unless you use foil for inner and outermost
layers.
I also suspect you'll find that MMC caps are really not all that
expensive in comparison... :^)
Good luck,
Bert
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