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Re: "Enhanced" Multi Mini Capacitors Part 2

Hi Reinhard,

Great to have you back!!  I have been busy as you can tell :-)

I will hit an a few points for your posts...

WIMAs derating system is rather odd.  The RMS current is what really
matters.  In my opinion they should give the RMS current rating at various
frequencies and be done with it.  I assume all cap makers found some way to
list deratings that their customers at the time liked...

I don't think Dv/Dt is an issue for us.  I think we are a few orders of
magnitude below the ratings of "good" caps.  At high pulse levels (like
when you short the leads of a fully charged cap) the internal connections
in the cap can get shocked mechanically and degrade over time.  We should
not reach that level.

Cap makers come up with their ratings doing just the some thing I am.  They
do a bunch of tests and then draw lines.  If you have five people they will
all draw five different lines depending on what they each think is best.
This is why you see variation from day to day in cap specs.  They are
guessing a bit...  Surprisingly, they count on customer feedback as to how
the parts perform under stress more than anything.  I know WIMA has learned
much from my company ;-)

You ALWAYS want foil caps with those big solid (looks like solder) ends
inside.  You can get WIMAs and others with big strap leads now too.
Surprisingly, the heat from those thin leads is not insignificant...

The size of the cap indicates how much heat it can dissipate.  That is why
they rate dissipation only on size.  I think Panasonic has a bit of an
advantage because their thin epoxy covering does not hold heat in like
WIMAs thick potted caps do.  I would think if a cap got too big it would
not be able to pull heat out of the center well.  Commercial poly caps I
have seen my have a problem there??  Of course, MMCs have far greater
dissipation do to their many little radiating surfaces.

The peak voltage rating of my resistors is a concern.  I'll check into that
more especially now that I think I may run them in air.  I know they arc
along the outside but without oil there may be a concern there.  I will
never be convinced not to use them.  I have seen to many problems with
series caps that were solved by adding them.  Another coiler found that
residual charges on his MMC without resistors was "shockingly" ;-) high
despite the fact he shorted the ends.  Of course, with only ten or so caps.
it is not a big deal to use them.

I would not recommend parallel connecting the caps since it will defeat any
self healing the caps may do as discussed in my other post today.

Of course, I now must wonder if my 28nF LTR cap could be made from 8 caps
instead of 50 hmmmmmmmm :=)  The actual coil is far easier service than any
of my torture testing...  I would always have about a hundred spares :-))
20kV 28nF $16 bucks!!!  Fun to try at least...  Perhaps I could "blow them"
up to 6kV (self healing) and them if they don't get too hot or I don't run
them too long...  I could carry it in my shirt pocket too :-)  On reason to
used drain resistors...  Imagine how deadly such a little thing could be!!!
 Almost 6 Joules this size of a pack of cigarettes!

We now probably know more than the cap manufactures about their parts in
some ways!!!  They are wonderful in that we can experiment, test, and
define how to use them.  The oil filled caps of "last year" never were good
for a lot of destructive testing.  We should really thank the manufactures
of these wonderful caps whose expertise has made all this possible.  This
is one area were modern technology has really had an impact.  Anyone who
has ever made their own cap, should cut across one of these things with a
knife and look at in through a magnifier.  

I posted a cross section of a WIMA FKP1 0.1uF 630VDC cap at the link below.
 You can just barely get a hint as to how find the wraps are (300DPI
scanner).  You can't see the individual layers without a magnifier...

	www.peakpeak-dot-com/~terryf/tesla/misc/wimacut.jpg

Glad your back Reinhard!

	Terry