Re: Cap Dissipation Factor, Plasic Capacitors for TC tank

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Sean,

Check out WIMA's neat new info site at:

http://www.wima-dot-com/navig/tech.htm

"I" could "argue" with a few things here, but they got it "mostly" right
:o)))))

Dissipation factor is critical in Tesla coil caps.  It simply is the
determining factor if our caps will burn up, or not...  Basically, it is a
resistance, combined with the primary circuit's RMS current, that
determines how much heat is generated in the cap.  A given cap can
dissipate a certain amount of heat...  A few equations later... we can
determine if the capacitor's poly layers see more than about 85 degrees C,
where they melt (a "bad" thing!!).

Very recently, the cap makers have come up with a new "trick" that extends
the melting point of polypropylene.  It must be a deep secret since "I"
have not figured it out yet :-))  But caps that used to be rated to 85C are
now rated to 100+C.  They may just be "feeling lucky", but I think someone
figure some new trick out...

MMC caps have a giant advantage in that they are cheap and easy to test
under "deadly" conditions to determine exactly how "much they can take".
Expensive commercial caps can't be tested with such wild abandon and we
have to trust the typically "scarce" information the manufactures give us.

But you really want to know if "your" cap will work...  The size is ok if
it is polypropylene and with heavy edge connected foil electrodes.  Sort of
rare unless they were thinking toward IGBT snubber caps or something like
that.  If it is rated for say "15 amps RMS at 200kHz", then you are set.

Many problems with caps blowing up are related to "us" not knowing critical
parameters the caps need to meet.  Voltage is actually a far far lesser
concern than RMS current and peak current ratings.  Voltage is easy, RMS
current is usually taken care of by using polypropylene.  Peak current
needs heavy foil electrodes and heavy end plate construction.  Now-a-days,
we actually now know enough about Tesla coils to know what "specs" caps
have to meet.  When we know our needs and the cap makers tell us the
details of their caps, we just "design" it to work fine.  Typically, we are
the "worst" users in that our specs are far higher than almost all
commercial needs.  25kV, 15 amps RMS, 1500 amps peak, 120 pulses of 3% duty
cycle per second at 250kHz, $35.00...  Those are the details that are
needed to make a cap that "fits".  Of course, these are some of the most
stringent requirements out there.  The cap makers think "Yips!!  What are
you doing!!  Making a Tesla coil or something :-))))  Of course, MMC's were
born from two commercial caps blowing their guts out on a Tesla coiler's
roof at Halloween...  Right away, you know, we are not "normal" folks
%:o)))  We also know a few tricks, like we can us DC ratings for "our" AC
circuits ;-))  In cases like that, we know more about caps than their
makers ;-))  And the makers listen real close...  Hehehe 

Cheers,

	Terry


At 11:09 PM 7/29/2002 -0700, you wrote:
>What exactly is the cap dissipation factor - i.e. is it just a
>representative figure used to tell how much a cap will dissipate? How does
>frwquency figure into it, and how do you measure the dissipation of a cap?
>I ask because a) I am curious, and b) I have a Plastic Capacitors 0.05 uF,
>20 KVDC cap and want to know it's suitability for TC tank use.  I doubt that
>it is any good for a TC tank because the thing is so tiny, well, about 2" x
>3" x 4" without insulators.  Maybe a mylar dielectric?  Any thoughts greatly
>appreciated!
>Sean Taylor
>