Re: Cap Question

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

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

Hi Dave,

At 04:28 PM 4/3/2002 +0000, you wrote:
>>Some are, yes. Doorknobs tend to drift in value as they heat up, and >also 
>>crack. They don't tolerate overvoltage too terribly well either. >The big 
>>metal cans are usually industrial PFC caps and unsuitable for >coiling (you 
>>need HV Pulse caps). Even if the cap is of the right type >and voltage, in 
>>coiling (where overvoltage of the cap is quite common, >especially among 
>>newbie coilers) those nice big metal pulse caps >almost universally become 
>>nice big metal doorstops in what is never a >long enough lifespan :) The 
>>cap could last you for 5 years, or 5 >minutes, you have no way of knowing.
>
>You seem to be almost evangelical about MMC caps (surely not a commercial 
>motive at work... ;-))  

The Geek Group sells MMC caps at cost ;-))  They are non profit.

>Sure they're cheap and offer decent performance, but 
>for lifetime and that last few % efficiency a properly specified HV pulse 
>cap cannot be beat.  You say a big pulse cap may fail 'at any moment'.  You 
>are wrong.  If properly specified they will (literally) last a lifetime.  
>Sure, if you under-rate the voltage and use any old surplus junk that 
>happens to be lying around in the local junk yard or surplus store, then 
>they may well blow up.  But, properly specified, high current RF rated 
>polypropolene dielectric EFO type pulse caps are the highest performance and 
>most reliable TC tank caps.  

Three factors affect performance, ESR, dissipation factor, and series
inductance.  Pulse caps do have a technical advantage in all these, but the
losses in either are so low that they are negligible.  In a Tesla coil, the
other primary circuit losses are far greater than the losses in a good poly
cap.  Modern cap losses simply are too low to matter.

I would say that if an MMC and a pulse cap are both equally derated that
they will have the same reliability in that they will both last "forever".
Statistically, MMCs should be "better" since they have all the big mass
production controls behind their manufacture where pulse caps tend to be
made "custom".  Unusual failures of pulse caps have been traced to
variation in quality that is expected with a very low volume product with
"hands on" assembly.  So if you make a million Tesla coils and run a big
study.  MMCs will win due to very good manufacturing controls.  The low
volume caps will show all kinds of odd problems.  Of the 5000 Panasonic
caps I sold, only one "seems" to have failed for no good reason.  It was
cheerfully replaced for free :-))

MMCs can be very easily adapted to one's coil requirements and you can keep
them on the shelf and quickly use them for most any coil depending on how
your string them up.  They can also be made adjustable.  If one does not
have a lot of money, you can overdrive them with a fairly predictable
reduction in reliability which is often acceptable to coilers.  For small
coils, MMCs are far cheaper.  There is a point at the mid size pig level
that pulse caps start to be cheaper than giant arrays of MMCs.  Then, big
pulse caps rule!  MMCs did inspire some nice equations and such for
predicting RMS currents which can be used to specify the right pulse cap
too.  If your building a coil like Bill's giant model 13, an MMC the size
of a truck is totally impractical,  Pulse caps are the only option "up there".

>
>To take a prominent example, Ed Wingate has 
>been firing CP tank caps for over 10 years without a single failiure, 
>despite having hit them with massive break rates and many, many hours of 
>runtime.

MMCs were inspired by some nice new pulse caps exploding all over Chip's
roof...  "Testimonials" are there for either case :-))  Chip's caps had
poor internal connections between the "packs".  A manufacturing error...

>
>>The MMC cap is the current state of the art in coiling. Stable value, 
>>tolerates repeated overvoltage, and completely variable to be as >unique as 
>>your coil (for such a simple things, I have yet to see two >duplicate 
>>coils, they're like a fingerprint). They also have the nice >benifit of 
>>being inexpensive. $3 get's you .15uF at 2kV, and we sell >them to coilers 
>>by the thousands.
>
>The EFO polypropolene pulse cap is the state of the art, and has been for 
>the past 30 years.  The mmc, thanks to its self healing capabilities, 
>achieves a useful cost/runtime compromise unachieveable with conventional 
>pulse caps.  In layman's terms - they cost less and die sooner.  So for the 
>casual coiler they are the best choice, however that doesn't mean they're 
>the best cap.

It may not be terribly hard to perhaps build self healing into pulse
caps...  It may increase cost and increase ESR/loss some.  Maybe need a
special oil too.  Probably not a big deal since pulse caps should be
operated "properly" :-))  One big killer of caps is over voltage.  Coilers
have a BIG habit of putting over voltage on things :-))  MMCs have the big
advantage of being able to take such abuse, to a point.  MMCs also can
withstand well over their rated voltage just as a matter of fact making
them even more robust when in the hands of coilers ;-))

The fact that Chris and crew can easily get us MMC caps by the thousands is
also super nice!!  Pulse caps are hard for the average fellow to find.

Of course, all these wonderful caps are inspired by the great achievements
of the capacitor engineers around the world.  Salt water, glass plate, poly
plate, rolled poly...  Have all come and gone.  It is nice we are
discussing which is "best" and only finding minor differences rather then
just trying to find something that works...  Between Pulse caps and MMCs
there really is no "best" cap, they are just different and each has its own
place.  It is interesting that even nice saltwater caps have come back into
being as a super cheap alternative.  I think the glass plates and rolled
caps are only a curiosity now, but the rolled caps could still be of use
for cheap caps in a high powered system...  So many choices :-))))

>
>-Dave-
>