Re: Cap Question

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

Original poster: "Christopher Boden by way of Terry Fritz <twftesla-at-qwest-dot-net>" <chrisboden-at-hotmail-dot-com>

A couple light comments interspersed :)

>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.
>

Terry's right, the Group is a 501(c)(3) nonprofit science and technology 
education group, but Dave is also right, I do have a motive for doing all 
the work we do to sell the caps, but it's not money :)
We have, in the thousands of caps we've sold, made less than $220, and 
that's only because most people are kind enough to buy a t-shirt or make a 
donation to the Group when they buy their caps, though it's not required. In 
the beginning we actually lost money becuase shipping ate us up.

The reason we work so hard on the caps is..

1. It helps us maintain a *very* nice relationship with the manufacturer by 
giving them a testbed for new technologies with tons of feedback. And having 
friends like that is a good thing for the Group :) CD *GAVE* us 250 caps for 
the Thumper project...that's 250 2000uF 450VDC High-Quality Electrolytic 
caps.....specificly for us to beat the hell out of them! :) This project 
would have not been possible were it not for the donation since the cost of 
the caps alone (not counting the 500kcmil cables to connect it all) is more 
than our combined budgets for the past 3 years!

2. Check our mission statement.. "to make the subjects of science and 
technology accessable.." that's why we're here. Caps are one of the hardest 
parts of a coil for people to get, they're bloody expensive. We originally 
tried to get caps from Terry back in the early days of MMCs and couldn't 
afford them. So we found a solution, and shared it with a few hundred of our 
closest friends :)

3. It's simple, good PR. Fully 30% of the members in The Geek Group learned 
about us from the Pupman list. Look how many people on here are sporting 
GeekIDs in their .sig files, there's a reason for that. The Group was 
practicly born here. I've been a member of Pupman since the Group had only 2 
members. Even our wonderful moderator Terry is a member, and has been for 
years.

> >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 :-))
>

We've had only 3 caps fail, period. One in testing (Terry did it on 
purpose), and 2 in a MMC of a newbie coiler who had his design *almost* 
right :) And he's getting a pair of replacement caps in the next shipment, 
on the house.


>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".
>

For the Avalon coil we're planning on a MMC type design (albeit with a 
little bit larger caps:) ) because, unlike the 13M the Avalon coil is 
designed to be wildly flexible in setup and use and MMCs are the ONLY way to 
do that.

> >
> >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 :-))

That's the big one right there. I have a growing collection of pulse caps 
all with one thing in common...they died in Coiling duty. I saw the look on 
Jim's face when his $$$ Maxwell went up in a flash at our Summer 2001 
teslathon.....this simply doesn't happen with the MMCs.

>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-
> >
>
>

Christopher "Duck" Boden Geek#1
President / C.E.O. / Alpha Geek
The Geek Group
www.thegeekgroup-dot-org
Because the Geek shall inherit the Earth!