Re: MMC or Maxwell? Which is better?

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

Original poster: Jim Lux <jimlux-at-earthlink-dot-net> 

At 04:11 PM 2/27/2004 -0700, you wrote:
>Original poster: "Crow Leader" <tesla-at-lists.symmetric-dot-net>
>Here is my main point.
>
> >
> > One other thing about MMCs ;-))  They were designed from the ground up for
> > "Tesla coils".  So all the fudge factors and over design was directed
> > toward the common mistakes and mishaps "coilers" make.
>
>Wrong. I don't see "designed for tesla coil use" in the cornell dublier
>catalog anywhere.


CDE doesn't sell MMCs for TC use, true enough, but they do sell 
polypropylene capacitors that can be used as a component in an assembly 
(i.e. the MMC). The assembly is what has been designed for TC use.  The CDE 
catalog gives RMS current ratings, etc.  That, along with some 
qualification testing, is used in the TC appropriate design for an MMC.


> > The huge over voltage and self healing features are the biggest advantage
> > to help MMCs survive those resonant rise events that have killed so many
> > nice commercial caps (Terry filters save the NSTs ;-))
> >
> > The next big advantage are the simple charts for all kinds of NST coils so
> > folks can get exactly what they need (minimal cost) without having to
>worry
> > about RMS current, voltage rating, and all kinds of nasty details that
>also
> > kill commercial caps.  All the hard work is done.  One just follows the
>
>These factors will kill any cap. Unless cornell dublier is some backwoods
>garage operation, their prodcuts are "commercial" as well. Worrying about
>currents seems valid in electrical projects. Why not ignore voltages too,
>and grounding and all that stuff.

The commercial cap vendors do not provide "applications notes" or 
"recommended usage" for tesla coils (there not being a huge market for 
them). They do provide ap notes for other applications (pulse discharge, 
high ripple switching power supplies, etc.).  Most people building 
something like to have (at least for the first go around) a fairly cookbook 
application note.  I'd venture to say that most people's first opamp 
circuit was a completely clean sheet design from scratch, but was modified 
from some handbook or apnote.

The web charts for MMCs fall in the same category.  For a NST of a 
particular size, you can figure out a fairly good first starting point that 
has a high probability of success.   As one gains experience, or as one 
decides to delve into the component optimization, one can move from the 
apnote recommendation.



> > charts and gets the caps and they will work...  When I get Tesla coil caps
> > nowadays, shipping costs and delivery times are the only worry...  I
>"know"
> > they will work perfectly fine once I get them...
> >
> > Of course, a commercial cap that is run on a Tesla coil within it's
> > voltage, current, etc. ratings will be fine aside from that defective cap
> > thing years ago...  But aside from the aluminum and plastic, there is also
> > a lot of testing and "work" that has gone into making MMCs "work" for
> > coilers.  In fact, I have never really thought about using them for
> > anything else!  Commercial caps make "you" do all the work, where with
> > MMCs, all the work is already done.
>
>If figuring out what specs you need is too much work, one needs to drop any
>sort of electrical hobby anyways. Even bowling as a sport requires some
>math.


It's a matter of which specs do you want to agonize over and which ones do 
you want to just take what's given.  I doubt anyone goes out and calculates 
the optimum diameter of their hookup wire based on circuit analysis derived 
currents, and then orders AWG 13-1/2.  No, you just use the "ampacity 
tables" and buy wire in whatever standard voltage seems to be closest to 
what you need. People who are winding transformers and coils commercially 
in large quantities DO analyze this stuff, and DO order custom wire of 
exactly the dimensions they want.

Likewise for a TC, there are plenty of other design points to worry about 
than the capacitor.  Sure, you could calculate the voltage reversal, and 
the peak voltages, and the RMS and peak currents, and then either send a 
sheet to Maxwell/GA for a recommendation/quote or hunt through the catalogs 
to find something that seems to work.  But why bother (unless you're 
interested in optimizing production designs).  Available time is finite, 
and there are a lot of more interesting (to me anyway) questions to 
contemplate than capacitor ratings.  MMCs are rated in "tesla coil 
compatible units". Maxwell Pulse caps are not.  This makes application of 
the MMC much easier.

If I were building a pulse discharge apparatus (like a quarter shrinker or 
Marx), I'd not fool with the MMCs.  They don't have ratings for those 
applications, I'm not interested in devoting the time to coming up with 
good applications models.  Nope, I'd use the Maxwell catalog and charts, 
and buy a cap specified in terms convenient for design a Marx banks. (even 
more likely.. I'd look at what Maxwell has built for Marxes, and use the 
same part they did, assuming requirements are similar)





> > Another thing "I" always stress is that MMC have (should have) drain
> > resistors which I think vasty increases safety...  No worries about
> > residual charges on an MMC...
>
>I recall they catch on fire alot, burning people's circuit boards. It shifts
>the problems elsewhere.


I doubt that any well constructed MMC catches fire.  As with anything being 
sold as a "kit" there are significant workmanship and design variations.  I 
would compare the MMC to a model airplane in ARF (almost ready to fly) 
condition.  You can still screw some stuff up, but the big design decisions 
(the schematic and parts selection) has been taken care of.


> > Oh yeah!!  MMCs are also sold pretty much at cost.  There are no big
> > markups.  I think I lost a couple bucks on the 4000 I sold and I don't
> > think Chris is using his profits to pay off the Geek Lamborghini :o))
> >
> > I know there will always be people that want to use the commercial caps
>and
> > that is fine.  They may be better on really high powered coils.  But I
> > don't think there is a lot of reason for the average coil builder to use
> > anything else unless they can't afford them.  In that case, the Geek
>Bucket
> > cap...  But I think there is no doubt that the MMC directly caused the
> > instant extinction of the oil filled poly rolled cap ;-))  Remember them
>:o)))
> >
> > An interesting thought, if one put a commercial 25kV 13nF commercial cap
>in
> > parallel with a 25kV 13nF MMC*.  How would one blow up the MMC without
> > hurting the commercial cap??  Of course, blowing the commercial cap
>without
> > hurting the MMC is easy ;-))
>
>That should be pretty straight forward. For starters, make sure you are
>using the self healing abilities with overstressed dielectric. As the
>capacitance of the MMC cap assembly drops while the metallization burns
>away, it will be stressed more and as the voltage across it increases.

In parallel, they will have the same voltage across the assemblies.  If one 
of the caps in the string degrades faster, it will take more voltage until 
it totally fails.  I think the degradation and failure mode would be 
similar in kind, but not details, to a series string of diodes.




>Again, cornell dublier only makes residential and recreational capacitors.
>This nonsense "commercial pulse cap" term needs to go away, it has no
>meaning on this list. Like on ebay, people assume anything that says "tesla"
>or "high voltage" is somehow "commercial" and "a pulse cap", or that
>anything from Maxwell is rated the same. I'm pretty certain they make more
>than one capacitor, and they all have different ratings. Try substituting "I
>want to buy a Maxwell" with something as silly as "I want to buy a
>Nichicon".
>
> > current in those shorts IS "cool"!!!  If you need those stunning currents,
> > you need commercial caps...  Coilers don't...
>
>Hmm, use electrolytic caps then, they are pretty "slow" and have low pretty
>low short circuit currents. Should be great for tesla coil.

That would be taking it a bit too far (unless you have a TC running at a 
few hundred Hz, and even then). Electrolytics also have fairly high 
resistance and huge dissipation  so the losses would be high.

Another cap suitable for TC would be a high rep rate commercial pulse 
discharge cap, with a series inductor/resistor to limit the peak 
current.  Maxwell/GA (as well as other pulse cap vendors of days of yore: 
Sangamo, Aerovox, etc) all have fairly good analytical models of their 
devices (some publicly available, some proprietary for internal use only) 
which could be used to design a suitable capacitor system. In fact, this is 
what GA's bread and butter is: designing pulse discharge systems using 
their capacitors.  I'm sure if you went to GA and said: "I'm going to build 
1000 tesla coils over each of the next 10 years, and I'd like your help 
selecting suitable capacitors, etc., and I'm willing to pay you for the 
consulting," they'd be more than happy to help you out.





>KEN