The MMC and Cross-linking

To: tesla-at-pupman-dot-com
Subject: The MMC and Cross-linking
From: "Reinhard Walter Buchner" <rw.buchner-at-verbund-dot-net> (by way of Terry Fritz <twf-at-verinet-dot-com>)
Date: Fri, 26 Feb 1999 06:38:06 -0700
Approved: twf-at-verinet-dot-com
Hello, Gary, Guy, all,

Just to make sure everyone understands what cross linking
means, here is an ascii art of what I mean (use courier font
to view).

*-||-*-||-*-||-*-.....*-||-*
|    |    |    | .....|    |
|    |    |    | .....|    |
*-||-*-||-*-||-*-.....*-||-*
|    |    |    . .....|    |
.    .    .    . ......    |
.    .    .    . ......    |
|    |    |    | .....|    |
*-||-*-||-*-||-*-.....*-||-*
|    |    |    | .....|    |
|    |    |    | .....|    |
*-||-*-||-*-||-*-.....*-||-*
|    |    |    | .....|    |
*-RR-*-RR-*-RR-*-.....*-RR-*
|                          |
|                          |
|                          |
o  <--- HV Connector --->  o


This idea of crosslinking originated on our german Tesla list.
It was my suggestion and here are some of my thoughts
behind this scheme:

Advantages:
----------------
a.) You only need a single string´s worth of equalization resisitors.
b.) Cross linking equalizes voltage distribution (as Gary already
mentioned).
c.) Current per cap is also equalized.
d.) ESR should be lower.
e.) If you DO cross link, the maximum number of caps that can blow
     is "X". X being the number of caps in one string. If they short,
     the matrix linking will protect all caps "underneath" the shorted
     cap. If it open circuits, the capacitance is simply reduced by
     one cap.
f.) Using heavy bus bars (flat copper strips) to cross link, should
    allow higher peak currents (more bang for the buck).

Disadvantages:
------------------
a.) Fine tuning is much more difficult (since you need to remove all
     connections on the string).
b.) If one cap in a string shorts, automaticly all caps (since one
     complete "set" of caps is "removed" from the string. I.e. a 15
     cap string will effectivly be reduced to a 14 cap string) see a
     higher voltage per cap. This might lead to quicker failure, if
     you can´t / don´t switch off fast enough.


So, the question whether to cross link or not, is not that easy to
answer (and I, for my part, haven´t decided on this yet). My
present "play around with" setup DOES NOT use cross linking.
(on the other hand, I am not using equalization resistors or
safety gaps, either) I haven´t gotten around to trying the
crosslinking (to see if it will increase performance) setup.

Either way around, I am sold on the MMC idea and WILL and
DO use this for my 8" coil setup. I am very pleased with the
results (41+" from a 7.5kV/75mA NST isn´t bad). As a matter of
fact, the MMC has given me the best results from all the caps I
have built so far.

Very important (I think): Look for metal foil MKP (polypropelene)
and NOT metalized plastic foil. The metalized plastic foil caps FAIL
almost immediately. Use caps of high quality. Usually the better
caps are larger in physical size (physical size vs. capacitance),
than the El-Cheapo brand.

Coiler greets from germany,
Reinhard