Re: MMC question

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

Original poster: Mark Broker <mbroker-at-thegeekgroup-dot-org> 

Well, let's put it this way....  We have never heard of a failure of a 
"Geek Cap" that wasn't directly related to poor design or 
construction....  The design chart on our mmc page is tried and true.  If 
you build an MMC using the suggested design on our site and follow our 
construction guidelines, your MMC should last forever in hobbyist duty.

If you really do feel uncomfortable using an "underated" capacitor, then go 
ahead and make it more beefy. :)  But, IMO, if "our" caps can hold off 
5000V, one could argue that 10 in series is plenty overdesigned.  IIR, a 
second high voltage test with a newer cap held off 5300V at the first 
breakdown, then more than 6000V at the second (that's a 2.7J bang inside 
the cap!).  Then the current draw got pretty high.  You have that info 
handy, Terry, or do I have to go digging?  (one of these days we'll just 
add that piece of information to our site)

http://www.thegeekgroup-dot-org/mmc/

FWIW, my first MMC was made from 1600V Panasonics (I think I managed to hit 
Terry's first bulk buy in early 2000), total voltage rating of 
19.2kV.  After a few hours of runtime, one dead NST, one dead "Terry 
Filter" (shorted the MOVs), a lot of toasted AOL CDs, and a lot of awed 
spectators, the MMC caps are still fine.  And IMO I've been gentle on those 
compared to some of the torture we've put our Geek Group MMCs through!  The 
Geek Group MMCs have seen many coil setups and many power levels, including 
running near-resonant value caps with a 14.4kV PDT overvolted to ~280V 
input on a 360BPS SRSG coil (half the Gemini project) and also running a 
static gap 12/180 setup with no topload (we tuned at about 2 turns - the 
gap was much less happy than the MMC).  So all this is just saying that, 
yeah, these are pretty tough little buggers.

Cheers

Mark Broker
Chief Engineer, The Geek Group


On Fri, 16 Jan 2004 22:46:01 -0700, Tesla list <tesla-at-pupman-dot-com> wrote:

>Original poster: "Gerry Reynolds" <gerryreynolds-at-earthlink-dot-net> Hi Luke,
>
>The 942C series of caps are great.  However, with a 15KV NST (this is RMS
>voltage), the peak voltage will be just over 21000 volts not counting other
>effects such as resonant rise and inductive kick.  A 10 cap string, IMO,
>will be insufficient for this transformer.  I would say a minimum of 12 and
>propose a 14 cap string instead.  LTR for SRSG will certainly keep resonant
>rise down and the SRSG will take advantage of inductive kick.  Also, for a
>given operating voltage, going to a larger string will reduce the peak and
>RMS currents in that particular string with everything else the same (BPS
>and Lp).
>
>Gerry R
>
>
>  > Original poster: "Luke" <Bluu-at-cox-dot-net>
>  >
>  > I have decided on the capacitors that the geek group recommend.
>  > The ones I have on order are the 0.15 micro farad and rated at 2KV.
>  > If I run these with sets of 10 in series they would have a voltage rating
>  > of 20KV
>  > I will be using a 15KV NST.
>  > The cap size will be LTR and I am going to use a SRSG
>  > Does it seem like I am cutting the voltage rating a bit close or will
>these
>  > caps hold up to that?
>  > Any one have any experience with these.
>  > I would rather drop my capacitance a little and put another one in series
>  > for a 22KV rating if it is needed
>  > But if I feel VERY confident they will hold up with 10 in series I want to
>  > go that way.
>  > I don't want performance at the expense of needed up keep due to cap
>failures.
>  >
>  > Luke Galyan
>  > Bluu-at-cox-dot-net
>  >
>  >
>  >
>