RE: MMC advice

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

Original poster: "Mudford, Chris" <chris.mudford-at-agresearch.co.nz> 

Hi Gerry

Yes, you were right in assuming 4 strings of 6 caps.  And, I am running
very close to resonance 14.7 nF cf ~16 nF for resonance.  Having
repaired my MMC again I reset the safety gap, again so that it was set
to fire at 105% of full power with no load.  I then added my main gap in
and ensured it was firing, which it was (but the safety was also).  When
all the coil was connected and I had checked all connections, including
RF ground, I then rechecked tune and plus or minus half a primary turn
made little difference, but, the safety gap was firing like a main gap
does.  At full power it is still pulling 15" arcs to the metal end of a
fluorescent tube (so performance isn't an issue).  After running for
30sec continuous the caps are slightly warm.

I will be following the rest your advice over the next few nights.  You
do mention construction of the MMC.  I'm not getting failure between
caps, it is only ever individuals squirting what would be described as
"high pressure flames" from the middle of them.  The caps and resistors
are seperated by 7 mm of formica  which they are mounted on.  Can you
expand on the high voltage techniques for solder joints and lead
trimming and how these can effect the performanc eof the MMC.

My conclusion at the moment is that I am getting resonant voltage rise
and that my MMC is built tooooo close to the edge, particularly as this
is my first coil  and it has received some punishment.  I used to have a
safety gap across the MMC (with no resistance) and this firing quite
often could well have done any amount of damage.

Cheers, Chris (NZ).

-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Friday, 3 October 2003 12:52 a.m.
To: tesla-at-pupman-dot-com
Subject: Re: MMC advice


Original poster: "Gerry Reynolds" <gerryreynolds-at-earthlink-dot-net>

I'm thinking that the safety gap firing all the time at 30% variac might
be a clue when the safety gap with no load was set at 105% variac.
Sounds to me like the system would have to be runing at (or very close
to) resonance for this to be the case.  With 4 strings of 6 caps (please
verify that this wasn't 6 strings of 4 caps), the Vdc rating would  be
9000V - very close to the peak voltage of a 6000Vrms transformer.  I
would bet that this 6000V tranny could put out more that 6000V (I've
measured a 12KV NST at no load to put out 12.6KV.  Opening up the safety
gap, I think, was the wrong thing to do.  Rechecking the safety gap and
looking else where for the cause for small streamers might have been
more prudent.

One suggestion might be to input the design to JAVATC version10.  The
tool will calculate the peak current for the physical geometry of the TC
primary and Cp and give a cross check to the tuning.  Another idea if
everything else checks out, is to scope the safety gap voltage (with cap
removed) and then run the coil for a very short time, turn power OFF,
and then see how warm the caps are getting.  Peak current will be a
function of the firing voltage, Cp, and Lp.  RMS current will be a
function of the peak current parameters as well as the BPS (and maybe
k).  It is difficult to determine unless you have a picture of the
current waveform and SW to calculate it. Measureing the temp rise of the
caps might be easier.  I would also recheck the construction of the MMC
array to ensure proper clearences are given (between caps and between a
cap and it's discharge resister).  Also make sure good high voltage
techniques are given to solder joints and lead trimming.

Gerry R
Ft Collins, CO


  > Original poster: Mark Broker <mbroker-at-thegeekgroup-dot-org>
  >
  > 1200A across 5 strings of caps is only 240A per string - just over
half  > their rating of 432A peak.  Even if using an "unrecommended"
design of 9x3  > .150uF caps for .050uF 20kV, the current rating is
still within  > manufacturer's specified limits....  >  > FWIW, I have
yet to come across a functional TC design that exceeds either  > the
peak or RMS current rating of the caps without also exceeding the  >
voltage rating by a good amount.  The only feasible designs I've been
able  > to come up with (using an Excel MMC sheet based off Terry's
MMC_Calc2 BASIC  > app) that would cause problems use extremely high
break rates (well over  > 500BPS), which cause excessive cap heating.  I
could see big coils run with  > very low surge impedances causing peak
current problems, but only if the  > secondary has, say, 400 turns and
micro-small toroid.  >  > That brings up a question with all the
magnifier discussions going on: how  > do the peak and RMS currents look
in the primary circuit of a magnifier -  > both optimally built using
Antonio's research or just thrown together a la  > Dave's Labor Day
Coil?  >  > Regards,  >  > Mark Broker  > Chief Engineer, The Geek Group
 >