Re: MMC advice

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

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



 > 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).

You may want to close the main gap slightly so it gets most if not all the
firings. With resonance, the power source could reach voltages 5 or 10 times
rating (depending on Q) if the spark gaps allow it.  It sounds like you are
now preventing this with your resetting the safety gap, but the main gap
need to compete with this gap.  The voltage will breakdown whichever gap
fires first.


 >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.

You may want to repeat temp test but for a longer time now (a minute, then a
couple of minutes) and see the temp rise trend.  The fact that it warms up
in 30 secs might mean something.


 > 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 thoughts here are to make sure you don't leave sharp points on your
solder joints and cover your cut leads with solder to round off any
resulting sharp edges.  The objective here is to prevent any corona.

 > 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.

This could very well be the case as the peak current was probably violated
when that safety gap fired.  If the failure mode is a result of this abuse,
you could probably expect more failures in the future.  If one fails as a
short, it could take out the next weaker one,  having an extra cap in the
string might prevent this cascading,
if you don't want to replace them all.  If the failure mode is heat, you
will want to reduce the RMS current that any one string sees.  This can be
done by reducing the peak current thru a string or by reducing the BPS.  You
can reduce the peak current any one string experiences by reducing the
voltage across the string (this could reduce performance and you may not
want to do this), decreasing the capacitance per string (adding more
capacitors in series to the string), or by increasing the inductance of the
primary.  Increasing the top load would  reduce the resonant frequency and
allow this.  Increasing total Cp with more strings, will lower the BPS but
not change the peak current per string but would need a change in the
secondary to track the change in resonate frequency (a longer or larger
diameter secondary, or larger top load.  There are many trade offs and
combinations to choose from and these are just some ideas.  Also be aware
that some changes may be counter productive, for example, increasing the
sparkgap to reduce BPS could result in a larger voltage to the array and
increase the peak current .

Gerry R
Ft Collins, CO

Cheers, Chris (NZ).
 >