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Re: MMCs, LTR coils, 45kv diodes and MOSFET flyback drivers

Hi Bryan,

At 06:18 PM 11/13/1999 -0800, you wrote:
>Hello Everyone
>
>	I have been thinking of constructing my first MMC recentlly.  This MMC
>would be 20kv at 33nf, using a single string of 10 2kv 330nf metallized
>polyproplyene caps. The power supply is two parallel 7.5kv 30ma NSTs. 
>WinTesla says that my resonant cap match is 21.2 nf.  How will the LTR
>cap affect streamers compared to a resonant cap?

Resonant caps contain less energy per bang but hopefully fire more often
giving about the same results.  Personally I think your 33nF value is
pretty good rather than the true resonant value.  A large cap that I used
was 17.5nF on a 15kV 60mA NST which would be equivalent to a 35nF in your
case.  A true LTR cap would be around 55nF but you would need a sync rotary
gap.

Always use safety gaps too!

Unless you have a high inductance primary (~200uH), a single string of caps
may die from over current and internal heating.  I would think you should
use at least two strings and preferably 5 stings for most coils as a
minimum.  Although, you may be lucky too.  MMC failures are very rare even
when highly overloaded...  If you try it and they work without getting real
hot let us know.  At about 40 C case temperature they are about to blow...
A large value cap like this is designed to take higher currents so you may
be ok...  A similar "high current" 0.33uF cap is rated for 13.5 amps RMS.
I can't say for sure, but you may be ok...  Smaller value MMC caps like my
0.056uFs are rated at 5 amps RMS so these may be as good as three strings
of mine.  I would definitly not fire them at greater than 120BPS.

Over voltage spikes on MMCs can be absorbed very well.  They are designed
to take an occasional "hit".  At about 2.5 times their rated DC voltage
they will simply start to uncontrollably arc internally and fail in seconds.  

Over current simply heats the poly in the center of the cap up until it
melts and fails.  Unfortunately, the entire string will heat about the
same.  Thus even though only one cap may blow, the others in a string are
probably toasted too.

Hope this helps.  Personally I would try the 0.33uF caps.  However, I would
not mind wasting $30 on the "experiment" if they blow either...

>
>	I have found more interesting items in the Mouser catalog.  They sell
>19.8kv, 22kv and 45kv diodes.  These are standard NTE replacement
>semiconductors.  NTE # 504, 505 and 513.  I do not know current ratings
>although I doubt they are very high.  Since it is NTE it should be no
>trouble to get the data sheet.  They also have NTE 522, 529, 531 and 538
>which are listed as Hi voltage multipliers.  They give no other specs. 
>Does anybody know something about these?

I think they are TV rectifiers like I use to use in the good old days...  I
think they are very low current, like 10mA, but I really don't remember...

Cheers,

	Terry