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first run of an MMC
Hello, All.
Last night I tried out my first attempt at an MMC. I've avoided
describing it before, since I am aware that it contains two big no-no's;
the dialectric is polyester and the individual cap value is .22µF. And I
only had 100, which meant I could only use three strings...
But the caps only cost me $0.20 a piece, so I thought if they failed, so
what?
Going by the cap rating, the whole thing should end up being about .027µF
with 25 caps in each string.
I also was trying out a new bifiliar-wound secondary, trying to make my
system use less of the large 17 5/6 turn primary.
Now, the fun begins..
As the coil had been sitting around disassembled for while, I reset the
saftey gap, hooked in the rotary, set the MMC on top of the SRSG's case
and connected it, tapped into the primary at about six turns, selected
two toroids,... ...switched on the SRSG, and ran up the variac. The gap
was firing... hurah! The cap was working.
I then decided to check the gap phasing, rather than fool around with
tapping yet, since the saftey gap was firing frequently. In the mercury
vapor light I was using to sync it, it was difficult to see, and I
decided to change it while the motor was running... ...big mistake.
The unpleasant sound of metal scraping metal at high speed and the top
chunk of one of the fixed electrode supports went flying out of the open
case.
No one was hurt, and, only semi-daunted, I set the broken support with
lexan glue and wired in my old RQ gap to replace the out of comission
SRSG. I then tried to tune the coil, and ended up only being able to
achieve smooth streamers out at only 3 1/2 turns, and they were really
short till I started stacking on the terminal capacitance. That MMC
certainly has a whole lot more capacitance than my SW cap bank ever did!
I ended up with a enough Al ducting toroids stacked on the coil that the
terminal was bigger in all demensions that the length ofthe secondary!
And even at that, the streamers were smaller than what I'd achieved using
the SW caps, and I was getting the best streamers at something like 2 or
3 turns.
.027µF is about double resonant for a 12/60 NST like I was using, and I
didn't expect the RQ gap to be able to really perform in such a system,
and was looking forward to using the SRSG in the morning to see just how
well the new system might work. But, saddly, at one point flames burst
out of one of the caps in the MMC, and coiling had to be suspended.
The destructed cap had one entire foil connection blown off, neatly
severing the end from the rest of the cap. The entire bank was warm, but
not worryingly much.
Is this bank going to last me any time at all? That first cap blew...
...Oh, that's right, it didn't blow untill after I tried ganging the
12/60 in parallel with a 15/30 to get longer streamers... I'd actually
forgotten that! But, the caps still blew after only about 10 minutes
total run time. I have spares... ...just so long as this thing lasts
long enough to perform at the MidOhio TeslaFest.
So, this morning, rather than play with the big system, I threw together
a bipolar coil from old components in ~5 minutes... ...6" streamers
from a 9/30 NST. Sigh... ...I can still smell the ozone in my clothes.
:)
TTFN
Grayson Dietrich
http://www.electrophile.8m-dot-com
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