Re: Failure mode of MMC's

[Terry Fritz <twftesla-at-uswest-dot-net>]

Hi Sundog,

The numbers for your set up are:
===========================
MMC Calculator  Ver. 2.1  2/19/2000  Terry Fritz
Transformer voltage =  12000 
Transformer current =  .18 
Firing voltage =  16970.52 
Fo =  250000 
Break rate =  120 
Cap value =  5.6E-08 
 
Strings  Caps/Str  Capacitance  Voltage   Temp C    Cost
  3        5         33.60         8000    19.75   21.75  :-((  :-(  
  5        8         35.00        12800     7.71   58.00  :-((  :-)  
  6       10         33.60        16000     4.94   87.00  :-|   :-)) 
  7       11         35.64        17600     4.08  111.65  :-))  :-)) 
--------------------------------------------------------------------
  8       13         34.46        20800     2.92  150.80  :-))  :-)) 
  9       14         36.00        22400     2.52  182.70  :-))  :-)) 
=====================================

so if your using an MMC larger than 5 x 8 caps (I assume your are using
Panasonic) you cap should be fine.

If they over volt, they either recover or blow the case.  Over current may
melt them down internally but the case should still cook as they short.  If
one has failed, you can rip it open to see if it was explosive (over
voltage) or melted (over current) damage.

Of course, your running the coil with a resonant cap so over voltage is a
real possibility and you should have safety gaps properly set.  The gap may
fire at more than 120BPS to depending...  Your coil really could handle
about 100nF!

I suspect you will find a dead leg on one of the NSTs.  If you are not
using safety gaps and filters, you may want to consider the following:

http://users.better-dot-org/tfritz/NSTFilt.jpg

If you did blow a cap, I would love to know the number of strings and
cap/string you were using and hear of the damage they had.  I can trade you
for some caps I can look at in person if the failure of the caps is some
MMC problem that I need to look into...

I often use washers as cooling fins on my rotary gap electrodes as is
fuzzily shown below:  I have a nice new camera now so future pics should be
far better :-)) 

http://users.better-dot-org/tfritz/newcoil/114.jpg

Glad to here your coiling is providing so much fun!!

Cheers,

	Terry Fritz



At 10:01 AM 8/11/00 -0400, you wrote:
>
>  Hi all!
>
>   Well, good news, bad news, and mediocre news...
>
> The good news is that I got up to 35-37" streamers yesterday evening
>(thursday) from my 4" coil (Thanks Dave!).  I changed the electrodes is my
>sparkgap to thin brass rod, as well as the stationary electrodes.   The
>flying electrodes worked beautiful, but the stationary got waaaay too hot &
>burned off some wire insulation from my tank wires....oops...No biggie.  The
>bad news is I'm running just over 2kva to do it.  Not too efficient.  The
>mediocre news is that I think I may have cooked my MMC or a few NST's.  The
>unrelated news is that on the 15th I go in for Lasik eye surgery and get rid
>of my glasses!!! WOOHOO!!!
>
>   I added a polycarbonate sheet above the primary, and for the most part,
>the streamers ignored the primary completely!  It was great!  I got an
>occaisonal strike to the sheet, and it just spread out like some weird vine
>on the polycarbonate.  Very cool looking.  I opened up the gap a little bit,
>then fiddled with the phase of the disc on the rotor, and believe that I
>toasted a chunk 'o my MMC.  My output dropped a lot, and my tuning went out
>a complete turn, and the srsg just sounded a lot wimpier.  The next run I
>only got ~3" streamers...definately a problem somewhere. Tonight I'll try to
>test each string in the MMC (5nf, 17kv), and also the trannies.
>
>if I *did* kill an MMC, it died peacefully.  No pops, no "exploding
>caps"...my output just dropped.  What is the normal failure mode for an MMC
>in TC use?  In stress-testing mine destructed by popping the red coating and
>letting out some smoke/hissing.  I'll be inspecting it later...
>
>
>My coil...
>  4" secondary wound with ~600something turns of 22 ga.
>  35nf MMC
>  (6) 12kv30ma NST's for ~2kva
>  6x14(?) toroid.  Looks about silly-big on the thing, but it makes it rock!
>  srsg running 120bps
>  tunes ~turn 4 on the primary.
>    Best runs were last night,  producing 35-37" streamers and 1 40" strike
>to a grounded rod.  I got ~5" by changing the gap electrodes from zinc and
>making the stationary electrodes out of brass rod.  Works *much* better.
>Also adjusted the gap for best operation between runs.
>
>
>Ideas?  Comments?  I think this coil can do better.  42" from a 15/60 just
>puts this to a bit 'o shame.
>
>									Caio!
>										sundog
>