Re: Another 942C failure mode?

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Jimmy Hynes <jphynes@xxxxxxxxx>

Hey,

Gerry mentioned a runaway phenomena where the failing cap loses 
capacitance, and therefore, see's more voltage, and pops again. People are 
generally told to separate the strings so that they can't all dump into one 
when it fails, but maybe it's better off that way, than to have it runaway 
because of the loss in capacitance.

My MMC was connected between strings (had to make it like that for other 
reasons), and I haven't had an overvoltage problem. I dont know if it ever 
broke through, but they should have seen 3kv/cap peak.

More comments interspersed...

On 6/12/05, Tesla list <<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx> wrote:
Original poster: Steve Ward <<mailto:steve.ward@xxxxxxxxx>steve.ward@xxxxxxxxx>

> It is interesting that you and Steve have damaged MMCs.  Perhaps the DRSSTC
> needs to have the MMCs looked into more.  The MMCs may need to be made a
> bit stronger than normal for DRSSTC use.


From the start, I was looking for high current caps, because I knew that 
overcurrent was more likely to be a problem in DRSSTCs than SGTCs. That 
doesn't mean a DRSSTC is harder on them for the same sparklength though...


 Of course, like me, the MMC was
> probably the last thing one was worrying about.  You can always just whip
> together another one.  I note, that my protectors with their single big
> caps get a little warm too.  My MMC is enclosed so I will not know if it
> gets hot until it is too late.  I never considered how it might like
> running at 6000 BPS :o))  I guess it has not blown up yet...
>
> Cheers,
>
>          Terry

Terry, i must point out that i redesigned my MMC for a 4kv rating
after seeing that Jimmy got away with it ;-).



Yay! I finally got him to stress his components a little ;-)

 I think we were both
pushing these caps just too hard.


I will admit that on that one night, I was. I didnt give it much rest 
between runs, and ran like 20 cycles (instead of 14), which did nothing but 
increase RMS current. Normally, they were fine though. I don't plan on 
upgrading my MMC :P

I dare you to build a spark gap
system that makes 6 foot sparks from 10 of those caps!  Or in Jimmy's
case, 93" sparks from 16 caps!



Exactly my point about not being tougher on them than SGTCs. It just 
presses them into a different failure mode (or can, I guess. Steve's 
failure mode was overvoltage, mine wasn't)

The caps were running uncomfortably warm (to touch), but CDE says they
are ok at 80C (which they likely were not, at least not on the
outside).

Mine were getting pretty hot on the outside. The thermo couple said 125f