Re: Any Geekgroup MMC failures out there??

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

Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>

Hi Bart,

Sounds like it failed where the plates meet the end cap.  Could have been 
from high current, but I suspect it just had a defect in it.

I don't see many cap failures being reported ;-))  Sound like all is well 
out there in MMC land!!

Cheers,

         Terry

At 10:54 PM 1/17/2004, you wrote:
>Hi Terry,
>
>Well, I've had one cap bite the bullet. This happened about a year ago. I 
>was the 1st to receive caps from The Geek Group many moons ago. These are 
>the 942C series 0.15uF caps. The one that that died was the cap next to 
>one of the brass bolts I used for a connection. There's a hole in the cap 
>about 1/8" from the end. I don't believe it an arc from the bolt to the 
>cap occurred since the hole is on the opposite side of the cap, however, 
>something odd occurred. It was "not" from over volting the cap but I 
>believe RMS current. I believe I was running 5 strings of 12 for 0.0625uF 
>total at the time. The caps were set on a pcb with 10M resistor soldered 
>across the cap on the opposite side of the pcb. Each cap from side to side 
>had at least 1/2" clearance. Plenty of room there. I suspect I exceeded 
>the rms current capability (25 amps).
>
>I've since rebuilt the entire cap bank onto an 8" pvc tube. Each resistor 
>is now soldered across the cap and each cap is simply placed on screws 
>protruding from the pvc tube (this way I can maneuver cap arrangements 
>anyway I like). It is currently 3 strings of 18 caps for a total C of 
>0.025uF. I haven't had a failure since.
>
>I'm not sure why the cap failed. It's still on the pcb board along with 5 
>neighboring caps. I just threw it up in a cabinet and left it there ever since.
>
>Take care,
>Bart
>
>Tesla list wrote:
>
>>Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>
>>
>>Hi,
>>
>>I was just wonder if anyone had any failures of Geek group caps for "no 
>>good reason"?  I have not heard of any problems, but it seems like a good 
>>time to ping the users out there to see if any problems have been 
>>noted.  Sort of just checking to see if there are any holes or unknown 
>>problems we need to check into.  Of course, if one does not follow the 
>>tips or runs them out of what is recommended, that is your problem 
>>;-))  But if you think you did everything right and they blew, I would 
>>like to hear about it to see if any common trends or problems are out 
>>there we don't know about that need fixing
>>
>>Perhaps additions to the "tips" too.
>>
>>
>>Cheers,
>>
>>         Terry
>>
>>
>>At 12:15 AM 1/17/2004, you wrote:
>>
>>>Well, let's put it this way....  We have never heard of a failure of a 
>>>"Geek Cap" that wasn't directly related to poor design or 
>>>construction....  The design chart on our mmc page is tried and 
>>>true.  If you build an MMC using the suggested design on our site and 
>>>follow our construction guidelines, your MMC should last forever in 
>>>hobbyist duty.
>>>
>>>If you really do feel uncomfortable using an "underated" capacitor, then 
>>>go ahead and make it more beefy. :)  But, IMO, if "our" caps can hold 
>>>off 5000V, one could argue that 10 in series is plenty 
>>>overdesigned.  IIR, a second high voltage test with a newer cap held off 
>>>5300V at the first breakdown, then more than 6000V at the second (that's 
>>>a 2.7J bang inside the cap!).  Then the current draw got pretty 
>>>high.  You have that info handy, Terry, or do I have to go 
>>>digging?  (one of these days we'll just add that piece of information to 
>>>our site)
>>>
>>>http://www.thegeekgroup-dot-org/mmc/
>>>
>>>FWIW, my first MMC was made from 1600V Panasonics (I think I managed to 
>>>hit Terry's first bulk buy in early 2000), total voltage rating of 
>>>19.2kV.  After a few hours of runtime, one dead NST, one dead "Terry 
>>>Filter" (shorted the MOVs), a lot of toasted AOL CDs, and a lot of awed 
>>>spectators, the MMC caps are still fine.  And IMO I've been gentle on 
>>>those compared to some of the torture we've put our Geek Group MMCs 
>>>through!  The Geek Group MMCs have seen many coil setups and many power 
>>>levels, including running near-resonant value caps with a 14.4kV PDT 
>>>overvolted to ~280V input on a 360BPS SRSG coil (half the Gemini 
>>>project) and also running a static gap 12/180 setup with no topload (we 
>>>tuned at about 2 turns - the gap was much less happy than the MMC).  So 
>>>all this is just saying that, yeah, these are pretty tough little buggers.
>>>
>>>Cheers
>>>
>>>Mark Broker
>>>Chief Engineer, The Geek Group
>>
>>
>>
>>
>>