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Registered Member #3775
Joined: Thu Mar 24 2011, 10:10AM
Location:
Posts: 6
Hi, need a bit of help with the MMC, i think i got things right but thought i'd double check.
7.5kv/30mA NST, static gap resonant cap = 0.0127 uF
After punching some figures into a calc (javatc) it spat this out : Recommended cap = 0.0191uF and a voltage of 26,517V Using deepfried calc i managed to get these figures using 47nF individual caps rated at 3000v Are these ok?
Total cap = 0.0208uF Voltage = 27,000V Caps per string = 9 Strings in parallel = 4
Sounds ok??? i tried messing around with the caps per string and strings in parallel but couldn't manage to get any closer figures This ok? also would these resistors be ok for cap drain off? Im assuming it would be alright as they are rated at 3500vdc and the caps are 3000v.
Registered Member #3775
Joined: Thu Mar 24 2011, 10:10AM
Location:
Posts: 6
Hmm ok after doing some research i found out that 942C series would be better suited then the 940? Although these only come in 2kv ratings which will quite significantly jack up the price And damn thought i had a nice setup going with the 940's
Although i had a hard time comming up with a decent array. i managed to comeup with this; Capacitors per String: 13 Strings in Parallel: 5 This gives Capacitance: 0.018uF and Voltage Rating: 26000v which is slightly under the recommended 26,517V and the capacitance is slightly under too (0.019uF) Adding 1 more string in parallel would give 0.0216uF but geez my wallet will explode. Any suggestions on this matter?
Registered Member #160
Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938
I've used the 940 series without any issues for a few years but everyone will tell you not to. If you are going to spend a lot of money, you may as well do it right and go for the 942 series. I don't see why you would need to rate the cap bank at 26kv when you are using 7.5kV input. I wouldn't recommend doing it, but I run 15kV NST through cap bank rated at 18kV. It is much larger than resonant though so no resonant rise.
Registered Member #3775
Joined: Thu Mar 24 2011, 10:10AM
Location:
Posts: 6
yeh true, the peak across a 7.5kv would be just over 10kv, that makes things alot easier, i guess ill aim for something around 12-14kv thanks for ya help!
Registered Member #2919
Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652
You want the safety margin to deal with bad things like voltage reversal and resonant rise; the lifespan of a pulse cap is highly dependent on how close you are running it to its absolute rating.
Registered Member #1408
Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679
bwang wrote ...
You want the safety margin to deal with bad things like voltage reversal and resonant rise; the lifespan of a pulse cap is highly dependent on how close you are running it to its absolute rating.
This had also been my experience. I have used 940's once with great success once with a problem. However I used a very poor choice of 940 and ran it close to resonance (in the MMC that was problematic). With what you have chosen, you are SO close to the cost of the 942, I would either buy the 942 or go to a lightly heavier cap (like 3k .15uf 940 -- 940C30P15K-F) if cost is within budget. If you are stuck for money you are so close to the standard .15 2k 942, I'd just buy those.
Registered Member #3775
Joined: Thu Mar 24 2011, 10:10AM
Location:
Posts: 6
quicksilver wrote ...
If you are stuck for money you are so close to the standard .15 2k 942, I'd just buy those.
Yeh seems you right, with these i managed to get some good values using 1 string only! thanks! 7 in series give 0.0214 8 give 0.0187 i think i'll go for the 0.0187
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MMC help
