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Registered Member #1653
Joined: Mon Aug 25 2008, 08:02PM
Location: Chicago, Il
Posts: 2
This is my first posting, so, "Hello to all!"
I am building my first Tesla coil. It is a relatively small one: NST is 7500v, 20mA, 150W; 3" secondary. I am deciding on how to go about building the capacitor bank. I am planning on using polyester film caps, but I am unsure of the layout. It seems that several coilers recommend having multiple strings (>4) of caps in parallel for the MMC. This is to better distribute the current. However, since my TC is rather low-power, can I get away with less strings in parallel? I am considering even making the MMC with 2 or 3 parallel strings. I am just trying not to use too many materials and, hopefully, save some money.
Any and all advise would be much appreciated. I will probably post more questions in the near future, mostly regarding this MMC. Thank you in advance.
Later, envytea
Note to moderator: I honestly tried to search previous postings for information relevant to my question. However, a key term "MMC" had too short of a string to be a legitimate query. I apologize if this question is redundant in the history of this forum.
Registered Member #1653
Joined: Mon Aug 25 2008, 08:02PM
Location: Chicago, Il
Posts: 2
Thanks Kizmo. I am now leaning toward using Cornell Dubilier metalized polypropylene caps.
My target capacitance is 0.007uF for the MMC. If I use 3 strings of 9 0.022uF/2000vdc rated caps, the MMC will have a value of 0.0073uF/18000v. Is this a decent choice? Would it be better to have the total capacitance slightly above or below the targeted value?
Registered Member #160
Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938
Your resonant cap value will be 7nF, so you should go 1.4 times higher than this to have a Larger than resonant cap value. Hence you should be looking for about 10nF in your cap. You could go 3 strings of 6 of those caps (.022uF 2000V) giving you a rating of 12000V @11nF.
Registered Member #1630
Joined: Sat Aug 09 2008, 11:36AM
Location: Seoul Korea
Posts: 115
Coronafix wrote ...
Your resonant cap value will be 7nF, so you should go 1.4 times higher than this to have a Larger than resonant cap value. Hence you should be looking for about 10nF in your cap. You could go 3 strings of 6 of those caps (.022uF 2000V) giving you a rating of 12000V @11nF.
actually I would be going 2.5-3x the input value of AC voltage. 7500Vac - 18.75- 21Kvdc rating.. this is standard practise. The capacitors you need are Polyethylene and on a personal bias CDE942Cs are the best suited for the job because of their self healing capabilities. Nice thing is your tank cap is pretty small. CDEs will last years if you treat them properly!
Registered Member #160
Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938
Seoul_lasers wrote ...
actually I would be going 2.5-3x the input value of AC voltage. 7500Vac - 18.75- 21Kvdc rating.. this is standard practise.
I believe this to be overkill, when using LTR the voltages are never going get that high on the capacitor. I use 24kV for my 15kV NST, and have even used 21kV. I have never had a problem. The CDE942C series is well tried and tested as the best for TC use. That said, I actually use the 940C30S47K-F which has a 3kV rating on each cap. The reason I do this is because those other capacitors aren't available from my supplier in Australia, and it works out cheaper.
Registered Member #1630
Joined: Sat Aug 09 2008, 11:36AM
Location: Seoul Korea
Posts: 115
Coronafix wrote ...
Seoul_lasers wrote ...
actually I would be going 2.5-3x the input value of AC voltage. 7500Vac - 18.75- 21Kvdc rating.. this is standard practise.
I believe this to be overkill, when using LTR the voltages are never going get that high on the capacitor. I use 24kV for my 15kV NST, and have even used 21kV. I have never had a problem. The CDE942C series is well tried and tested as the best for TC use. That said, I actually use the 940C30S47K-F which has a 3kV rating on each cap. The reason I do this is because those other capacitors aren't available from my supplier in Australia, and it works out cheaper.
I would stay away from 940C caps as they tend to fail at the axial leads (ends) . If you use the JAVATC calc and go to the section on MMC design you will see what I am reffering to about the voltage rating needing to be at least 2.5x. There is a lot of stress from the capacitors rapidly discharging and this translates into heating of the dielectric. It is best to have some extra voltage rating than not, less stress on them/
Registered Member #1589
Joined: Sun Jul 13 2008, 06:40PM
Location:
Posts: 70
So, what is the resolution on the caps listed? I ordered some from Newark and I received them, but completely forgot about how to construct the MMC, do I use VDC(3 000V), VAC(750V), or the average of the two values(1 875V)? In my scenario it runs to be, using VDC rating, 6 caps per string, giving me 18 000V with a 8 000V transformer. It's not >8 000*2.5 giving me 21 000V, but more than the peak available(V*sqrt(2)), which gives me 11 313.7085V. Am I doing it correctly?
If it feels like I am taking over the tread, say so and I will make a new one. I thought it would be okay since this one is dead, and my question is relevant to these specific cap's specs.
Registered Member #1517
Joined: Wed Jun 04 2008, 06:55AM
Location: Chico CA
Posts: 304
In an oscillating system you need to use the VAC rating for the caps + the safety factor. It appears as if you have done that, but are a tad bit under the rating for safety (aka cap longevity), IMHO.
There have been several discussions on what the appropriate amount of overrating is for a MMC voltage ratings. I'd say that you're just about right with your rating.
You can build it cheap and worry about the caps failing or you could build it so you never even consider the caps :)
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The development of an MMC...
