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Registered Member #1630
Joined: Sat Aug 09 2008, 11:36AM
Location: Seoul Korea
Posts: 115
envytea wrote ...
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?
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.
Coillah wrote: 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 :)
Coillah is totally right, over building your MMC is a good idea for prolonging the life of the capacitors. In any TC circuit there is what is known as peak voltage. On a 12Kv NST for example you are peaking around 18Kvac just off the XMFR alone, you then add some induction and your tank voltage could be as high as 25Kvac-30Kvac. As Coillah points out build cheap, it dies quickly $$$. Use TC calc for MMC and build it to recommended specs your caps are going to last! Like I said it has been 2 years and I am still running the same capacitors, now in a bigger MMC for my 6.25" coil! - Yes, EXACTLY.... CDE942C series capacitors are VERY durable and are what you need. Problem, these capacitors are so sought after you may have some problems getting them. This of course depends on the supplier. Alternatively ask CDE directly in an email. I built my first MMC for free by asking for 21 samples. CDE was kind enough to donate the 680nF 1.6Kvdc rated caps (new in 2007 style, with cylinder case) as they previously had never been used in TC service to their knowledge. These capacitors not only perfomed beautifully, they were WAY over built. I phoned CDE back and let them know about operating temperature, overall performance issues... I phoned back in early 2008 saying I needed 50 caps. I got the caps in under 2 weeks.
Registered Member #1630
Joined: Sat Aug 09 2008, 11:36AM
Location: Seoul Korea
Posts: 115
I have both Java MMC designer and TESLA map 5... both explain clearly that you need AT minimum 1.41x the rating of your capacitors DC rating for your MMC to actually survive it's first firing. This means that your capacitors will likely fail due to heating or a spike of it happend. To be safe many members (on the tesla coil mailing list) suggest you should have 2.5-3x your voltage rating in DC. You do know that a 12Kvac neon actually puts out 18Kv when you use it in service... so that is already ~1.5x your rating. You build an MMC that is just rated for 18kv you are going to be on the edge with it. --- when I typed 24Kvdc MMC for use on a 12Kv neon the result was an Orange rating or "ok" - under that it suggests that may suffer from heating. 2.5x the rating and the result was an excellent "green". I sent a few professionals emails on this and they have all emailed me back clearly telling me 30 -36kvdc (~32Kvdc) for a 12Kv NST is safe and that anything less isn't exactly recommended! In that case it is ~2x peak value on the xmfr not the actual transformer rating.
Registered Member #160
Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938
D.C. Cox suggests otherwise. By your theory my MMC bank should be dead as I have run it at 21kV rating on my 15kV NST. I'm sure it is better to be safer than sorry, but these capacitors are pretty good and they self heal... except when they go...BANG!
Registered Member #1630
Joined: Sat Aug 09 2008, 11:36AM
Location: Seoul Korea
Posts: 115
Coronafix wrote ...
D.C. Cox suggests otherwise. By your theory my MMC bank should be dead as I have run it at 21kV rating on my 15kV NST. I'm sure it is better to be safer than sorry, but these capacitors are pretty good and they self heal... except when they go...BANG!
True, safer is better. I guess it is who you listen to that determines your MMC construction method. I guess your capacitors are lucky? dunno, but I have read that post by DC Cox and he suggests 2x as you say. Interesting. I will run , from 4XMFRs at current, an upgraded XMFR bank to 6 NSTs 3 wired on 1 hot of a 240V service and 3 on the other. Total draw will be ~4.5-5Kw at peak. This type of power you need the extra voltage capability! Like I said 2years out of the same caps is pretty good. I like it when I can reuse my caps for different projects saves $$$!
Registered Member #160
Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938
Yeh, mine have been going for 2 years in a month and four different coils, one of which was a magnifier. TDU also uses 24kV rating on his large 18" run by two PTs. All I'm saying is that we tend to overbuild when there is not that much need to. Sure these caps are expensive but they are not crap, they can take a lot of beating.
Registered Member #1630
Joined: Sat Aug 09 2008, 11:36AM
Location: Seoul Korea
Posts: 115
Coronafix wrote ...
Yeh, mine have been going for 2 years in a month and four different coils, one of which was a magnifier. TDU also uses 24kV rating on his large 18" run by two PTs. All I'm saying is that we tend to overbuild when there is not that much need to. Sure these caps are expensive but they are not crap, they can take a lot of beating.
You say you built a magnifier, cool! I was always wondering about building one. I find these are quite interesting to read about. You use a sync rotary spkg? do you have pictures of it?
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
envytea -
You mentioned using Cornell-Dubilier "metallized polypropylene" capacitors.
You DON'T want capacitors with metallized film construction, you want hybrid polypropylene film/foil construction, like the CDE 942 series.
The metallized polypropylene film caps use a micron-thick coating of vaporized aluminum on polypropylene film as the metallic plates. Film-foil construction uses actual metal foil for the plates. The weak point with the metallized film construction is at the ends of the capacitor "roll", where the lead wires are attached. The ends of the roll are sprayed with a low-melting point metal alloy to make contact to the edges of the plates, and the lead wire is then attached to this sprayed-on metal layer. In the film-foil caps, the metal spray connects directly to the edge of the metal foil, and can carry high current. On the metallized-film caps, the connection of the sprayed metal is just to this micron-thin layer of vaporized aluminum, and this cannot handle large current pulses. The metallized film burns away, and the cap goes "open circuit".
Registered Member #1630
Joined: Sat Aug 09 2008, 11:36AM
Location: Seoul Korea
Posts: 115
Herr Zapp wrote ...
envytea -
You mentioned using Cornell-Dubilier "metallized polypropylene" capacitors.
You DON'T want capacitors with metallized film construction, you want hybrid polypropylene film/foil construction, like the CDE 942 series.
The metallized polypropylene film caps use a micron-thick coating of vaporized aluminum on polypropylene film as the metallic plates. Film-foil construction uses actual metal foil for the plates. The weak point with the metallized film construction is at the ends of the capacitor "roll", where the lead wires are attached. The ends of the roll are sprayed with a low-melting point metal alloy to make contact to the edges of the plates, and the lead wire is then attached to this sprayed-on metal layer. In the film-foil caps, the metal spray connects directly to the edge of the metal foil, and can carry high current. On the metallized-film caps, the connection of the sprayed metal is just to this micron-thin layer of vaporized aluminum, and this cannot handle large current pulses. The metallized film burns away, and the cap goes "open circuit".
Regards, Herr Zapp
I have seen a few of these KATO before. Somtimes they put on quite a fireworks display. Polypropylene makes great deal of white smoke that smells like candle wax.
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The development of an MMC...
