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Registered Member #1054
Joined: Wed Oct 10 2007, 10:51PM
Location: Bulgaria
Posts: 29
Hello, I'm searching for a suitable capacitor bank for my medium-sized RSGTC. I'm at a low budget, so I aimed for "WIMA" caps, they are cheap and seem to stand high frequencies. I plan to buy 50 pcs of these shown on the picture. When in series I'll get 44nF@12,500V which I guess is going to be Ok with a triple MOT PSU. Do you think these will work, also is 250V the rating for both DC and AC? (I'll use AC)
I've heard these are very effective... I think I've even seen videos of MKP4 caps being used and they actually do very well... I think more ideally you could build your own capacitor with a nice dialectic like polyethylene or polypropylene and you could easily achieve 30kv... Can you give more specs about your coil just out of interest and really I've always heard that it's best to have the cap rated 40-50% above the supply.
Edit - more on building your own if you are on a budget:
Registered Member #1054
Joined: Wed Oct 10 2007, 10:51PM
Location: Bulgaria
Posts: 29
Hi twist2b,
Thanks for the reply, that's rising the hope I'll finally get good caps for my TC :) I've been through that issue for a long time... I've tried beer bottles, plastic bottles, but they require large space, and I just don't have it in the garage. I've tried also these: ,_3.5_nF I would say they are the best option for homemade HV capacitors. I even did a nice bank immersing them in transformer's oil:
The capacitance they give is about 50nF, but then a new trouble arose. When I tried to feed them with the triple MOT PSU at the spark gap (a tester static gap) there was just a tiny little spark, way too weak for what is expected to be. It was almost nothing. Then I fed them with my ZVS driver from curiosity - the result was big and loud bangs at the spark gap! I was totally confused. Why they would work with the ZVS, but fail to do so with the MOT supply? Is the higher frequency of the ZVS which makes them work? I don't want to use a ZVS for PSU, because it gets hot relatively quickly. That's why I decided to put away homemade stuff and buy some manufactured caps. The secondary's specs are:
wire diameter - 0.6 mm coil diameter - 11 cm coil length - 47 cm turns - ~786
The primary coil is made of 2.1 mm thick wire, which will be replaced with a copper tubing. For a toroid I use 11 cm in diameter air duct pipe. The outer toroidal diameter is about 37 cm.
HUH! Those photos are awesome! HAHA! From reading up on the subject everyone seems to run into problems with rolling them but less issues with plates: But yours look awesome! Hmm, I don't know a lot about MOTs but using a spark gap topology makes it really hard I would think... but like I said I don't know. Are you ballasting with the 3rd MOT? You sure you achieve the proper resonance with the CAPs? Bah, I wouldn't really be able to help you much on the supply side... ZVS's are cool though, that was my first HV project.
Registered Member #1054
Joined: Wed Oct 10 2007, 10:51PM
Location: Bulgaria
Posts: 29
Thanks, it isn't an easy task, but after the first five pcs you get specialized :) I put an effort to be good, because I've already had four of them and they work really well on my small TC (the avatar picture). It seems the problem is arising when they are stacked in large numbers, the MOTs just don't charge them. The MOT PSU is powered by one phase of a three-phase system (240V/50A), so I don't need any ballast. Without capacitors they produce nice big arcs. Please don't offer me another homemade caps :D I'm really done with them. I assume that since noone else's been written here the WIMA caps are a good choice. I'll purchase them today :) Thanks again!
Registered Member #4074
Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335
hyparh wrote ...
Thanks, it isn't an easy task, but after the first five pcs you get specialized :) I put an effort to be good, because I've already had four of them and they work really well on my small TC (the avatar picture). It seems the problem is arising when they are stacked in large numbers, the MOTs just don't charge them. The MOT PSU is powered by one phase of a three-phase system (240V/50A), so I don't need any ballast. Without capacitors they produce nice big arcs. Please don't offer me another homemade caps :D I'm really done with them. I assume that since noone else's been written here the WIMA caps are a good choice. I'll purchase them today :) Thanks again!
I've used similar MKP WIMA caps in the past for a tiny micro-sized spark gap coil, as well as some other high frequency and HV applications. Compared to a home made capacitor the WIMA caps will have much lower losses and much better general performance, plus they usually aren't too expensive (at least compared to the favoured Cornell Dubilier PP caps). I've also had success with an MMC constructed from Vishay MKP capacitors, which were even cheaper.
That's right, indeed the MKP capacitors are an excellent choice. Besides Mica Caps which are possibly the best (though expensive and difficult to find), Film Polypropylene capacitors are generally regarded as the go-to capacitor of choice.
The Wima MKPs are Metallized Polypropylene which will work excellent, are cheap and have self-healing properties. E.g. if the dielectric is punctured, the metallized deposits 'vaporize' locally, allowing the capacitor to continue. They are also cheaper in general than the well regarded CDE 940 series. For greater RMS power handling, Metal Film (not metallized) polypropylene capacitors are used like the CDE 942 series which can handle higher peak currents. Either way, they should work much better than your rolled caps (which looks amazing btw).
However, I see you are using the 250V version all in series. For a more robust cap bank especially with the MOTs you will be running at, I would recommend finding higher voltage versions and putting a few strings in parallel to handle the power and to reduce the ESR of the cap bank.
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"WIMA MKP4" capacitors for an RSGTC?
