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Re: help me put my coil on a diet; its capacitor appetite is expensive!

Original poster: "resonance" <resonance@xxxxxxxxxxxx>

Doorknobs will definitely not work reliably with a pole xmfr power system, especially at those power levels.

The 942C series will work if you keep them at Erms x 3 value for each string. 15 in series gives you 30 kV DC rating at .01 uF per string. Strings are then paralleled for the value your coil requires.

I will dig up the part number we use. They are rated at 50 kV pulse duty, much larger than the 37 series with it's 35 kV rating. Larger for more heat dissipation and better edge foil insulation on the dielectric.

If you are coming up to our Teslathon you will see them in action on a 25 kVA coil in operation with a 10 ft. dia. toroid.

Dr. Resonance

I have now toasted one of my two maxwell's, as well as one of my four strands of CDE 940C's. In the interest of not blowing up too many more capacitors, I am trying to do my research this time before ordering their replacements (and not using any of the remaining caps, for this coil anyway).

unfortunately finding a few roadblocks to that end:
1. I have a possible source of doorknob caps, in the form of another pacific northwest coiler who I recently bought a pole pig from, who is going to try to acquire some soon. However, I have heard doorknob caps might not be as effective, not to mention they are going to be HEAVY to get enough of them for a 25KVA pig powered coil, which is using a 120BPS synch rotary. 2. Have a rell.com shopping cart, containing 90x CDE 942C's (the -F version, which they have in stock, not the non -F which has a lead time of weeks). however, that is a $300 order that will still only have the same voltage rating as the 940C's which failed QUICKLY when I used them last week. A safer voltage rating would require more caps, and would still be less than the resonant cap voltage by about 1/2. .06 uF is what the 2 37667's I was using added up to, with great results, so that is what I was aiming for this time. LTR according to JavaTC is something closer to .48 uF (hmm... $2400 worth of 942C's, even if I only use strands of 15! I think I am going to pass on that option). 3. Maxwells- I would love to find out more about Dr. Resonance's better Maxwell caps, than the 37667's which are known to fail easily in a 25kva pig powered coil. (Between Aaron and I we have fried 2 of the 3 maxwells that I have ever seen in my life, both within the last month or two. ) Unfortunately all I can find is part numbers and ratings, nobody selling anything other than the 35 kv .03 uF 37667's, and even those are $225 where I have seen them. Does anyone know of a current source for better maxwell's, and/or have other recommendations in this area? 4. Any other options? I haven't found any that don't involve a custom order from CSI (which doesn't seem to take me seriously, judging from their lack of a response to my recent request for quote). I didn't even mention that I would be using it for a hobby!

So, in lieu of having many other options, I am starting to lean in the direction of building my own. I have tried this in the past but didn't know what I was doing, at all, last time. This time I am a bit more educated, prepared, and have a more impending need now that everything else in my coil seems stable including for the first time ever a synchronous rotary. For building this cap, I have a vacuum pump that will pull 29+ inches of mercury, and a t-shirt press which may help between the two of them to remove air bubbles. I was thinking that if I ordered a roll each of these two, I might be able to make enough caps that are both fairly efficient, and high capacitance/ voltage, that I could replace the maxwell/ MMC with something not too much larger, in the overall scheme of things. <http://www.papermart.com/Templates/47-0-40.htm>http://www.papermart.com/Templates/47-0-40.htm (a 100' roll of 30" wide metallic .8 mil polypropylene sheet) <http://www.papermart.com/templates/47-0-15.htm>http://www.papermart.com/templates/47-0-15.htm (a 500' roll of 30" wide clear polypropylene 1.2 mil sheet) (multiple layers of, between each layer of the above.)

I figure if I do a careful job in the overlapping, and use some mineral oil or something to pot it once the whole thing is rolled up in a tube and wires attached, I would be able to make it fairly robust. I even thought about making it in a sandblasting chamber that was completely cleaned out, and had an argon tank hooked up to it or something to purge the air (and corresponding particulate) out of the chamber after putting the parts in it, then sealing it before working on assembly. but that is way overkill considering it wouldn't even fully do the job- better just to use multiple layers of clear polyprop between each metallized layer, and leave large areas of clear overlapping all 3 edges of the metallized, and most of all removing every single air bubble by pulling a full vacuum for a few days after filling the tube with oil. then maybe a little more oil, an over-pressure safety valve for when the oil gets hot enough to raise the pressure inside it, and it should be decent enough for a run or two of the coil, right?

Definitely has to be better and smaller than a fishtank with 2 half-racks of saltwater filled/ motor oil covered corona beer bottles in it.

please give me feedback if you think I am nuts or could do this better another way or using something off the shelf. Regardless of what option I choose, I only budgeted myself about $400 to spend on replacement caps and I am hoping to use them more than once. I also don't feel like waiting more than a couple weeks for something which is on backorder so that rules out at least one option.

thanks in advance,