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Re: [TCML] Mylar capacitor

I used to make my own caps back in the day, and tried both LD Polyethelene and mylar. I ran them with coils upto about 15kw power level, but even with the smaller coils found that long run times (several minutes) eventually led to failier. I used the then common method of having a few layers of insulation between the two foils and rolling them, or stacking layers up in a plastic tub under oil. I tried adding thicker layers of insulation, and got longer runs, but they always failed eventually.

I noticed that the failier was always somewhere around the edge of the foil, so I set up an experimental cap with an open lid and watched it run until it failed... At first the coil ran smoothly, but after a while I noticed bubbles forming in the oil around the foil edges. (the coil started running rougher at this point) Corona was firing into the air bubbles, heating the dielectric and lowering the voltage rating until BANG! A spark would break through the insulation. There was no air in there to begin with, but the high voltage near the edge of the foil causes the oil to "off gas" and creates pockets of air.

I started making caps with multi segments so that the voltage per foil was much less, this way, if you keep the voltage across each section below 10kv you dont get any corona in the oil...
No corona means no localised heating spots, which stops them from failing.
It allows you to use VERY thin layers of insulation, which helps to give you more capacitance...

foil------------------ ------------------- ------------------- ------------------
           foil    ---------------------      -------------------      ----------------------

I built MYLAR caps this way, and noticed that they did warm up slightly after long runs. The POLY caps were bigger for the same capacitance, but never even got warm after long runs...

The segemented caps are more work, but definately worth the effort.
In saying all that, if you can buy a pre made pulse cap in your budget, I would go for it.
It will save you HOURS of time... :-)

Hope that helps.


----- Original Message ----- From: <jhowson4@xxxxxxxxxxx>
To: "tesla list" <tesla@xxxxxxxxxx>
Sent: Saturday, August 20, 2011 3:37 PM
Subject: [TCML] Mylar capacitor

Hello everyone,

I wish to pick your brains on the use of Mylar in TC capacitors.

yes yea i know MMC, blaa blaa, I would need 400 caps... so its not happening.

I am in college, and I have been scratching my head trying to figure out how i am going to get a cap good for .1uf @ 50Kv cap on the cheap and still have significant head room to safeguard against the brutality of TC use. Apparently if i go the poly route its still going to cost me 250-300 bucks depending on where i bought the rolls of poly. making it advantageous to just attempt to go for a pulse cap (if anyone is selling feel free to chime in) but i realize they are pretty expensive themselves and I am having a hard time finding one with the specs I desire in surplus.

So I was staring at the list of common dielectrics i might be able to substitute and saw Mylar, after some quick math I see that I could potentially drop the costs off significantly and have a reasonable (but lossy) cap that could last me couple years of tinkering, until I can afford the good stuff that is.

I have found stuff in the list archives describing that mylar is very lossy and becomes more so lossy as its temperature increases, leading to its inevitable failure, etc etc. But I have also found a forum in which Peter Terran claims to have used Mylar caps for a museum coil and has had virtually no problems for 20 years. (correct me if I am wrong Peter). That's pretty fantastic for museum use.

so hear me out on this

I was thinking of ordering a small role of 10mil clear Mylar, and using 2 layers to get a maximum voltage of 150Kv (well above anything I could throw at it) Just for fun, divide the well known puncture voltage by 2 and you still have a 75Kv cap, which is a voltage I would ballpark for when designing a rolled poly cap anyway. Doing so would cost me about 150 bucks, almost half the cost of the poly I would need to do the same.

But wait I think I can do better, lets take another approach, rather than one big capacitor that is way over designed, how about 5-10 lower voltage capacitors in series that are still way over designed. So in my case anywhere from 5-10Kv per cap. this means I need 3-4 mill for a ballpark overrated value. 4 mil Mylar is cheaper than the 10 mil. meaning I could pull it off for more like 50-100 bucks if I played my cards right.

Roll some little guys up, use my vacuum pump to impregnate them in mineral oil (to aid in cooling and corona issues) and put them in a vertical open ended PVC pipe (so it does not explode). Also Mylar apparently does not absorb more liquids, as i just discovered, which is a good thing.

All in all it seems like a reasonable plan to me, i just have to make sure to over design enough so that even if the caps do get hot, they will have enough head room to not fail readily. I will also have to make sure to not run for more than a minute or so at a time, which i don't do anyway unless i am trying to capture a video of something.

What do you guys think? Does this idea have even a hint of being a good one? or is the stuff just too darn lossy.

John "Jay" Howson IV

"Why thank you, I will be happy to take those electrons off you hands."

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