[TCML] Mylar capacitor

[jhowson4@xxxxxxxxxxx]

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. 




Thanks, 
John "Jay" Howson IV 


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


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