[TCML] Mylar capacitor
glau1024 at gmail.com
Mon Aug 22 18:58:17 MDT 2011
I respect anyone who puts out the effort to thoughtfully plan and roll his
own caps. Much will be learned in the process, no matter what the outcome.
Before the day of MMC's, I did likewise, with LDPE.
But as many will report, despite my best efforts to vacuum impregnate under
oil, the cap failed not long after it was done (long story here:
The real issue with rolling your own, is that no matter how thick and
bomb-proof you believe your dielectric to be, unless you make so many
smaller rolled caps in series that the corona inception voltage per cap
(Google it) is not exceeded, the edges of the dielectric will degrade and
I don't think I saw what final cap value you were shooting for or power
supply size to be used?
Regards, Gary Lau
PS - I also used one of the "Fair Radio Sales .01/100kV mylar caps, and it
did indeed work. It did get somewhat warm, so I had to restrict run length.
On Fri, Aug 19, 2011 at 11:37 PM, <jhowson4 at comcast.net> wrote:
> 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
> 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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