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rolled caps: was Re: Coil Grounding

At 10:43 PM 2/14/2000 -0700, you wrote:
>Original Poster: "Adam" <adamsmith-at-mediaone-dot-net>
>
>on 2/13/2000 5:24 PM, Tesla List at tesla-at-pupman-dot-com wrote:
>
> > Original Poster: "Jason  R.  Johnson" <hvjjohnson13-at-xoommail-dot-com>
> >
> > Even if budget is a problem I would think that you should be able to do
>better
> > than beer bottles. A rolled cap or an mmc would be much better and not all
> > that
> > difficult/expensive. For a rolled cap I know that you can find info in the
> > archives and for an mmc (also info in the archives) you can get caps from
> > Mouser Electronics- www.mouser-dot-com or Digi-key- www.digikey-dot-com.
> >
> > Don't anybody let me give Troy bad advice :-)
>
>Well, there are some folks coiling on a low budget, for which even an MMC
>could prove unaffordable.  Beer bottles cost $0.05 each--any MMC will be at
>least 2 orders of magnitude more expensive than this.
>
>And poly caps are certainly not cheap in terms of materials cost or labor
>hours.  My 0.018uF 0.090" poly cap cost me about $200 in materials, which
>includes the 1/2" plexiglas case, but not the gas to go and pick up all the
>pieces from the various suppliers (aluminum flashing, polyethylene sheets,
>plexi, transformer oil, brass nuts and bolts, polyethylene tubing, nylon
>screws, rubber gaskets, etc...)
>
>I got enough poly for some rolled caps too, and for these I needed to buy 4"
>thin-walled PVC, 4" end caps, PVC cement, neoprene washers, brass hardware,
>more copper flashing, and mineral oil from the drug store since I didn't
>like the transformer oil all that much.
>
>And you need a vacuum pump to do it right.  I would not do any of this
>again.  Anyone considering building more than one poly cap should seriously
>consider just biting down on the leather strap right now and forking over
>the dough for a commercial cap.  You'll get a much smaller, neater, and more
>robust product for the same $$$ if you shop smart.
>
>-Adam
>adamsmith-at-mediaone-dot-net

I have built some rolled poly caps that work well, seem to be rather stout 
and were not terribly expensive. They were made from the following parts
10 ' section of 4" sched 40 PVC        $5.50     you only use about 32"
2  4" female adapters  with screw in plugs  $6.00 ea
1    roll of clear plastic sheeting  .006 $10.00
2  2inch pieces of  5/16 all thread   any size will do  1/4 -1/2
8 nuts
2 flat washers
1 Gal vacuum pump oil   $10.00  you use <1/2 gal  ( I have also used non 
detergent motor oil  and  unscented lamp oil  successfully)
some  tin foil

optional: The boiler drain valve and flare fitting are for venting and 
vacuuming
1/2" boiler drain valve $2.00
1/4"mpt X 1/4"flare fitting 1.50

the first one will cost you about $40.00 but the second will cost you less 
than $25.00 because you already have the oil and pipe. you may even have 
enough plastic left over for the second cap which would then only cost 
about $15. Scrounging in the right places and talking to the right people ( 
construction dumpsters.  plumbers)  you may cut cost even more.
         Its hard to calculate the final capacitance because there are too 
many variables ( how tightly you can roll them  quality of the plastic 
ect.) but my experience has been using 12 layers of plastic ( for a total 
dielectric thickness of .072 )and 12 inch wide tin foil  the caps come out 
to be about 8-10 nf ( measured) and increase slightly over time( as the oil 
seeps in and fills all the air spaces)
         the main disadvantage to these caps that I can see is they are 
rather big and heavy ( picture a 3foot section of 4" pipe filled with 
plastic and oil) and should probably be kept upright.( you don't have to 
though) Hard to get the exact capacitance you want. If you pay full price 
for everything they are almost, if not as expensive as an mmc

The main advantage is they can be "rewound". If you blow one up, all it 
takes is another $10 roll of plastic, some tin foil and a few hours of your 
time and you are back in business. Most of the material is "scroungeable" 
and they are lot more portable than bb caps. (they don't leak or splash.)
         The reason I went with rolled poly instead of an MMC was I had 
some material on hand and the rest was readily available. The MMC that 
Terry shows on his website sure looks a lot lighter than a 4" pipe! :-))

         If anyone is interested in building such caps contact me off list 
and I can send you some pictures and detail sketches that may be helpful in 
their construction.

Good day all
Bruce