Re: Caps: Stacked vs Rolled

            Re: Caps: Stacked vs Rolled
            Fri, 11 Apr 1997 07:50:51 -0700
            Bert Hickman <bert.hickman-at-aquila-dot-com>
            Stoneridge Engineering
            Tesla List <tesla-at-pupman-dot-com>

Tesla List wrote:
> Subject:
>            Caps: Stacked vs Rolled
>       Date:
>            Thu, 10 Apr 1997 08:22:15 +0000
>       From:
>            Chris Gardner <gchristo-at-clt.mindspring-dot-com>
>         To:
>            Tesla List <tesla-at-pupman-dot-com>
> References:
>            1
> First off - I have set aside the DC coil project until I complete the
> 556 cascade controll circuit. I am now using a 10kv furnace trans with a
> small quick and dirty cap which I think works pretty well for the 15
> minutes of careful craftsmanship that went into it. I can only run the
> unit at about 30 on the big-ol-variac, but its a start and tons o fun
> with purple glowing from the cap and a 2 inch sparc.
> I am now determined to manufacture a serious capacitor and would like to
> post my current design thoughts and would ask if this seems sound;
> I have read some comentary that smaller caps in par. discharge quicker
> that larger caps (Fr. McGahee), My interpretation of this is that the
> same plate area will discharge quicker when divided into a greater
> number of smaller units, this is the first reason I have decided to go
> with a stacked design. The second is flexibility, I think a stacked
> design lends itself to an adjustable set up.
> I intend to obtain several sheets of .05 copper, and have these sheared
> to 5" x 6.5", then take these to a machine shop buddy of mine (arent
> they great?) and have him stack-em, clamp-em and rabbit out 1.5" x 4.1"
> or so with an eighth inch round on that inside corner. This should leave
> me with 25sq inches per plate face. Im thinking about 30 plates.
> These would be alternated and placed in a custom plexi box. For
> dielectric Ill be using 3 or 4 sheets of polyetheline between each
> plate. I havent settled on a thickness yet (hey, I'm still designing)
> But all will be mounted and clamped, filled with a good transformer oil
> and topped off with a half inch of parafin. (Do these things get hot?
> Ive seen no mention of running temps for caps)
> The tabs will then be fixed so that two buss bars can be attached to a
> variable number of plates.
> I am a strong believer in versatility and already have 4 secondaries of
> varying sizes im playing with.
> As they say there is nothing new under the sun, and Im sure this has
> been tried. Am I doomed or does this seem a good path to pursue?


Flat plate caps work great and if made properly have very low
inductance. However, you can save some money by skipping the copper
plates and using either aluminum flashing or heavy-duty aluminum foil.
There will be NO measurable performance difference but you'll see a
sizable cost savings. 

Your total dielectric thickness should be at least 90 mils to 120 mils
(for a 15 KV source). By using LDPE, your cap will stay cool even if you
ran it continuously. Don't clamp it too hard - the oil needs to
completely displace any trapped air - mounting the cap so that the
plates are aligned vertically will help this process. I'd also skip
using paraffin - make a top out of plexiglas and screw it down using a
buna-n or neoprene gasket. 

Safe cappin' to ya!

--- Bert --