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Re: [TCML] 8" Secondary Woes

On 9/3/13 7:29 AM, Greg Peters wrote:

Hi all,

Well I've done something stupid. I wound a beautiful spiral primary with a
12" diameter inside turn on nice polycarbonate supports. I assumed that I
would be using an 8" (200mm) secondary and that 8" was a standard pipe
size. I was aiming for 2" between the primary and secondary. Never assume
anything! The nearest PVC water pipe OD is either 160mm (6") or 250mm (10").

8" OD tube is as rare as hens teeth in Australia. The only exception is 8"
acrylic tube which is difficult to cut and work with. I'm not a big fan of
acrylic, as I always seem to get my saw stuck, etc and I can see my
beautiful former being destroyed.

I have called nearly every pipe and plastics supplier in the country and I
am out of luck. No polycarbonate, styrene, PVC, polyethylene, polypropylene
or other plastic in 8" OD! I even called some of the gas pipeline suppliers
- I CAN get a length of blue 200mm LDPE pipe for about $500....not ideal
(expense wise).

I noticed that McMaster Carr has very cheap polycarbonate tubing in 8" OD,
and placed an order only to be informed that they will not ship to
Australia. There are a few other not so cheap US suppliers who may be able
to assist if necessary though they seem significantly more expensive.

Anyway, the closest thing I have found in Oz for a reasonable price ($200)
is a length of orange PVC electrical conduit at 225mm (9") OD.

So, my question to the list. Do I persist with trying to find an 8" tube,
possibly even from overseas, or settle on the 9" conduit and tolerate
having only 1.5" between my primary and secondary? I expect I will need to
lift the secondary quite a bit to prevent overcoupling and even flash overs
from the primary which will be running 24kV. Is anyone else running a
largeish coil with this kind of spacing?
You may not be as overcoupled as you think. Run the modeling program and see. Also, you can always put the tap point for the primary on the inside rather than the outside, which essentially moves the "mean radius" of the primary out.
You could always *make* a former of your desired diameter by using cardboard or foam as a core, and laying up fiberglass on it, then removing the cardboard or foam. A couple layers of fairly lightweight glass cloth would be *way* stronger and lighter than most pipe.
Or, you could call around to plastics fabrication shops and see if they will make a 8" diameter polycarbonate tube from sheet. If you don't need the "exact" diameter, they can bend the sheet around a form and weld it. You'll get some seam, but good shops can make an almost invisible seam.
You can also buy a 10" diameter pipe, cut a chunk out of it lengthwise, and weld/glue it to the smaller size. PVC, in particular, softens quite nicely with hot air or water, and glues very well.
There's always cardboard, too. You can either buy a premade tube, or make your own from thin sheet stock and white glue. Same works for using thin polycarbonate or polyethylene sheet. Make some circles of the right diameter on a shaft, wrap multiple turns of 1/32" or 20 mil stiff plastic around, with an appropriate adhesive. by the time you get to 3 or 4 layers, it will be quite rigid. The problem is that you will have a visible line where the last wrap ends. Cardboard tubes do a spiral wrap that has a butt joint to fix this. 



I'd run the models first, before panicking..


This was meant to be a museum quality build and the idea of having to
significantly lift the secondary is frustrating, but realistically it's
looking like the only option! The coil has been painstakingly designed to
sit exactly in the middle of my shed space for maximum spark length in all
directions. Oh well, next time I will be more careful and assume nothing!

Cheers,

Greg.
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