Secondary
and Toroid
Grid Dipping the Secondary
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Notes:
Well, the secondary winding was definitely a challenge. I used a broom handle between two chair backs as an axle on which to spin the PVC form. I made another quick holder for the spool of magnet wire. This was a slow, tedious, time consuming process. I spent about 4 hours over two days working on just the winding. I found it easiest to wind about 5 turns, then make sure they were properly aligned and tight to the form, wind another 5 and so on. I used masking tape about every inch to secure the windings until the urethane could be sprayed on to seal the windings. And yes, the cheap masking tape did leave an adhesive residue on the wire that I later had to remove.
I
suppose some coilers out there may oppose the use of the smaller 24 AWG
magnet wire and lean toward
22
AWG or even 20 AWG for a coil this size. But once again, this is
what I had access to, so this is what I used.
Two things to watch out for here...one I knew, the other I did not. First, PVC pipe (as well as most plumbing supplies) are rarely the size they are advertised. In my instance, I knew that 6" PVC drain pipe is actually 6.275" OD. This may not sound like much, but that adds about (.275 x pi) = .864" additional length per turn x 1141 turns = 986" overall length. That's roughly 82 more feet of wire. So, if you wish to calculate inductance or length of wire needed, this may make a small, but noticeable error. Secondly, no sooner had I wound and coated the secondary and I read a page about "lossy" materials -- how some materials have a tendency to become conductive at radio frequency...PVC being one of them. The article went on to recommend several coats of urethane to seal the PVC and eliminate the conductance before winding. Too late for me! If I ever do see the need to coat the secondary form, I will definitely build a jig to wind the coil. I don't think I would have near as much luck sliding the wire over several thick coats of urethane as I did over the smooth PVC.
The toroid is 4" diameter black polypropylene flexible drain pipe. I paid about $2.80 for 10'. To form the toroid, I wrapped the pipe into a ring and melted the end seam using a soldering iron. Technically, this is called "plastic welding", and it's really the way to go with the polypropylene. Not too many solvents make a good weld with this stuff. No need to even try the PVC cement, it won't work at all. With the ring formed, I pushed a 12" diameter disk made from 1/2" plywood into the center. Then, I ran a bead of "liquid nails" construction adhesive around each side of the plywood. After this dried, I used 2" x 12" strips of aluminum foil with a spray adhesive to cover the toroid. The adhesive does make a thin insulative layer between the foil strips, but at 1 million volts, I don't think it will make much difference.
In the future I'll probably be moving the toroid closer to the secondary to cut down on some of the corona around the top windings.
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2-6-99 - Well, the future is now! I have upgraded to an 8" x 24" toroid. As a result of additional voltage reflected back down the coil, which created enough interwinding arcs on the secondary that I thought the PVC was going to be incinerated, I have added several additional layers of urethane clear coat. Additionaly I have added a small toroid approximately 3" x 8" directly atop the secondary. The new 8" x 24" toroid sits directly atop the smaller toroid. This is said to provide better "voltage grading" of the secondary - per the R M Craven IEEE paper on "Design Improvements in Tesla Coil Performance"