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6Nov/110

Professor Kill A. Volt


So I built a Tesla coil for Halloween. Me and Ash came up with the idea a week before, so I didn't have much time.

I already had most of the components for a coil ready. Already had neon transformers, a handful of doorknob capacitors, totalling 17.5nf. Since this is all I had available with capacitors, I was forced to keep the power down to 12kv @ 30ma (360 Watts). I had an old primary and secondary coil I built years ago for a static gap coil. But the secondary was only about 3" diameter.

I decided to build a synchronous rotary spark gap to get the most out of the little power I was putting in. I used a universal brushed food processor motor. I ran across a neat trick discovered by Clive Penfold that allows you to convert a brushed motor to synchronous with just a diode soldered between 2 opposed brush commutators. Synchronous Rotary spark gaps are a great way to get a efficient, and solidly timed spark discharge in the primary circuit. It allows the capacitor to discharge @ 120 beats per second, and also discharges at the peaks of the AC wave. The phase of the discharge time can be changed by rotating the motor in relation to the tungsten electrodes on either end of the gap. It worked quite well for me, as shown in these vids/pic:








The rotor is just a 1/4" barbed nylon "T" fitting for hose. I stuck a couple of set screws in it to lock the fitting on the motor shaft, and lock the tungsten electrode in position. I dremeled slots in the white plastic mount to allow for phase adjustment. (The slots were made bigger after that pic) Using my Variac I found it ran nice @ about 40 volts. I found a transformer @ Raelco that made it more permanent.

I'm mounting everything in a poly rolling cart that I found at the DI for $10. The cap is just a handful of 40kv doorknob caps, all in parallel for a total of 17.5nf. I used aluminum strip to mount them together, and a couple of IKEA plastic shelf brackets to mount it to the cart. A simple safety gap was built using a couple of carriage bolts (not shown).

Now the Transformer, Cap, and Spark Gap is taken care of, I start fiddling with the primary and secondary coils.

I already had old stuff available, and gave it a shot:


The sphere at the top is a pair of IKEA Bowls soldered together. Actually worked pretty well. The streamers were about 12"-14"

I decided to go ahead and wind a new primary/secondary. Due to only running 360 Watts, I know the performance wont be much better in terms of streamer length. But at least I'll have a better set up for next year after building a better cap, and increasing the current up to 120ma.

The new secondary was wound on a 5.25" poly post cover from NPS. Wound 24" of 23awg magnet wire. Here's some pics of the winding process.



It came out pretty well, although I had to tweak it a few times to work out the loose windings. Over the next few days I put approx 6-8 coats of Polyurethane from a rattle can on it.

The primary took a few attempts. I tried using clear acrylic boxes from IKEA, and threading copper tubing into holes I drilled.. It didn't work out so well. So I started over, and decided that zip ties and drilled PVC on a temporary plywood base should work.


I used 3x 20 foot spirals of 3/8" copper tubing soldered together. This was probably the single most expensive part, but turned out well!


I think the winding spacing is a bit too wide, but it works. I soldered together 2 more IKEA Bowls, this time for a 14" diameter sphere. And gave it a test!


It was marginally better, as expected. Not enough current, remember? Probably about 18" streamers. I put some hardboard covers over the spark gap area to cut down on noise and UV light hitting people. Also wired a foot operated switch from Harbor Freight to make operation easy. The Rotary Gap stays on continuously, and the transformer is kicked on with my foot on demand.

But wait, we need more! I also tossed together a few 12kv transformers for a 120ma Jacobs Ladder!


Also gathered some props.


I built a pair of grounded huge aluminum "tongs" with IKEA strainers at the end to use as a Faraday Cage. I let the kids put the candy in the tongs themselves, sometimes yelping really loud to make em jump. :D
I then zapped the candy with the coil, before giving it back to them. My wife Ash put together a spider web covered workbench with bubbling concoctions, sheets in the background, and other spooky bits. We both dressed in lab coats, her being my clipboard wielding "Lab Assistant". I'm also wearing a pair of welding goggles, and 40kv lineman gloves (for looks).

It all worked out well, and everyone enjoyed it!





A friend recorded this one.


This will be a new tradition each year. Each year will be bigger. I should be running a nice MMC Cap, and 1440 watts input next Halloween!

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