New 6-Inch Coil


This is my new 6-inch x 30-inch coil doing its thing. For the first firing, I erred on the side of caution and excited the coil with a bank of NSTs delivering 9kV at 180ma. Results were somewhat disappointing, with streamers up to about 30 inches or so--not quite what I had hoped for. I did a hasty tuning check, and got best sparks with the primary tapped at turn 7.

For the second run, I bet the farm and hooked it up to my 11.5kV pole transformer inductively limited to 4400VA. This time the coil went totally nuts! It sprouted two or three spastic 5 to 6 foot streamers simultaneously! It was intense! I could actually feel the hair standing up on my arms, as if I was charged with static electricity. The garage reeked with the stench of Ozone and NOx within a few seconds--I've never experienced the smell getting so bad so fast before.

I got a couple of primary strikes, and also one or two hits into the top of my beer bottle cap. In each case the safety gaps on the pole xfmr fired. No problems though. Most streamers seemed attracted to the ceiling, and a few arched over to connect to the concrete floor. There was no evidence of top turn corona, racing sparks, primary to secondary breakdown, or any of the other gremlins that seem to infest new Tesla coils. Likewise, the tank components, including the "lossy" saltwater caps, remained cool to the touch. The spark gap electrodes got warm, but not uncomfortable to touch. This is the best behaved coil I've ever built.

Here's the technical stuff. The secondary is 6.25" OD SDR-35 PVC drain pipe wound for 30" with 22AWG green enameled magnet wire. The flat spiral primary is 9-turns of 3/8" copper refrigeration tube, spaced 3/8" between turns, 9" ID and 23" OD. The bottom turn on the secondary is on the same plane as the primary. Cap is 42nF of beer bottles. Toroid is 20" x 4" corrugated drainpipe-type stolen from my 4" coil and balanced on top of a plastic cup for standoff. Spark gap is single static "sucker" gap mounted atop 120V vacuum cleaner motor. All tank wiring is 4AWG THHN. Ballast is two 120V MOTs in series with each other and in series with the 240V pole xfmr winding. My performance goal is 75-inch streamers. Judging by the results of first light, I don't think I'll have any problem reaching that level.

My design was generic in nature, incorporating features of many successful Tesla coils that I've read about on the Pupman mailing list and seen on the Tesla Coil Web Ring. I went with a low-risk approach in order to maximize the likelihood of early success. To keep costs down, I raided several key components, including the spark gap, tank cap, and toroid, from my 4" coil. These were thoroughly tested, proven components, further reducing the risk of failure. Really, the only part of this Tesla coil that I didn't copy is the cable tie primary design. I've never seen or read about that particular technique before. Even so, I have no illusion about the cable tie system being my personal innovation. I'm sure somebody, somewhere, has built a primary like mine before.

This thing really wants a bigger tank cap and a wider, fatter toroid. I'm working both issues at the moment. I'll soon begin construction of a 68nF MMC based on 1600V polypropylene film capacitors. I've also acquired a car tire inner tube for use as a toroid form. I'm hoping these two improvements will push out a few more inches of spark.