Salt shaker shown for size reference.As I've mentioned in past posts, of all my passing nerd interests none capture my heart like sparks and explosions. Which currently brings to mind the internal combustion engine, but somehow that's a bit too contained to be really exciting. About four years ago I made this mini tesla coil, and as of three years ago I'd been planning on re-designing the secondary. Of course one project spawns another, so I wound the secondary coil (a blog post in its own right, here) which has now been sitting on my shelf, collecting dust.
Anyway I recently found myself with some spare time, and with a new year's resolution to catch up on old projects, I finally finished what I started four years ago. So let's start at the beginning.
The primary coil is wound from 3/16" copper pipe. There are three reasons for this. Visually, copper pipe is a winner. Mechanically it's very malleable yet holds its shape well. Electrically, a hollow copper pipe has almost twice the surface area as solid pipe of the same diameter, and by the skin effect at high frequencies, poses a much lower resistance. Check out Litz wire, if you don't believe me. The form is 1/4" acrylic, notched and glued to an acrylic cylinder. To the left, you can see the secondary standing around.
Here you see the primary section, waiting to be re-wired. The upper right is the 4kV transformer. I have no idea what that was originally for. It's not a NST, because it has no internal current limiting, and wasn't designed for 100% duty cycle. My best guess would be an OBIT, since it provides fairly high current, and was just meant to be pulsed on. Below that is the homemade RF filter, the spark gap (minus electrodes) and capacitor bank.
The secondary ground return is shown, a small screw post. Yes, as the glue is currently drying, the thing is resting on a box of kitty litter.
The secondary and primary mounted together. I haven't put the arc shield between the primary and secondary yet, as I hadn't powered it up.
1/4-20 mounting bolt for the topload. As you saw in the first photo, I opted for a simple spherical topload rather than the toroid I was going to make. I don't have access to the Alpha CNC lathe at the moment, as soon as I do I'll turn that.
With the thing properly grounded in dry weather I'm getting 4" sparks, so by the rule of thumb that's around 120kV. I'm still having problems with strikes to the primary, so next time I either make the primary more dense, or put another shield between the two. Click for high resolution images.
I put a CD on the topload, and a ground terminal above it. Sparks are very pretty, if you ask me. If only I were a better photographer...
