Craig's Tesla Coil Design
Based on various sources like Richard Quick and others I followed this path:
I had the Neon Sign Transformer (NST) So I started there. The nameplate says 13,000 volts 100 ma. (about 1300 watts) This is a fairly hefty transformer. My father tells me this was probably used for cold cathode lighting rather than a sign application. Using Ed Sonderman's spreadsheet it calculated a .02 microfarad capacitor would be a good match to the transformer.
I wanted an impressive looking coil and I had a good transformer so I wanted an 8 inch form. Richard Quick suggests a 3:1 aspect ratio for a secondary 8 inches or over. I followed this fairly close. I wound 25 inches on an 8 inch acrylic tube. Using the suggested #22 wire this resulted in about 925 turns.
My first top load was too small but I measured a resonant frequency of about 150 kHz. The smaller toroid is 14 inches in diameter and the dryer duct is 4 inches in diameter. I later built a larger one and got much better performance.
Given the 150 kHz resonance of the Secondary I then looked at what Primary I would need to tune to the same frequency with a .02 microfarad capacitor. The spreadsheet and Win Tesla both showed a pancake coil of about 12 turns would be right. I made a primary coil with 15 turns.
Heres the schematic:
Nothing new here compared to what youv'e seen elswhere.
The Variac is a variable autotransformer. (Thanks Jib)
The RFI filter is a gadget to reduce the radio frequency interference from getting back into the power lines. I used two 15 amp units in parallel.
C1, C2 and R1, R2 and the safety gap make up the transformer protection circuit. This circuit prevents most of the high frequencies from getting into the neon transformer where they could cause a failure. C1 and C2 are 450 picofarad 40 Kvolt "doorknob" capacitors. R1and R2 are 1K Wire wound resistors. They are actually two 2K ohm resistors in parallel.
The Cap is a MMC (multi-mini capacitor) It's made of 7 strings of 20 each .056 microfarad 1600 vdc 1500 p-p volt caps. This results in a .0196 microfarad capacitor. It should be able to handle 30,000 volts. I used Panasonic polypropylene caps. Digikey part number P10516-ND. (I'm afraid these caps cost about $300, but it seems to be durable.)
Currently the main gap is a multi-gap unit made of 1 inch copper pipe couplings. This was temporary of course but its been that way for about a year. I'm experimenting with a rotary gap described elsewhere on this site. I'll probably build a forced air multi-gap to install in the base of the coil.
I believe the best advice is to match your transformer to the discharge cap so your getting the most bang for the buck.
System Architecture
I wanted the coil to be well integrated and fully contained in the base of the coil. Another goal was to make it easily transportable. I designed the basic structure in a 3D modeling program that is very CAD-like called Rhino. (http://www.Rhino3d.com) Here is a screen shot:
I've tried various 3D modeling/rendering programs and I've had a long experiance with another popular CAD program through my work. Rhino isn't cheap but it's priced very resonable for it's advanced features.
Working in Cad let me try out a number of configurations before cutting or winding anything. I still use a quad pad and a pencil for sketching before I go to CAD. (hence the background)
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