Tesla Coil Transformer
HV RF Filter

This page is the area where I explain some of my projects I am currently working on, as well as those that have been completed in the very recent past but are relevant still. Any questions about them are welcome and can be submitted by way of the feedback page and will be answered in some way or another... Since it was obvious that the high voltage RF signal was coming back up the line into the transformer on the Tesla coil, I decided (and other people decded for me) that I should build an RF filter between the transformer and the tank circuit of the coil. I was considering building a copy of "Terry's Filter" except that it would require me to buy a bunch of parts that I don't have sitting around the lab. Since I only buy things as a last-ditch effort, I decided to make my own from scratch using what I did have on-hand. What I did have was a bunch of 4-15 Henry chokes, and a few 460pF 30kV doorknob capacitors, among many other things.

This is the schematic for the Terry style filter for NSTs. I'm not buying 16 metal-oxide varistors just to make a filter. I'm not sure why the person drew the varistors as current sources, but whatever. This is the physical setup of the prototype of my filter. The concept, put simply, is that high inductances block high frequencies and pass low frequencies linearly, and small capacitances pass high frequency and block low frequency and DC. A simple filter to pass low frequency 60Hz and block high frequency RF would have high inductance in series and low capacitance to ground in parallel. I might put a safety gap in also, but safety gaps are stupid so probably not anytime soon. This is my a shot of the filtered 500kHz versus the unfiltered 500kHz. The unfiltered is the odd-looking sine wave (yes, the university's function generator is a junk heap, my good one was at home) and the filtered signal is the nearly-zero ripple. I think it works, and all the components are good for the voltages. I'm looking forward to the full-power test tomorrow.

This is the schematic for my filter. I am not sure what the inductances total to, I am going to solder and measure tomorrow. I mounted all the inductors on a piece of plywood that I had cut to an appropriate size, which I had intended on mounting on the front of the Tesla cart vertically, but I am seriously reconsidering now, given the approximate 40 pound weight to it. The red doorknob capacitors are above on the lab bench, out of the photo. This is version 2.0 of the filter, since at higher powers the filter tended to burst into flames. These spark coils (hopefully) at worst will only have the mineral oil boil over and make a little bit of a mess. That's the plan, anyhow.