DRSSTC 1

A Compact Musical Tesla Coil
Double Resonant Solid State (revision 2)

10.25" x 2.4" Secondary, 60N65 halfbridge at 340VDC, 222kHz, >22" Sparks


 Jan 2013 

Index

Introducing my first Double Resonant Solid State Tesla Coil, which will be referred to as DRSSTC 1 from now on. In many ways, this Tesla Coil represents a critical signpost in my Tesla Coiling journey since 2003 when I first built Tesla Coil 1. Last year in 2012, I completed my first Solid State Tesla Coil 1, which was a great learning experience. Through the project, I learned many things about SSTCs and decided to embark on my first DRSSTC.

The DRSSTC was invented sometime around 2005, and differs from a traditional SSTC with the addition of a primary tank capacitor. The primary current flow is now much higher, and these new breed of SSTCs essentially represents the most efficient, controllable and efficient Tesla Coils to this day at time of writing. Nikola Tesla would have been pleased, I think!

This webpage serves to document the design, construction, lessons learned and results of my first DRSSTC, and I hope it will be a source of information and inspiration to the Tesla Coiling community - a community which I've had the pleasure of interacting with and learning so much from. The goal at the end of the project is hopefully to design and construct one more powerful DRSSTC with a spark length exceeding my venerable Tesla Coil 2. For now, the goal of DRSSTC 1 is to produce a reliable, beautiful and powerful table-top DRSSTC.

I do not know of any other DRSSTCs built in Singapore by any fellow friends in Singapore. DRSSTC 1 may be the first of it's kind in my little sunny country! Thank you for visiting my page and if you have any questions, wish to share your projects, or feel that my projects have inspired you in one way or another, feel free to email me at loneoceans[at]gmail(dot)com.

Page Contents

1. Introduction
2. Design
3. Construction
4. Results & Media
5. Useful Links

Updated DRSSTC 1 Specifications and Status

  • Current Project Status (Mar 2013) - DRSSTC 1 has now gone through revision 2 and has spectacular performance!
     
  • Max spark length – I don't know yet, does >22″ sparks to free air and ground with ~100us on-time.
    More testing to come soon. Right now it's limited by hitting the floor.
  • 222kHz resonant frequency (within 1%)
  • 2.4″ Diameter PVC Secondary, ~2000 turns of AWG36 for a 10.25″ length
  • 3.5″ Diameter Acrylic Primary, 6 turns of AWG 14 for about 0.8″ length
  • Single 0.101uF 2kVDC 942C CDE Primary Capacitor
  • 340VDC (120VAC rectified and doubled)
  • Roughly 100us on-time
  • 2.2″ x 8″ AmazingOne spun toroid
  • Half Bridge of 60N65 IGBTs (seems to be similar to the 60N60 TO247 IGBTs)
  • Coupling = 0.22
  • Energy Transfer time = 9.87us

Overview Video

 
DRSSTC 1 rev 2 plays music - as controlled via a Midi keyboard, ~100us or less on-time per pulse.

For much more videos and images of the coil in action, please scroll down to Results & Media!


 Feb 2013 

Introduction

The double resonant solid state tesla coil perhaps the pinnicle of Tesla Coiling at least for now (though very fancy QCW models are on the horizon). Having finally built my first SSTC last year, I figured that it would be a good time to build my first DRSSTC.

As my first DRSSTC, this was primarily another learning project for my future up and coming brick-IGBT coil. I wanted to go for something simple, small, and mangeble. I've had the fortune of being in the good company of other brilliant coilers (like Bayley, Tyler, Kramnik), and managed to get an older pre-production version of the oneTesla driver PCB, which runs this coil. The circuit was inspired by Dan McCauley's EVR single-board MicroBrute DRSSTC, and is essentially based on the fantastic DRSSTC 0.5 design pioneered by Tesla Coil wizard Steve Ward. This coil currently has a tiny built-in 19VDC laptop power supply for the electronics which uses the famed UCCs from TI as gate drivers to a GDT driving the gates of a halfbridge of 60N65 IGBTs. These transistors have been pushed to about 300A and seem to run reasonably reliably.

Over the past few weeks constructing the coil, I've modified several parts and faced several setbacks, but have finally managed to get the coil working very well! Originally, the coil used a 0.068uF 940C capacitor but that made my primary frequency about 20% too high and I lost a 60N60 half bridge. Increasing the primary to 8 turns of 14AWG (k=0.27) turned out to be a bad idea and I had a lot of racing sparks. After a few more coats of Polyurethane on the secondary, a return to 6 turns of AWG14 and a larger tank cap, the coil is in tune with a performance I'm very happy with.

There are many people I'd like to thank for giving me invaluable help on this project. Most of all I'd like to take the folks at 4hv.org for providing a wealth of information over the years on Tesla Coil design. Also, many thanks must go to Steve Ward for his invaluable DRSSTC designs which have become the basis of many coils. The driver is based on oneTesla's older driver, as is the MIDI interrupter first designed by Kramnik. I'd also like to thank Bayley for helping out with much of the debugging of the coil, as well as for lending me a lot of materials and parts for the project. Finally, I'd like to thank Philip and Rob for their advice and inspiration with my Solid State Tesla Coils.

(.. more to come soon! )


Design and Schematics

The design of the SSTC can be basically split into: [1] The Logic Control Circuit [2] The Power Circuit [3] The Interrupter and others. This section explains my design choices, as well as what I have learned through the progress of this project.

This section will contain some technical jargon. If you're unfamiliar with electronics in general or Tesla coil operation, please visit my Introduction to Solid State Tesla Coils page to learn all about how a Solid State Tesla Coil works and basic introductions to the electronics workings! (Under Construction, March 2013)

1. Logic Control

More to come soon.

2. The Power Circuit

More to come soon.

3. The Interrupter

More to come soon.


Construction Details

This section details the construction specifics of DRSSTC 1. Being a coil of small stature, it took surprisingly little time to piece together. The main enclosure was made from two pieces of green acrylic, held together via four threaded rods. Two end-caps were machined to fit perfectly in the secondary, which are screwed on via two nylon bolts each. The end-caps are tapped to accept a 10-24 nylon bolt which secures the secondary coil to the base.

Over the course of several weeks, DRSSTC 1 went through a few revisions, each time improving the design both mechanically and electrically.

 01 Feb 2013 

Revision 0 -Preliminary Designs

Based on the design choices above, the first design of the DRSSTC 1 used a single 68nF polypropylene metallized film capacitor as the Primary Capacitor. This was a 3kVDC 940C series CDE capacitor, well suited for use in Tesla Coils due to its robustness and proven reliability. The primary coil was set at 6 turns of coarse-stranded 14AWG wire which was available on hand, wound on a 3.5" diameter acrylic cylinder, with the bottom of the primary coil just about half an inch above the bottom of the secondary coil.

 

I was still waiting for the varnish to dry on my just-wound secondary coil, so I borrowed a friends' secondary coil which was wound on the same-sized PVC former. This secondary coil was 2.4" in diameter, wound to a length of just about 10.5". Although it was wound with AWG36 wire, it was wound quite loosely with an estimated 1700 turns of wire. For the topload, I used my home-made form toroid, measuring about 2.2" x 7.5".

This combination produced good results as you can see, with the interrupter set to a maximum of 145us on-time. However, it was clear that there was something amiss - the coil needed to be tuned. Right now, the coil was struggling to produce ground strikes, which should be doable given that the breakout point is just over a foot off the ground. Therefore, I made a few revisions to the coil.

 21 Feb 2013 

Revision 1 - live and learn

My freshly wound secondary coil was finally complete - the specifications are as follows: 2.4" diameter, 36 AWG wound for 10.25", just about 2000 turns. This was one of the most challenging secondaries I've wound, taking just about 3 hours to wind with some rest breaks in-between. It was coated with about six-layers of spray-on Polyurethane varnish. Being overly excited, I swapped the old secondary I borrowed and replaced it with the new secondary. This was a problem.

I turned on the coil and it did work well, producing sparks, but the coil was producing much smaller sparks than expected, without any ground strikes even at 145us pulse-width. Eventually, the coil gave up and lost it first set of 60N60 IGBTs. :( There was also no fuse installed. With the IGBTs failing closed-circuit, this led to one of the PCB traces burning up.

After some calculations with JAVA-TC, I came to the realization that the Tesla Coil was very out of tune. The primary resonant frequency with 6 turns of AWG 14 and the 68nF capacitor gave a resonant frequency of about 270kHz, while the new tightly-wound secondary gave a resonant frequency closer to 220kHz. This meant that my system was greater than 20% out of tune, producing disastrous results with the extra energy sloshing back at my IGBTs and killing them. In a bid to salvage the situation, I did some recalculations on JAVA-TC.

The calculations showed that if I kept the same tank capacitor but increase the number of turns from 6 to 8, I should be able to get both my primary and secondary frequencies to match up at 220kHz. I replaced the primary as calculated, and did a few improvements to the power circuitry - which means cleaning up the burnt trace and fixing it with a wire and adding a 10A fast-blow fuse to the primary circuit. I also decided to make the coil even more portable by integrating its own 19V power supply unit (a 19V 2.1A SMPS for small netbooks I bought off ebay for $4). In addition, I installed a power-bus switch to turn on the bus capacitors only after the 19V power supply had kicked in. The 60N60 IGBTs were also replaced with 65N60 IGBTs, which, according to the datasheets, suggest that they are essentially similar, but the 65N60s are a dollar cheaper and come in a slightly different package. I used a larger heatsink and replaced the fan with a more powerful one. Finally, I borrowed a 8" x 2.2" AmazingOne aluminium spun toroid from Bayley to finish off the coil.

As a result, the entire setup is now very compact and clean, with only 1 IEC calbe required for power and 1 fiber optic input. It was time to test the revised coil!

I plugged in the coil and slowly ramped up the power - it's working! It was obvious that the coil was much more in tune, producing far larger sparks than previously. However, there was immediately a huge problem - racing sparks up and down from the secondary to the primary coil. Indeed, while 8 turns had produced the correct resonant frequency, it also upped the coupling from k = 0.22 to k = 0.27, probably far too much for this geometry to handle.

The photograph shown here shows the only photo I got before the racing sparks forced the test to end. Notice the corona glowing off the top turn of the primary. Note that this was running at very-low pulse widths, perhaps around 75us or so, and already producing sparks similar to revision 0. Unfortunately, it's back to the drawing board..

 23 Feb 2013 

Revision 2 - math saves the day

I spent a whole day sitting in front of Java TC trying to find a way to optimize the coil, and finally came up with a solution. I managed to find a spare 0.1uF 940C series CDE metal-foil polypropylene film capacitor from another tesla coil project. Turns out that if I replaced the primary capacitor with this larger one, and rewound my primary with a new 14AWG multi-stranded cable with exactly 6 turns, I should be able to bring the primary resonant frequency down to 222kHz, which should in theory match my secondary within 1%! Of course we usually want to tune our primary such that the secondary frequency is slightly higher than the primary. During operation, sparks form which increases the capacitance of the secondary circuit and decreases the resonant frequency. This phenomenon is known as streamer loading. However, I decided to go with the near perfect resonant combination and hope this would be good.

This new geometry drops the coupling to k = 0.22. In addition, the use of a very-multi-stranded primary wire should help in handling more current due to a slightly greater surface area (due to the skin effect at 222kHz). I checked and made sure everything was good and hot-glued the primary in place.

I tested the coil with trepidation. Fortunately, all the calculations had paid off and the coil performed remarkably and far better than I had hoped. With the interrupter set to a modest 75us, I was easily getting ground strikes! The sparks are now far more intense, louder and brighter than before with even less pulse-width. It looks like I might have hit the sweet spot of resonance! I decided to set the interrupter to a max of 100us to increase reliability of the coil. After many weeks of hard work, I am now happy to say that the coil is now performing as I had hope to be, and some more!

This revision is about as good as this coil will get for now, given that it's already producing sparks wayy bigger than itself. The coil is now working as designed! Continue below to see more photographs of it in action!

 

DRSSTC 1 rev ii is now working as planned!


Results - Photographs and Videos

 02 Feb 2013 

First light! After several weeks of construction, DRSSTC 1 is ready for its first light.

Note that the photographs above are of revision 0, with the coil running at around 270kHz resonant frequency with a half bridge of 60N60SMD IGBTs.

The above shows a video of the coil in action. Notice how even with 145us on-time, the spark length is not terribly impressive.


Most interesting here is a close up of some of the sparks hitting Tim the Beaver! Note that you can see how each 'note' is actually comprised of many individual sparks, each created with one pulse event lasting about 145us. Each spark heats up the air around it, hence the second spark is likely to follow a similar path as the partially ionized air-path is easier to go through. This results in sparks forming in parallel. Fascinating!

 24 Feb 2013 

The following photographs show revision 2 of the coil running at < 100us with much better performance than revision 0.

Performance is very impressive and I am surprised as to how well the coil is performing! The photographs speak for themselves.

I also managed to get a few photographs of the ground-strikes on concrete, which look absolutely fascinating!

I also tried to set up a quick and easy spark-length test. The metal rod on the left is simply balanced on a push-cart with rubber wheels, so it's not exactly the best ground-strike test, but it does show how easily DRSSTC 1 makes 22 inch sparks. Right now, most of the sparks are simply hitting the ground. I will need to do a more accurate ground-strike test by aiming the breakout point upwards.

 09 Mar 2013 

More tests of the coil in action now featuring a light bulb!

I also made a quick overview video of the coil in action. It's the same video as above, but shown here again for convenience!

Enjoy!


Good Links

Here are some nice and very useful web pages.

More to come soon.


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(c) Gao Guangyan 2013
Contact: loneoceans [at] gmail [dot] com

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