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DRSSTC 2 – mini DRSSTC

Introduction

Most Tesla coils I’ve built so far were rather bulky devices that weren’t easy to move around for demonstrations. I wanted a small, low power Tesla coil that can still produce some formidable spark display, and a small DRSSTC was a perfect choice for this purpose. Steve Ward’s basic DRSSTC driver was again used as a base for the project, with some minor modifications from my side. The interrupter has been improved upon the basic design into a version with separate NE555 timers for repetition rate and pulse length control – again inspired by, who else but Steve Conner. However, I’m pretty certain this is the last time I’m utilizing this traditional approach in a DRSSTC: my future designs are going to be all-digital.

Project description

I originally intended this coil to be a single-board design, housing all the control and power electronics (apart from the interrupter) on a single PCB. However, my affinity towards through-hole components prevailed and I simply couldn’t fit all the required components onto a board size that wouldn’t look ridiculous against the expected size of the coil. Hence I split the power and control section into two PCB’s sitting atop each other. The coil was wound with wire about 0.1mm thick around 4x16cm form, resonating with a small topload at a frequency over 700kHz. This is a very high frequency for a DRSSTC and I’m currently looking for ways to bring it down in order to minimize losses. The power section is a half bridge of HGTG30N60A4D IGBT’s which have long history of success in DRSSTC’s. Schematics and the construction pictures follow below:

Control circuit schematic

Control circuit schematic

Someone familiar with Steve Ward’s designs will easily notice the extra “QCW comparator” I used in the control circuit. This was an experiment to test the proposal of achieving ramp modulation of the current envelope by simple replacement of the interrupter signal by a comparator fed with ramp signal – the flip flop on the output would limit the on/off behavior to full cycles, resulting in a form of bang-bang style regulator that could hopefully force the coil to make straight, sword-like sparks. The input was intended to be from sound card, but I ran into heavy problems with it’s AC coupled output and all I got in the end was a set of blown gate driver chips due to pure CW operation at 700kHz. I gave up on this idea after I decided to move my Tesla controllers into microcontroller and FPGA world. However, the comparator may still serve, for example, as a simple square-wave audiomodulation input from sound card.

Control board

Control board

In the power section, IGBT’s have an option to be used with external diodes in case they lack internal diodes. The board also houses a set of two current transformers (primary feedback and overcurrent detection) as well as the 1:2 gate drive transformer. In the early days of DRSSTC’s people tended to push IGBT’s well over their datasheet current specification, and attempted to prevent them from desaturating by overvolting the gates with 24 to even 30 volts instead of the usual maximum of 20V. I would heavily discourage both of these practices; I’ve had IGBT gates break down immediately in my circuit merely from ringing and overshoots at 30V drive voltage! Adding 33V anti-parallel zeners to the gates (not shown on schematics) and reducing supply voltage to 12V stopped the IGBT’s from dying, but regardless of that I continue to think this practice is rather unnecessary and it’s far wiser to invest in a bit bigger IGBT if there’s doubt.

Power section schematic

Power section schematic

Power section PCB

Power section PCB

And let’s not forget about the interrupter, which is, as usual, housed in a separate handheld enclosure:

Interrupter schematic. Mains supply switches are separate from the PCB and are not shown. Coil only fires while button is pressed!

Interrupter schematic. Mains supply switches are separate from the PCB and are not shown. Coil only fires while button is pressed!

Interrupter board. Potentiometers are mounted vertically in reality!

Interrupter board. Potentiometers are mounted vertically in reality!

The IGBT’s were mounted with positive pressure right over their dies, directly to heatsinks with no isolation in order to get maximum possible heat transfer to the small heatsinks used. A generous amount of airflow is also provided by a cooling fan. The energy storage capacitors are 2x 680uF, 200V which I think may be a bit borderline for this application; while they store enough energy, I’m afraid their ESR may be high enough to hamper operation in a circuit that requires 100’s of amps surge current. The primary resonant capacitor consists of 3 sturdy CDE 942C20P15K capacitors in series for a total of 50nF at 6kV.

Assembled PCB's

Assembled PCB’s. Notice the aluminum clamps used on the IGBT’s! A piece of rubber sheeting is used to even-out the pressure over the IGBT die

Front view - connectors, led's, 230V switching relay

Front view – connectors, led’s, 230V switching relay

Current and gate drive transformers

Current and gate drive transformers

Capacitors and the cooling fan

Capacitors and the cooling fan

Initial test assembly

Initial test assembly

First light

First light was achieved with another control board, that I’ve made according to my future plans of big DRSSTC revival; the small coil served as a good test.

I later constructed proper primary supports out of acrylic and welded together my toroid, forming a robust and quite nice looking assembly.

Enclosure assembly

After finishing and testing the logic PCB, I finally boxed up the coil into a pretty acrylic enclosure, with carefully crafted supports for the primary coil. Here is another video showing it making some already-satisfying sparks:

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