SSTC design guide

Published: July 15, 2019. Updated March 19, 2020.

This article is to point out some of the design decisions and calculations that is different from a DRSSTC, so this article will not contain a description of all parts used in a SSTC. The following topics are covered by the DRSSTC design guide as the guidance and best practices is the same.

As an example in this guide, I will use the dimensions and properties of my Kaizer SSTC 2 Tesla coil. It is a full-bridge IRFP460 running at 250 kHz, making up to 47 cm sparks, see all construction details on the link.

Primary peak current calculation

To do an easy primary current calculation, to know if the MOSFETs can drive the load, it can be simplified by ignoring primary resistance and DC blocking capacitors reactance as they are very small factors.

First calculate the primary coil inductance for your helical or spiral primary coil. Use one of the two links for the online calculator.

Next step is to calculate the primary coil reactance. f is frequency in Hertz and L is inductance in Henry. Values are written in kHz and uH for ease of reading. The 8 turn helical coil with a diameter of 115 mm, with 1.78 mm wire and 2 mm spacing has a inductance of 10.16 uH.

X_{L}=2 \cdot \pi \cdot f \cdot L

X_{L}=2 \cdot \pi \cdot 250 kHz \cdot 10.16 uH = 15.95 \Omega

The peak current for the peak-to-peak square wave voltage envelope can now be calculated using Ohm’s law. I supplied my coil from full-wave rectified 230 VAC, which multiplied with square root (2) for the peak voltage is about 320 VDC.

\text{Primary peak current} = \frac{Voltage}{Resistance X_{L}}

\text{Primary peak current} = \frac{320 VDC}{15.95 \Omega}=20 A_{peak}

Conclusion on primary peak current. We now have a basic measure for how much current we are trying to push through our MOSFETs and primary coil. The MOSFETs should as a minimum be able to withstand this, with a safety margin added on top. Voltage rating should have around 33% head room, so if you are feeding the inverter 320VDC, a 600V MOSFET is to prefer.

Primary Geometry and Coupling

There is generally four shapes of primary coils.

  • Flat spiral coil (Used in SGTC and DRSSTC)
  • Helical coil (Used in VTTC, SSTC and DRSSTC)
  • Cone coil (Used in SGTC, DRSSTC)
  • Half-circle coil (Used in QCWDRSSTC)

A SSTC will almost always use a helical coil with a high and tight coupling to the secondary coil. Due to the relatively low primary circuit current it is necessary to have a primary geometry that gives a high coupling to get a good energy transfer.

In my SSTC constructions I have often used regular machine tool wire wound directly around the base of the secondary coil, with nothing more than 2-10 mm of insulating material in-between and also to make it able to adjust coupling by moving it up or down. The insulation needs to extend further than the primary coil to avoid flash over damages, as I have experienced.

DC Blocking Capacitor

The DC blocking capacitor is named after its purpose, to block any DC component of a signal to enter the transformer being driven by a half- or full-bridge. A DC offset current may cause an unbalance of the transformer that initiates a runaway process that ends up with the transformer saturated and the large current drawn in this mode will damage both transformer and MOSFETs. [1]

The DC blocking capacitor can either be in series with the primary coil for a full-bridge or in series with the primary coil for a half-bridge that connects to ground. For a half-bridge it can also be two capacitors forming a voltage splitter with a midpoint where the primary coil connects to, this voltage splitter can also be used in a voltage double configuration. The most important factor is that it is a very low ESR capacitor, in order to minimize the switching losses across it. Generally this means that the same type of MKP capacitors used in a MMC is suitable for DC blocking capacitors, given the capacitance is suitable. Below the DC blocking capacitor is the blue RIFA sitting in series with the black wires going out to the primary coil connectors.

There is two factors to calculate for the DC blocking capacitor. The capacitor reactance ratio to the inverter output impedance and the resonant frequency of the primary coil L and DC blocking capacitor C is much lower than the resonant frequency of the secondary coil circuit.

First we can calculate the DC blocking capacitors reactance. f is frequency in Hertz and C is capacitance in Farad. Values are written in kHz and uF for ease of reading. I used two 0.68uF X2 MKP capacitors in parallel.

X_{L}=\frac{1}{2 \cdot \pi \cdot f \cdot C}

X_{L}=\frac{1}{2 \cdot \pi \cdot 250 kHz \cdot 1.36 uF}=0.468\Omega

So now we can check the resonant frequency against the Tesla coil secondary circuit frequency. f is frequency in Hertz, L is inductance in H and C is capacitance in Farad. Values are written in kHz, uH and uF for ease of reading.

Frequency=\frac{1}{2\cdot\pi\cdot\sqrt{L\cdot C}}

Frequency=\frac{1}{2\cdot\pi\cdot\sqrt{ 10.16 uH \cdot  1.36 uF}} = 42.8 kHz

Being 5 times lower than the resonant frequency, there is no risk at the primary LC circuit resulting in a DRSSTC condition which would destroy the MOSFETs.

The reactance of the two capacitors was 0.468 Ω and the inverter output impedance should be higher than this.

\text{Inverter output impedance}=\frac{\text{Voltage output}}{\text{Current output}}

\text{Inverter output impedance}=\frac{320 VDC}{20 A}=16\Omega

It is worth noting that the inverter output impedance should be almost identical to the primary coil reactance as the pure Ohm resistance of the primary coil is very small.

The DC blocking capacitor reactance is 32 times smaller than the inverter output impedance and the design requirement is fulfilled. A lower capacitance would result in even smaller losses in the DC blocking capacitor.

Conclusion on DC blocking capacitors is that they should behave like a dead short at the resonant frequency of the Tesla coil or inverter. So anything below 2 Ω reactance should be considered to work.

Gate Drive Current

Many different driving methods are used for SSTC’s and their half- or full-bridge of MOSFETs / IGBTs. Unlike the DRSSTC universal drivers, there is a wide variety of gate drive ICs, transistors or other implemented in different SSTC driver circuits.

It is important that there is enough current and fast enough rise time when driving the gates of the MOSFET/IGBT that switching losses are minimized as much as possible.

Use the MOSFET / IGBT Gate Drive Calculator to estimate the required peak current needed and RMS power consumption for the power supply design.

References

[1] Alexander Gertsman, Sam Ben-Yaakov, “Zeroing Transformer’s DC Current in Resonant Converters with No Series Capacitors”, Department of Electrical and Computer Engineering, Ben-Gurion University of the Negev, 2010.

How I came to build a 4 meter spark generating Tesla coil, a technical story from 2008 to 2016.

Here is the recording of the live stream I did on 2018 January 31, Wednesday at 2000 CET, I performed a live stream on youtube …

SSTC and DRSSTC musical modulator

Since Martin from ctc-labs.de decided to close down his website, which was all written in German, I asked him for permission to translate and publish …

Musical SSTC/DRSSTC interrupter

All credit for this article goes to Martin Ebbefeld from ctc-labs.de, after he closed down his website we made an agreement of me hosting some of his content. I translated his article as best as I could from German to English and added a few more details or information. The pictures are unfortunately in a low resolution, but it is for now all that is available.

A first few thoughts on how music can be played with a DRSSTC. The high current primary waveform that is used in a DRSSTC can not just be frequency modulated to amplify a analog signal as played back from a regular music source. The stress would be too great, the heat dissipation would destroy the IGBTs.

So what do we do?

A lot of clever and experimenting amateurs around the world have implemented different methods to be able to reproduce tones with a DRSSTC, tones that are equal to what a piezo speaker can produce, but almost all music is not that simple.

The high frequency resonant signal that is used to drive the bridge comes in small packages of 100 to 300 us length, this does not leave us much head room to modulate in. What we can do instead is to modulate the time that lies between the pulses, the so-called off time. Either by MIDI signals or from a simple analog to digital conversion signals as it is done in this article. (To be able to understand the following text and piece it all together, it could be necessary for the reader to study basic operation of a DRSSTC)

The simple circuit used here is a development of a circuit earlier made for other purposes, it could now be customized to work as music interrupter. We can not simply play back our favorite piece of music, we either need it in MIDI format or as a raw melody where we will have to edit it by hand.

Although there are already programs freely available for micron controllers to process MIDI signals in real time to a series of pulses to drive a DRSSTC over fiber optic cable, that method can be too complex for some experimenters. This article uses old and easy to understand analog components.

A normal DRSSTC will operate with a break rate of around 120 to 200 Hz (off time). This means that the time between pulses is 1/200 = 0.005 = 5 milliseconds. The characteristic buzzing sound is a product of the 5 millisecond off time and if we adjust the off time to be less, the distances between the pulses will be shorter and the sound will rise in pitch and appear as higher tones.

The idea is to match the input to a list of real tones, a awesome and brilliantly simple method. So how is this implemented in the modulator, the following examples and diagrams will explain this.

Tabel 1: Almost entire piano key list. Middle C (C4) and Tenor C (C5/treble C) with their respective frequency in Hz
Note C1 C2 C3 C4 C5
C 32.70 65.41 130.81  261.63 523.26
Cis 34.69 69.29 138.59 277.18 554.37
D 36.71 73.42 146.83 293.66 587.33
Dis 38.91 77.78 155.56 311.13 622.25
E 41.2 82.41 164.81 329.63 659.26
F 43.65 87.31 174.61 349.23 698.46
Fis 46.25 92.49 184.99 369.99 739.99
G 48.99 97.99 195.99 392 783.99
Gis 51.91 103.83 207.65 415.3 830.61
A 55 110 220 440 880
Ais 58.27 116.54 233.08 466.16 932.33
H 61.73 123.47 246.94 493.88 987.77

Middle C, called C4 is the octave called one-lined and Tenor C (treble C) is the octave called two-lined. The above table shows the frequencies in Hz for the different scales and are within those that a Tesla coil could play without going into extreme conditions. The inability to play higher tones lies with the ratio of on time and off time that becomes very unfavorable for the system.

A proportional limit between on- and off-time should be kept to maximum on-time to be 10%. This means that the highest playable note that a Tesla coil should play to avoid any damage would be maximum 10 times the maximum allowed on-time.

To understand this, lets look at the following illustration.

The simulation above shows a interrupter signal with 10% duty cycle which represents pulses that are 200 us long, which approximately corresponds to the pulse length in a normal DRSSTC, this is the period of time where the coil operates at its resonant frequency. Some coils operate with shorter pulses due to faster ring up time in low impedance primary circuits or some need longer pulses if they have a primary circuit with a higher impedance.

With a duty cycle of 10% and on-time lasting 200 microseconds, the length of the off-time ,until the next rising edge of the on-time, is 1.8 milliseconds.

\mbox{1.8 ms}=\dfrac{1000}{1.8}=555.55 Hz

555.55 Hz almost corresponds to the tone Cis in C5.

The reason to not play notes higher than 555 Hz has to be found in power dissipation limits, tolerable temperature rise and overall stress of the IGBTs, read my IGBT design guide for more details on this. The closer together the on-time pulses are, the more often the bridge is on feeding energy to the resonant circuit of the DRSSTC. More heat is dissipated, it is a larger load and power consumption rises dramatically. If the off-time is short enough, it may happen than that pulses start to overlap the ring down period of the damped oscillation and the current in the primary circuit no longer settles back to zero, but oscillates between the lower and upper limit which eventually will lead to failure in one way or another.

Modulation

We need to use a source that can feed our modulator with tones according to the sheet in table 1. This can be a synthesizer, whether it is software or hardware does not matter. We can however not use a normal music CD, it would only result in a chaotic mess of pulses that would not produce any good results on a DRSSTC. To get the best result we have to take out a part of the song, preferable the melody or lead guitar, something that is simple and recognizable for the song. With the method this modulator uses it is possible to convert the length and volume of each tone to a pulse length and exact pitch that the can used to drive a DRSSTC into playing audible tones.

Another example will follow.

In the above illustration the signal can be seen, from which the modulator has its pitch and tone length. Using the internally controllable on-time of the modulator it can vary the volume.  The highlighted area in the illustration is about 2.154 milliseconds long and that corresponds to:

\mbox{2.154 ms}=\dfrac{1000}{2.154}=464.25 Hz

464.25 Hz almost corresponds to the tone Ais in C4. With a deviation below 2 Hz.

These pulses was created in a music software where it is possible for a synthesizer to output square wave pulses. The tone lasts for 65.034 milliseconds and thus corresponds to a 1/16th note, since 1000 divided by 65.034 gives 15.376 and it roughly is 16 bangs per second, if you include the short breaks between each stop note.

In the above illustration we will take a look at a low tone for comparison.

The length of the note corresponds to the earlier illustration, it is also about 1/16th note, but with a much more coarse structure.

The length of one period of the pulse is here 8.66 milliseconds

\mbox{8.66 ms}=\dfrac{1000}{8.66}=115.47 Hz

115.47 Hz almost corresponds to the tone Ais in C2. With a deviation below 1 Hz.

This is a very low tone and DRSSTC does actually handle low tones quite well, also the so-called base line notes known from base guitars.

However these square wave pulses lasts for too long and the duty cycle is up around 50%, which is much higher than our initial design specification of a maximum 10%. So we need something in the modulator to make a more useful signal out of the very high duty cycle signals.

Circuit

Generating the tone sequences described above by the means of software is the most difficult part of this modulator. Without properly prepared signals from soft- or hardware there will be no singing Tesla coil from this modulator.

The simplicity of the circuit makes it easy to debug and possible also to expand with more features. It consists of a operational amplifier that works as a pre-amplifier so that it is possible to use low level audio signal from a CD, PC or keyboard.

The input audio signal is filtered and amplified by the LM741 operational amplifier. It is further amplified by the two BC547 transistors and converted to a square wave equal to the output of a Schmidt trigger. So it is possible to feed this modulator with sinusoidal signals and get a proper square signal to drive the DRSSTC with.

The most import part of the circuit is the 555 timer, the main task of the modulator is handled here, which is to ensure a correct length of the pulses as to not exceed the 10% duty cycle.

In the above illustration we have the input sound signal in green, the signal that is fed to the LM741 operational amplifier. This signal is conditioned, amplified, inverted and inverted again before it lands at the trigger input of the 555 timer.

If the trigger input of the 555 timer is held high, nothing will happen on the output and the oscillator circuit output made with the 555 timer is on standby. When a series of pulses are put into the modulator, the second BC547 transistor connects the 10 nF capacitor to ground and the trigger input of the 555 timer is momentarily pulled to ground. This causes the 555 timer to output a pulse of precise length determined by the potentiometer and the 22 nF capacitor. The signal shown in red is the output signal from the 555 timer.

The distance between the rising edge of the red pulses corresponds exactly to the distance between the rising edge of the green pulses, that was our input, so the important information as pitch is followed through. Everything that is done with this circuit is to cut off the unnecessary part of the square wave. The on-time, the width of the red output signal, can be regulated with the potentiometer to determine how much power the coil is allowed to use.

In the following illustration is can be seen that varying pitch and sequence of the input signal is used to generate a series of pulses that can be used to drive our DRSSTC with.

Chords

A last sensitive issue needs to be addressed. Very pleasing and cool sounds can be made by playing more notes simultaneously, also known as chords, but it does present some interesting issues with this simple modulator.

A square wave in itself is not able to express a chord as it can only represent one state. Different synthesizers overcomes this problem in different ways and some uses a intermediate frequency where it alternates between the two notes used in a chord so that for each X’th part of a chord it plays the two notes like this A, F, A, F, A, F, A etc. This method can also be simulated in some software synthesizers.

Chords does however also last longer and a short allowed on-time might give interesting results where the sound is not faithfully reproduced to the source. Chords are not recommend with this modulator, but experimentation could get interesting.

 

Demonstration

Music is played from the speaker output of a childrens toy keyboard connected to the input of the audio modulator. The Tesla coil demonstrated in these videos it is the Kaizer SSTC 1 in a state where it was being rebuild to the Kaizer SSTC 2 where the implementation of the modulator is shown in the schematics.

Topload design and selection for Tesla coils

Published on: Sep 24, 2015. Updated on Oct 03, 2018.

This is chapter 10 of the DRSSTC design guide: Topload

Intro

The topload of a Tesla coil serves more than one purpose. The most important and why it is there in the first place is that it acts as a capacitor that can store the high voltage charge and it will also lower the resonant frequency of the secondary circuit.

The magnetic field around a topload will prevent sparks from forming on the top winding of the secondary coil and also in some degree prevent sparks from going inwards towards the secondary coil, primary coil and strike rail.

It also adds more distance out from the center and height to the entire assembly to avoid spark length being limited if it would always strike to the ground because of a short distance.

 

Shapes

Round and smooth surfaces are preferred to get the longest possible sparks when corona losses is avoided. A uneven surface will result in many small sparks, barely visible to the naked eye where energy is dissipated and this means there is less energy to be put into the spark you are aiming to get.

Round and smooth surfaces almost only leaves us with two shapes, spheres and toroids.

Spheres can result in problems with sparks going downwards to strike the primary coil or result in racing sparks along the secondary coil. This is caused by the electromagnetic field shape from the sphere. If a Sphere is used it is best to place the break out point on the top of it so sparks would be forced as far away from the secondary coil as possible.

Toroids gives the best electromagnetic field shaping where sparks are much less likely to travel inwards towards the secondary coil. If the diameter of the toroid is very large a secondary smaller toroid can be used underneath to ensure that there is no breakout from the top winding of the secondary coil.

 

Solid vs skeleton

From an efficiency point of view it doesn’t really seem to matter whether you use a solid metal toroid, or a piece of flexible ducting bent round into a ring, or a skeleton frame made out of tubing, they all work much the same. However, if you are wanting to run the coil without a breakout point, you’ll find that a smooth surface gives a single large streamer that wanders around the topload, whereas a rough one can generate several shorter streamers.

 

Do use these materials

Copper: One of the best conductors in terms of price/resistance ratio.

Aluminium: About 2 times the electrical resistance of copper.

Corrugated aluminium air duct is made entirely from aluminium that is twisted to form a continues lock seam.

Two frying pans mounted opposite against each other can form a small, uniform and cheap topload. The goes for two savarin cake molds, mounted opposite against each other, they come in a wide variety and some even have a filled middle which makes them easy to mount on a secondary coil.

Styrofoam torus/donut/toroid shapes from a hobby store covered in aluminium tape makes for a light weight and cheap toroid, it can however be hard to get a very smooth surface.

Two bottom parts from beer or soda cans can form a tiny topload.

Brass: About 4 times the electrical resistance of copper.

For connections through a threaded rod or bolts to mount the topload on, brass is the prefered material as its resistivity is still good compared to how rigid and strong it is.

 

If possible, avoid these materials, they are not optimal

Metallized polyester flexible air ducting: The polyester film is very thin and only rated for a maximum operating temperature of 71 C. A break out from the surface of the film could easily damage or melt the film. The spiral is made from spring steel.

Stainless steel: About 40 times the electrical resistance of copper.

The stainless steel salad bowls from Ikea are cheap, almost round and easy to build a sphere from.

 

Current and losses in toploads

Udo Lenz at the 4hv.org forums made some very specific measurements on topload and arc current on a DRSSTC on three different power levels, where the highest was producing 80 cm arcs. His measurements for the DRSSTC was inspired by the measurements by Greg Leyh from the worlds largest SGTC Electrum.

UPDATE: these links are now dead, try archive.org to find a copy, pictures of the coil can be seen here: https://www.lod.org/electrum.html

Hydron at highvoltageforum.net has made some other very specific measurements with a battery driven data recorder: https://highvoltageforum.net/index.php?topic=117.0

The current through the topload is

\mbox{Power dissipation}=\mbox{Current}^{2}\cdot\mbox{Resistance}

The current through the topload is given (at a given operating point of your coil). If you increase the resistivity, the dissipation in the topload goes up (see equation above) and so your Q factor goes down. If the topload was superconductive, it would not dissipate anything and won’t do anything to your Q factor.

 

Choosing minor and major diameter

A rule of thumb for choosing the minor diameter for a DRSSTC topload toroid is to take the secondary coil diameter and use the same value.

Kind of the same rule can be used for the major diameter, take the length of the secondary winding and that is your toroid major diameter.

These rules apply if you follow the general design guidelines for sizing of the secondary coil.

The minor diameter will affect the breakout voltage, so if the minor diameter is too large then the coil will not break out without a break-out point. Too small a minor diameter will cause it to break out at a lower voltage depriving the coil of some of the voltage build up on the toroid, usually resulting in multiple smaller streamers.

The major diameter affects the field control under the toroid, so too small and you will get strikes to the primary coil / ground rail or if way too small strikes to the secondary coil which might destroy it!

 

Choosing distance to top winding of the secondary coil

The underside of the toroid should align with the top winding of the secondary coil or be raised a distance at up to secondary coil diameter.

 

One or two toroids

If you want to raise the topload higher above the secondary than the above figure, it will be a good idea to add a second smaller toroid for field shaping. It can also be necessary to add a second smaller toroid if the main toroid is very wide so field shaping around the secondary coil top is weak.

 

Calculating the capacitance of a sphere

K is the dialetric constant 1.01, R is the radius of the sphere. This formula is using inches for measurements, 10 mm = 25.4 mm. Result is in pF.

C=\left ( \frac{K\cdot R}{9\cdot 10^{9}} \right )

 

Calculating the capacitance of a toroid

D1 is the major diameter of the toroid and D2 is the minor diameter of the ring of the toroid. This formula is using inches for measurements, 10 mm = 25.4 mm. Result is in pF.

C=2.8\cdot \left ( 1.2781-\frac{D2}{D1} \right )\cdot \sqrt{\frac{2\cdot \pi ^{2}\cdot \left ( D1-D2 \right )\cdot \left ( \frac{D2}{2} \right )}{4\cdot \pi }}

 

Breakout point

A breakout point should be a good conductor with a even surface to avoid a large corona spray all around the wire, rod or what is used for it.

The tip of the breakout point can be tungsten if heating or even melting damage to the tip is a problem.

The breakout point has to stand out far enough from the very close and heavy magnetic field that is close to the topload surface. At least 10 centimeter would be a good start, even longer and upwards pointing can be used to get arcs further away from striking down in the primary coil, earth rail or just get longer arcs to ground from the bigger distance.

Running a DRSSTC without a breakout point can be very stressful on the IGBTs as the Tesla coil have to build up a large potential on the surface on the topload for it to break down from the very even surface. The IGBT inverter is much happier when it can just keep feeding energy into a arc.

Tales of running a DRSSTC without a breakout point tells that nothing happens until it almost hits full input voltage from f.ex. a variac. Suddenly large arcs will start lashing out in unpredictable and random directions. Half of the time the sparks would either be from topload, racing sparks on secondary or even inside of the secondary.

 

Construction

The art of making a metal toroid is called metal spinning. A sheet of metal is formed into shape with a metal rod against several different wooden master models mounted in a lathe.

Aluminium ducting is easy to mount around two circular pieces of wood with spacers between them.

 

Previous topic: Secondary coil Next topic: Tuning

Kaizer SSTC III

Introduction

The idea was to build a very small and compact Tesla coil as a gift for my mother that works in various science classes for the lower grades in public school.

This driver circuit is very similar to the one used in Kaizer SSTC I. This time I have made a PCB containing both driver circuit and bridge.

 

Safety

WARNING!: Working with electricity is dangerous, all information found on my site is for educational purpose and I accept no responsibility for others actions using the information found on this site.

Read this document about safety! http://www.pupman.com/safety.htm

 

Considerations

I knew this would get claustrophobic with so little space for a complete interrupter, driver and bridge.

Using the enclosure as the heat sink is the reason why a low break rate is chosen, to avoid excessive heating.

 

Specifications

Bridge 2x IRFP460 MOSFETs in a half bridge configuration
Bridge supply 230VAC directly from the wall, 4A rectifier bridge and 330uF smoothing capacitor
Primary coil Rev 1: 55 mm diameter, 1.38 mm diameter isolated copper wire, 10 windings.Rev 2: 80 mm diameter, 1.38 mm diameter isolated copper wire, 10 windings.
Secondary coil Rev 1: 50 mm diameter, 200 mm long, 1430 windings, 0.127 mm enamelled copper wire.Rev 2: 75 mm diameter, 165 mm long, 1500 windings, 0.1 mm enamelled copper wire.
Resonant frequency Rev 1: Self tuning at around 470kHz.Rev 2: Self tuning at around 180 kHz.
Topload Rev 1: Made of two bottoms from beer cans, 65mm diameter and 30mm in height.Rev 2: 45 x 152 mm turned aluminium toroid.
Input power Interrupted mode: ?W at 230VAC input voltage.
Spark length Rev 1: up to 140 mm long sparks.Rev 2: up to 250 mm long sparks.

 

Schematic and PCB files

PCB file for ExpressPCB

 

Construction

21st July 2009

I designed a compact single sided PCB that contains both driver and bridge section on a mere 65 x 75mm board. Here is newly etched board, traces are a bit shaky as I have drawn them all by hand.

The MOSFETs uses the enclosure as a heat sink, I sanded down the paint for metal contact and use pads to isolate between MOSFETs and enclosure.

BPS is kept low, but can be varied from 4 to 20 BPS, to avoid excessive heating as the enclosure is not an optimal heat sink.

In the bottom of the following picture you can see the bridge rectifier mounted to the enclosure and the input filter for 230VAC in. The red wires lead to the 330uF/400V smoothing capacitor and the 100nF/1600V Rifa capacitor is the DC blocking capacitor in the primary circuit.

The coil is connected directly to 230VAC without any kind of voltage regulation and also requires a external 12VDC supply for the driver.

Antenna and primary coil connections are temporary solutions for the sake of demonstrating the Tesla coil in working order. A fold out antenna from a small radio or such will be added later. Some kind of support with banana jacks with a secondary and primary coil mounted on will be added, to avoid wrong phasing of the primary coil.

Here the complete setup is size compared to a 330ml beer can

 

Sparks

Here is one of the more spectacular spark pictures I have taken, in my eyes it looks like a demon waving its arms over the head which also have a distinct face with glowing eyes and a open mouth, or maybe I am just seeing things from inhaling too much ozone 😀

24th July 2009

I borrowed a expensive macro lens for my Canon 350D camera and took some pictures with great details of the sparks, very sharp pictures!

 

Revision 2

1st August 2009

Doing a short demonstration I adjusted the antenna with my hand while the coil was running, this resulted in unstable oscillations and the bridge was short circuited. I am now replacing the destroyed MOSFETs and here I can feel the disadvantage of servicing on a compact design.

A new secondary coil is in the making, it is wider, shorter and have half the resonant frequency of the first. It will be fitted nicely on a piece of acrylic for a complete look.

 

19th August 2009

The new secondary is finished, it took me about 8 days to do the winding as it is very intensive to wind with such a thin wire. Keeping the wire tight, windings close to each other, not pulling the wire too hard from the spool, watch for jams and overlaps and it all have to be done with a bright light very close to get a good view.

It uses the topload from my VTTC I, a 45 x 152 mm aluminium toroid, with this it have a new resonant frequency around 180 kHz.

Top of secondary was filled with epoxy to insulate the brass bolt from the inside of the secondary and the bottom earth connection is fastened with a nylon bolt.

It is all fitted onto a piece of acrylic with additional protection around the primary connections so it no longer possible to touch any conducting part of the primary circuit.

 

Audio modulation

I use a audio modulator made by the user Reaching (Martin Ebbefeld) from 4hv.org.

For sound input I use a cheap children’s keyboard from a toy store, its far from perfect for the job, especially because its waveform is highly distorted and its not clean tones but seems to involve a lot of modulation inside it to simulate different instruments. But its cheap and expendable.

Watch the film and look at the schematics for more about the audio modulation.

 

Conclusion

I am very satisfied with the final result, that I got to fit everything and use the enclosure as a heat sink turned out real good. Heating is not a problem with run times at about 2-3 minutes which is also the durations its been built to be demonstrated for.

Enclosure dimensions are 125W x 80D x 50H mm.

Revision 2 looks even better, performs better but was also a lot of work to wind the new secondary with such a thin wire.

Demonstration

Revision 1

Revision 2

Kaizer SSTC II

Introduction

This is a modified version of the first SSTC I built, the Kaizer SSTC I. It uses the same secondary, topload and driver board. New things is a full bridge of IRFP460 MOSFETs, audio modulation, shielded drivers and a new casing.

 

Safety

WARNING!: Working with electricity is dangerous, all information found on my site is for educational purpose and I accept no responsibility for others actions using the information found on this site.

Read this document about safety! http://www.pupman.com/safety.htm

 

Considerations

In the quest for longer sparks I decided to use a full bridge to take advantage of the full voltage on the bridge.

The MOSFETs will be mounted on top of a heat sink so its easy to change them by only removing the secondary platform and solder them off.

The drivers will be shielded in order to avoid the EM field generated by the Tesla coil itself to inject noise into the drivers.

 

Specifications

Bridge 4x IRFP460 MOSFETs in a full bridge configuration.
Bridge supply 0 – 260 VAC from a variac, 8 A rectifier bridge and 1500 uF smoothing capacitor.0 – 365 VDC on the bridge.
Primary coil 115 mm diameter, 1.78 mm diameter isolated copper wire, 8 windings.
Secondary coil 110 mm diameter, 275 mm long, 1000 windings, 0.25 mm enamelled copper wire.
Resonant frequency Self tuning at around 250 kHz.
Topload 100 mm small diameter, 240 mm large diameter, toroid.
Input power Continues Wave mode: 2000 – 4000 Watt at 200 VAC input voltage.
Interrupted mode: 100 – 2000 Watt at 260 VAC input voltage.
Audio modulated mode: 300 – 400 Watt at 150 VAC input voltage.
Spark length up to 475  mm long sparks.

 

Schematic

The UCC3732X are MOSFET driver ICs, one non-inverted output and the other inverted, in order to get a push-pull drive of the gate drive transformer. A gate driver IC can deliver the high peak currents needed to drive MOSFETs efficiently.

The 74HC14 is a inverting hex schmitt trigger, it is used to get a proper solid 0-5V square wave signal from signals that are not perfectly square, the antenna feedback can vary a lot in waveform and amplitude, the 74HC14 converts this to a clean drive signal for the MOSFET drivers.

All unused input pins of a 74HC14 has to be tied to ground, floating inputs and a noisy environment is a recipe for trouble. The noise can couple between the gates internally and make the whole IC not work properly.

The music modulator works by amplifying the audio signal in the LM741 and at the BC547 transistors. The 555 timer ensures that the signal length of the generated square wave is much shorter than the audio signal, in order to not have too long on-time and thus damage the MOSFETs / IGBTs from over-current.

 

Construction

15th March 2009

I took apart a 19″ LCD monitor and a 24″ CRT monitor, from these respective computer parts I salvaged a good piece of acrylic from the LCD monitor and a fairly sized heat sink from the CRT. I cut the acrylic in half for a 2 level platform and the heat sink was cut in 4, its necessary to isolate the MOSFETs from each other as their housing is also a conductor.

19th March 2009

Driver electronics and audio modulator are installed under a metal casing from the CRT monitor to shield it from the heavy EM field surrounding the Tesla coil, this is to avoid problems with the driver being interrupted by its own EM field.

The bridge is made out of four IRFP460 MOSFETs, four MUR1560 diodes, four 5R resistors. The power supply is a 8 A rectifier bridge with a BHC 1500 uF/450 V smoothing capacitor, a 27K 7W bleeder resistor is added in the final build.

The audio in jack was later removed due to it making a short through its metal housing to the ground rail, I had overlooked that the audio in negative was not common with the ground rail, but there is a capacitor inbetween.

The secondary is held in place by a crate for ventilation on houses, its an easy and quick way of taking the coil apart for transport or storage, and it holds the secondary firm and tight.

A acrylic tube is added to support the antenna, in this way it is possible to adjust the coupling of the antenna to the secondary simply by pulling the wire.

The new shielding of the audio in signal is made from a piece of shielding from a industrial cable pulled over it and grounded.

The secondary with terminations. 110 mm diameter, 275 mm long, 1000 windings, 0.25 mm enamelled copper wire.

The complete coil looks, except maybe the electrical tape used to hold the topload together.

 

Sparks

Interrupted mode

At 250 VAC input voltage, 350 VDC on the bridge, it was possible to reach 475 mm long sparks, in interrupted mode, to a grounded object.

More pictures of sparks in interrupted mode, it is running at about 4 – 5 BPS.

3rd May 2009

Continues Wave mode

At 200 VAC input voltage, 280 VDC on the bridge and a power consumption around 10 A, peaking at 20 A, the coil was drawing somewhere in between 2000 to 4000 Watt. This resulted in very hot, thick white arcs punishing the dead iPod shuffle which remarkably left the player relatively unharmed considered what had just taken place.

These flame like sparks are 250 mm in length.

18th August 2009

I constructed a new topload from two cheap aluminium frying pans from Ikea. With handles cut off and screw from it grinded away it had a smooth surface and was fixed with a long screw through both of them.

6th September 2009

During a run of CW at full input voltage, the full bridge blew apart completely, with a loud bang.

 

Audio modulation

I use a audio modulator made by the user Reaching (Martin Ebbefeld) from 4hv.org.

For sound input I use a cheap children’s keyboard from a toy store, its far from perfect for the job, especially because its waveform is highly distorted and its not clean tones but seems to involve a lot of modulation inside it to simulate different instruments. But its cheap and expendable.

Watch the film and look at the schematics for more about the audio modulation.

 

Conclusion

Upgrading the SSTC I with a full bridge was a absolute must. It is small changes compared to the better performance and the driver have no problems at all driving four MOSFETs instead of just two.

Getting sparks at 475 mm length in interrupted mode and white power arcs at 250 mm length is truly satisfying for this little coil, the secondary winding itself is only 275 mm in height in comparison.

Enjoy the demonstration.

 

Demonstration

Demonstration of different modes.

New topload, running in interrupted mode.

New topload, running in CW mode.

New topload, running in interrupted mode and closeup of sparks.

Kaizer SSTC I

Introduction

This is my first solid state Tesla coil, so I went with a sturdy and proven schematic made by Steve Ward. A lot of other coilers have replicated this circuit with great success and therefore it is easy to find information how it works and how to troubleshoot it.

 

Safety

WARNING!: Working with electricity is dangerous, all information found on my site is for educational purpose and I accept no responsibility for others actions using the information found on this site.

Read this document about safety! http://www.pupman.com/safety.htm

 

Considerations

One of the differences from the original circuit is that I use 230VAC input instead of 115VAC. So capacitors and MOSFETs have a higher voltage rating.

The interrupter will be changed to go down to a very low break rate.

 

Specifications

Bridge 2x IRFP460s MOSFETs in a half bridge configuration
Bridge supply 0 – 230VAC from a variac, 8A rectifier bridge and 330uF smoothing capacitor0 – 325VDC on the bridge.
Primary coil 115 mm diameter, 1.78 mm diameter isolated copper wire, 10 windings.
Secondary coil 110 mm diameter, 275 mm long, 1000 windings, 0.25 mm enamelled copper wire.
Resonant frequency Self tuning at around 250 kHz.
Topload 100 mm small diameter, 240 mm large diameter, toroid.
Input power Continues Wave mode: 1000 W at 230VAC input voltage.
Spark length up to 250 mm long sparks running interrupted.

 

Schematic

All unused input pins of a 74HC14 has to be tied to ground, floating inputs and a noisy environment is a recipe for trouble. The noise can couple between the gates internally and make the whole IC not work properly.

 

Construction

22nd January 2009

I began the construction of the half bridge circuit in a small plastic box, the heat sinks are a Pentium II heat sink cut in half. The bridge is made from copper wire size 2.5mm² / AWG14.

The bridge is made from a 8A bridge rectifier with 330uF 450V smoothing capacitor, two IRFP460 MOSFETs with MUR1560 diodes, two 0.68 uF 400VAC film capacitors for the voltage splitter and 10R gate resistors.

The driver circuit is made on vero board with a external 12VDC power supply.

 

Troubleshooting

23rd January 2009

When I first tried to run the driver circuit separately to test the driver before connecting it to the MOSFETs, it only resulted in the MOSFET driver chips (UCC37321/UCC37322) catching fire and burning up like a small volcano. This did of course upset me when it happened once more when I had changed the chips. This led me to seek help and I learned that running the driver chips unloaded, without a MOSFET or GDT connected to the outputs, the chips will oscillate into oblivion and burn them self down.

With the complete circuit put together it all worked except the primary coil was phased wrong, but it was no problem since I used banana plugs for the primary connections.

I ran the coil as CW (Continues Wave, non interrupted so its switching at its resonant frequency) to stress it to its maximum, which also did result in failures at 230VAC in, drawing around 4 to 5A.

The secondary coil was grounded to the mains ground in my house, but by accident I were using a plug without a earth connection in, so the secondary earth was arcing to the phase and neutral in my power bar. Pushing around 1 kW into this rather small circuit with passive cooling became enough combined with HF noise on the phase and neutral and one of the MOSFETs exploded violently and the other died silently. Here I discovered my design did not make it easy to change the MOSFETs, a important thing to consider in future constructions.

For the next couple of days I could not get the coil to work again. Everything in the driver circuit was changed and measured with a oscilloscope without finding anything out of order. It was first when I by accident measured short circuit connections with a DMM that I discovered one of the secondary windings on the GDT was not connected to the MOSFET, it was because the gate resistor was destroyed from the short circuit of the MOSFETs. Changing the 10R resistor made the whole thing work like a charm again.

 

Sparks

Here are some pictures from the first light, input power is from 30VAC to 230VAC at up to 5A.

 

Audio modulation

I use a audio modulator made by the user Reaching (Martin Ebbefeld) from 4hv.org.

For sound input I use a cheap children’s keyboard from a toy store, its far from perfect for the job, especially because its waveform is highly distorted and its not clean tones but seems to involve a lot of modulation inside it to simulate different instruments. But its cheap and expendable.

Watch the film and look at the schematics for more about the audio modulation.

 

Conclusion

Building this clone of Steve Wards SSTC5 was a great introduction to solid state Tesla coils, I now have a understanding of how it works from interrupter to driver to bridge.

Further projects with this circuit will be a complete rebuild with audio modulator and a full bridge of MOSFETs, this will be a separate project.

 

Demonstration

In thew following videos, the SSTC I is playing music from the interrupter shown in schematic for the SSTC II.