3x MIDI videos from DRSSTC1 demonstration

All MIDI files played can be found in this thread: https://highvoltageforum.net/index.php?topic=118.0 DRSSTC1 information: http://kaizerpowerelectronics.dk/tesla-coils/kaizer-drsstc-i/ which I will soon update with the rebuilded bridge Popcorn Star Wars – Imperial …

Building the OpenTheremin V3

Introduction I received a bare board and some of the SMD components for a OpenThereminV3, it is a semi-assembled kit that can be bought from …

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.

Kaizer DRSSTC II

Introduction

I wanted to build a small DRSSTC in a few days without having prepared anything, most parts are reused or scrapped from things I have found and saved.

 

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 was nervous that the metal enclosure for the driver was too close to the primary coil and would absorb some of the energy, first tests show no sign of heating of the metal.

I made the bridge section entirely on a normal one sided PCB and was not too sure if the traces were thick enough to withstand the high peak currents or keep a low inductance layout. It all seems to work without problems.

 

Specifications

  Revision 1 Revision 2
Bridge 2x G12N60C3D IGBTs in a half bridge configuration 2x IXGN60N60C2D1 IGBTs in a half bridge configuration
Bridge supply 0 – 160VAC through a variac, 6A rectifier bridge and 2x Aerovox 410uF 430V filtering capacitors in parallel.  0 – 210VAC
Primary coil Flat primary. Inner diameter 70 mm, Outer diameter 187,36 mm. 6 turns 1,78 mm copper wire (2,5 mm²), turn spacing 8 mm. Tapped at 4.8 turns.  
MMC 2 in series Cornell Dubilier (CDE) 942C20P15K-F capacitors for 0.075uF at 4000VDC rating.  
Secondary coil 50 mm diameter, 200 mm long, 1430 windings, 0,127 mm enamelled copper wire.  
Resonant frequency Around 327 kHz.  
Topload 40 x 215 mm aluminium tape on a Styrofoam toroid.  
Input power 250BPS, 250uS on-time, 68 primary cycles, 300A limiter: 120VAC in.  350BPS, 120uS on-time, 35 primary cycles, 280A limiter: 210 VAC in at 2A. 420 Watt.
Spark length Up to 240 mm long sparks.  Up to 370 mm long sparks.

 

Schematic

The driver is a variation of Steve Wards universal driver and beneath you can see the bridge schematics.

 

Construction

13th August 2011

Optimize driver PCB, design bridge PCB, toner transfer to PCB.

14th August 2011

Etch PCBs, assemble bridge PCB completely, half done with driver assembly. Heat sinks, electrolytic capacitors, voltage splitting capacitors and rectifier bridge are all salvaged components. Materials for building a enclosure and base is found. Lexan salvaged from LCD monitors is used for the base and some normal house wiring is used for the primary coil.

 

15th August 2011

Winding CTs for feedback and OCD, building enclosure and base, driver PCB assembled.

 

16th August 2011

Driver PCB fault finding and testing, enclosure and base building. I had forgot to add the trace that resets the OCD on the driver board after I moved it while optimizing the board layout.

 

17th August 2011

Complete construction and ran first test, no first light.

Some more detailed pictures of topload, secondary with terminations and primary coil.

Flat primary. Inner diameter 70 mm, Outer diameter 187,36 mm. 6 turns 1,78 mm copper wire (2,5 mm²), turn spacing 8 mm.

50 mm diameter, 200 mm long, 1430 windings, 0,127 mm enamelled copper wire.

40 x 215 mm aluminium tape on a Styrofoam toroid.

 

28th August 2011

First light, phasing of feedback transformer was wrong.

3rd September 2011

24 cm sparks, running 250uS, 68 primary cycles, 120VAC in, 268A primary current. 250BPS.

 

Here is a scope shot of the primary current waveform and a zoom of the same.

 

4th September 2011

I blew up both IGBT transistors running at the same settings as above on the 3rd September 2011, but at 160VAC input, nothing violent, just a flash and all was silent.

I will rebuild the bridge with IXYS IXGN60N60C2D1 IGBTs, which will hopefully make the bridge indestructible compared to the size of this Tesla coil.

11th September 2011

Rebuild the bridge and spent the day fault finding on the circuits as it did not work, turned out to be a 33V Zener diode on the bridge board that was short circuited.

 

16th September 2011

I recorded some data from different settings and came up with following before admitting that my heat sink is just too small. Further tuning is till needed, I hope it can do better and it seems to never go higher than 280A primary current.

I peak Voltage in, AC Current in, AC Burst length BPS Watt, AC Spark length, mm
280 210 0,5 120 200 105 274
280 210 0,75 120 300 157,5 290
280 210 2 120 350 420 354
280 210 4 120 500 840 370

200 BPS, 274 mm sparks.

 

350 BPS, 354 mm sparks.

 

500 BPS, 370 mm sparks.

 

Here is the current waveform which pretty much stayed the same doing these tests, also a better quality picture of the sparks.

 

Additional tuning gave me much better results in the form of almost the same spark output at lower on time, lower peak current and lower power in.

Here it is a screenshot of a spark going out to 337 mm running 70uS on-time, 260A peak current, 300 – 400 BPS at 260VAC at 0.5A.

 

Conclusion

Revision 1

It is no problem building a small DRSSTC in a few days with some previous knowledge and a off the shelf secondary coil.

In the future I will properly not make another Tesla coil with TO-247 IGBTs, I need some more overhead with the way I push my Tesla coils.

 

Demonstration

28th August 2011

First light

 

3rd September 2011

Came out a bit dark, but shows 24 cm sparks, running 250uS, 68 primary cycles, 120VAC in, 268A primary current. 250BPS.

 

16th September 2011

37 cm sparks, running 120uS, 35 primary cycles, 210VAC in, 280A primary current. 200 – 400 BPS.

 

16th September 2011

33,7 cm sparks, running 70uS, 260VAC in, 260A primary current. 300 – 400 BPS.

14th February 2012

Playing Doom 1 – Episode 1 soundtrack with my new midi modulator.