Solid StateTesla Coil
Improved version of the classic Tesla coil which, thanks to the use of actual technologies in power electronics such as isolated gate bipolar transistors (IGBTs) and the digital circuits such as microcontrollers, allows a degree of control until now unimaginable in this always fascinating device and can, for example, play pieces of polyphonic music in MIDI format.
This is the electronics that controls the bridge of IGBTs and generates a sinusoidal current through the primary LC circuit system with its natural frequency being interesting to approach as much as possible to the situation of soft switching, i.e. should the IGBTs switch when current
next to zero thus minimizing losses by switching and allowing to handle intensities than otherwise they would destroy safely semiconductors. This part works at low voltage (5V, 15V) against the part of power 320V and consists of a feedback loop, a logic of synchronization, input optical control signal (switch) and over-current protection circuitry. It is a variation of the original design by Steve Wards, simple and effective, as well as being easy to debug circuit. Thank you very much Steve!
For development of the inverter control circuitry in this occasion I opted for double sided PCB design, using to this purpose electronics design software named Eagle, whose evaluation version allows to work with circuits like this
As designed, the system would fail if we do operate in continuous mode, since the power semiconductors can only support the enormous current for short periods of time (1ms). On the other hand, although there are Tesla coils designed to work in continuous mode or mode CW (continuous wave), the arcing produced by this type of devices do not emit any sound.
For this, thanks to an external circuit called interrupter, the system will work interrupted way, with ON times around the 100us and less than 10% duty cycle. These pulses will be repeated in time with a certain frequency producing sound with that particular frequency. One of the pictures above shows a basic interrupter, based on the known 555 IC, useful when it comes to fine-tune the system but only capable of producing a simple musical note.
With the help of several microcontrollers, it is possible to read the information transmitted from a PC by a MIDI port and generate from it several musical notes simultaneously, i.e. polyphonic sound. The result is quite interesting...
Source code for programming cascade microcontrollers is available here:
Fugue in Dm from J. S. Bach
Each coil interpretes one hand of this exceptional organ masterpiece acording with its partiture. This means 8 notes poliphony and a great right to left hand dialogues.
Below I will explain some of my Dual Resonant Solid State Tesla Coil or DRSSTC.
The DRSSTC consists of a clasic Tesla coil, with its primary LC circuit and the secondary LC circuit (both tuned, being this the first resonance). However to control the current flow through them bridges of MOSFETs are used or, better still, of IGBTs that are controlled by electronic drivers designed to achieve a soft switching, right in the zero crossing of the primary current (English "Zero Current Switching" ZCS). Depending on the semiconductor used and the success of the driver in getting soft switching can work with several hundred amps peak currents. These drivers include a feedback loop of primary current (which could be considered as second resonance) as well as optical input to control the status (ON or OFF) of the device and is usually advisable a simple circuit overcurrent protection if we are to make the machine last us enough to be able to continue learning with it.
The following images illustrate the process of design and construction of what we call Coil 1, a modest system that has more than fulfilled its initial purpose which was not to serve as making contact with this technology and training in the understanding and debugging of circuit.
Based on a full bridge of 4 IXGH60N60C2 TO-247 Package IGBTs with nominal values of 75A, 600V, quite modest for these purposes even though they support 300A at 25°C in 1ms pulses. The truth is that they can work safely handling 400A pulsed with a low duty cycle and always looking for a soft switching. The use of heat sinks and, if possible forced ventilation, is imperative.
The primary and secondary windings together with the primary capacitor and the toroidal terminal form two electromagnetically coupled LC circuits and that, with the appropriate values can be in resonance, then maximizing energy transfer between both.
This part of the system corresponds to the clásic design of a Tesla coil where the breaker or gap and high voltage power supply have been replaced by the previously described electronics.
The following video shows a test run of the system at reduced power through the use of a 3kVA variable autotransformer (variac), very useful for fine-tuning a delicate system like this. This transformer is configured around 35% of its maximum voltage (230 VAC) by what the DC bus have no more than 100V.
The MIDI controller is using 2 arduino Nano modules in cascade configuration, playing only 4 notes simultaneously and its performnce amply meets my expectations.
The piece of music chosen for this test is the third movement of "the summer" from Antonio Vivaldi's "four seasons".
After acquiring some skill in the design and debug of electronic circuits that govern a modern Tesla coil I have decided to take a new step with the incorporation of IGBT modules which can handle much higher powers than discrete transistors in TO-247 package or To-264 used in the previous design.
After finding a pair of Fuji 2MBI150SC-120 IGBT modules at a great price (10 eur/PC) I have decided to build two similar coils whose power electronics is based on half-bridge which, thanks to a rectification with voltage doubler is fed to some 650VDC. This power is used in total 4000uF of capacity for its filtering, using also a snubber comprising two 2.2 uF / 630V polyester capacitors in series.
IGBT module has as nominal values 150A / 1200V and perfectly supports several hundred amperes peak currents. I've also used large size aluminium heatsinks and, of course, forced convection.
The design of the DC bus has been ensuring that IGBTs and capacitors are the most next to minimize parasitic inductance that can cause unacceptable overvoltage peaks.
Below are some videos of various musical pieces performed by "Coils" based on the previously explained designs, but with some improvements in circuit debugging and a more correct tuning.
Theme executed by two DRSSTCs running at 60% of their rated power.
The Coil on the right is based on a full bridge of discrete IGBTs in TO-264 encapsulated (slightly bigger than the TO-247). It is the SGL160N60UFD, with better performance than those used previously in the coil #1.
Primary capacitor is formed by several polypropylene caps in series-parallel configuration to ensure that they support both voltage and current that will be subjected in operation at rated power. This configuration is known as MMC (Multi Mini Capacitor). Total capacity is 64nF.
Secondary winding is formed by 850 turns wound on PVC pipe 125mm with 40 cm of height. I used 0.4mm enameled copper wire insulated with double polyurethane bath. The inductance of the coil is 32mH.
DC Bus is being filtered by 4 X 1000uF/450V electrolytic capacitors.
On the other hand, the coil on the left uses an IGBT module in half-bridge (Fuji 2MBI150U4A120), a bite more advanced than the previously mentioned in the coil #2.
Rectification uses voltage doubler, since this module has a nominal voltage of 1200V, while the discrete IGBTs used in the other coil only support 600V. The primary capacitor (MMC) reaches a capacity of 136nF and the secondary winding an inductance of 53mH.
According with these specifications, and as you can see in the video, the coil on the left presents an output significantly more powerful.
Overcurrent protection is configured around 500A for both models.
Bright side of Life
Same coils performing this final Theme from Monty Python famous movie "The life of Brian" at about 70% of rated power.
Cello Suite n.1 in G (BWV 1007) from J. S. Bach - Prelude
Left Coil playing solo the entire prelude of this marvellous compositiòn. Power setting at 80% but OCD is triggering constantly. I think it is a relatively conservative setting (aprox 500A).
Four Seasons from Antonio Vivaldi - 3rd movement - Presto
Left Coil playing solo the movement, executing 4 note poliphony. Power setting at 80% but with higher frecuency notes and more short t_on, OCD is actuating only a few times . Global performance is very impressive.
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