Harrison Wieting
PHYSICS ENTHUSIAST
Audio Modulated Tesla Coil
Specifications
Secondary: 10" winding of 24AWG on 4" OD cardboard mailing tube form.
Primary: 4 turns of 14AWG stranded insulated wire wrapped tightly around the bottom of the secondary windings.
Toroid: No toroid is necessary. I have 6" x 3" toroids that I have tried, but they make no difference in performance or output
Feedback: Secondary feedback using an antenna
Power Electronics: 100V MAX from the variac, full-wave rectified and smoothed by a 560uF cap
Bridge: Half-bridge of IRFP260M mosfets with 5ohm gate resistors
Control Electronics: One inverting and one non-inverting UCC mosfet driver's. Includes an On-board modulator. BNC jack for use of an off-board modulator and audio modulation.
Grounding: Secondary is grounded to mains earth
Spark Length: 6" at least. Exact length is to be determined. Spark Length is not the main objective here. Obviously the more power you give the system, the larger the sparks will be. But also the system will get louder. It already is very loud so there is no reason to push the limits and go over 100V input.
This is my first audio modulated Tesla coil. I used a MIDI freeware on my laptop interfaced by an M-Audio Uno USB to MIDI. The MIDI interface is connected directly to the MIDI Modulator which is the Flexibrute MIDI Modulator (polyphonic fiber optic and BNC) By Eastern Voltage Research. I do not have a keyboard or other synthesizer so I resort to downloaded MIDI files. This works decently well, the audio quality is superb. However, the strong RF field generated by the tesla coil has a tendency to cause hang-ups in the notes. This is a very common problem and the only way that I know of to fix this is good grounding of the Tesla coil secondary and keep the MIDI source (laptop, modulator, synthesizer, keyboard, etc..) as far away from the Tesla coil as possible, and shielded with a faraday cage.
How it Works...
The typical interrupter for a tesla coil generates a square wave with a set pulse width and pulse repetition frequency. This frequency is audible in the output of the spark. See the SSTC4 operation video to hear the frequency range of my modulator from 100hz - 450hz.
The duration of the PW (pulse width) is the same as the ON time. The period is the ON time + the OFF time. The duty cycle or duty factor is the ON time divided by the period. The period is also called the frequency. Well, sort of. In physics, period and frequency can be different. The period is measured in time while the frequency is measured in Hz (hertz) or cycles per second as they used to call it in the old days. But wait a minute.. isnt cycles 'per second' a unit of time? Yes it is! So for the sake of this explanation, I'm going to say that period and frequency are the same. So the photo above and to the right has a frequency of 1 Hz at various duty cycles.
So.. what does this all mean? Say we want to play do re mi fa so la ti do. Then we set our frequency to match that of the note we want to play. Simple as that! But what about the PW? What does that have to do with anything? Controlling the
PW is a way to control power. This is important for many audio
modulated DRSSTC's. If the PW is too long, then the duty cycle
becomes too great, thus creating a power overload, and we
risk blowing the IGBT's or mosfets. Usually we want to limit
duty cycle to less than 5%, sometimes this is even too much.
Typically, in SSTC's this is not a problem because mosfets can
be switched really fast and abused like no other. In most all
SSTC's, you can drive the mosfets at any duty cycle you want.
In fact many audio modulated tesla coils utilize what is called
CW mode or continuous wave mode. CW is a duty cycle of 1 or
100%. That is on 100% of the time! Check out my plasma tube
videos to see this.
So where does this come in to play with the MIDI audio
modulation that I use?
What I do is use a MIDI freeware called Anvil Studio. I have
mixed feelings about the software, but that's beyond the scope
of this topic. I download MIDI files from the internet and load
them to Anvil Studio. I have my flexibrute MIDI interfaced
from my laptop to the MIDI modulator using a 1x1 USB to MIDI
interface. The type I use is the M-Audio UNO. It cost me $40
dollars new from guitar center and it works just as it is
supposed to. Beware of the $5 ebay sales! Spend the extra bucks and buy yourself something that will work.
So here is exactly how the connections are:
Laptop > M-Audio UNO USB device > flexibrute MIDI controller > Tesla coil
Okay, what happens when I press play? The MIDI signal is sent from the laptop to
the flexibrute MIDI controller via the USB device. The flexibrute controller converts these MIDI signal into square waves. These square waves are then sent to the tesla coil. And there you have it! The transistors are being driven at the frequency of the note that you tell it to play.
So that's all cool and awesome, but HOW is there sound being generated out of plasma?!
This is exactly what makes the tesla coil such a unique apparatus. It is one of very few electronic instruments that can produce sound without the use of a speaker. The Tesla coil itself is the speaker. The spark creates a variation in air pressure through the sudden heating, cooling, and rapid expansion and contraction of the air around the spark. This vibration is the audible sound that we hear (exactly the same as when you hear thunder), which is the same as the frequency at which we drive the tesla resonator. Tesla coil's can be designed to create a wide spectrum of sounds, even full bandwidth. When It comes to audio modulation of the spark using MIDI, all we are really doing is changing the pitch to match the notes we want.
