In this project I will describe an amazing phenomena using which we can generate sound/music from a high voltage plasma arc. Yes you read it right! Literally its music generated from fire. Plasma arc is formed when a high voltage is applied across two conductive surfaces insulated by air. Sufficiently high voltage and low distance between the surfaces will result in dielectric breakdown of air between the surfaces, further resulting into a current flow between the surfaces. This phenomena is also known as corona discharge. You can see this during lightnings and around the high voltage electricity poles.The temperature inside a plasma can reach several hundred degrees.
This plasma arc acts as a diaphragm layer similar to the one present in speakers, but the difference is that it is massless. This corona arc diaphragm generates air pressure waves which we perceive as sound, and if we can vary the frequency of current flowing through the plasma we can generate different tones and even music. Basically, that’s what a plasma speaker do. It generates a plasma and varies the frequency of current flowing through it. Isn’t that cool? Yes, but its dangerous as well.
**Warning** Formation of plasma happens at very high voltages which can kill you in microseconds. It’s not advised to play with high voltages when you don’t know what you are doing. But if you are dumb and still wants to play with high voltages, please go ahead. Some day you might help humankind by virtue of natural selection and getting yourself killed. HaHa
Motivation and Background
Few days back, I was trying to make a high voltage pulsed power supply for a special application when I stumbled upon a circuit that used a flyback transformer from an old TV as a source of high voltage. These flybacks, now obsolete, were used in cathode ray tube (CRT) based TVs and computer monitors to accelerate a beam of electrons onto the screen before LCD and LED TVs became mainstream. That beam had to refresh the complete screen at-least 30 times a second so that we don’t perceive the delay between every frame.
These flyback transformers are a cheap source of generating plasma arc as they output voltages in the range 20-30 kilovolts. Even the input frequency of the transformer can be easily varied which helps us in generating tones at the secondary output. This makes the flyback ideal for making a plasma speaker.
Why the name flyback? That’s because if you touch the high voltage output of a flyback transformer you will probably fly backwards and fall few feet away. haha just kidding! Although that might be true, but the main reason why they are called flyback transformers is because the output wave from the transformer had to scan the TV screen line by line from left to right and then fly back to the left of next scan line as quickly as possible so that the screen refresh rate is fast. More common names for this transformer is Line OutPut Transformer (LOPT) and Integrated High Voltage Transformer (IHVT). That’s what the mechanic told me when I went to buy a flyback at a TV repair show near my home. They don’t understand the term flyback and I had to show him an image if it. Here is the image of the flyback I bought. The Thick red wire is the high voltage output and goes into the anode terminal of the cathode ray tube. The thin black wire at the bottom is the ground.
Now a days the best source to get a flyback is to get it from an old TV because most big companies like Samsung and Sony have stopped manufacturing CRT based TV sets so these transformers are now out of production. If you are in Bangalore you can get them at a TV repair shop for less than ₹ 100. I bought 4 of these at SP road for ₹ 200. Quite cheap!
The Driver Circuit
The voltages applied to a flyback’s primary winding are high frequency rectangular pulses. This is also a distinguishing factor between a flyback and a normal transformer; a normal transformer is driven by sinusoidal waves. Flyback is basically a pure inductor which stores energy in its ferrite core during the duration for which the rectangular pulse is on and releases that energy while the pulse is off. This results into a sawtooth shaped current waveforms at the primary and secondary side of the transformer. And the turns ration between primary and secondary increases the output waveform to several kilovolts typically anywhere between 10 to 35kV.
One of the easiest way to generate high frequency rectangular pulses to drive a flyback is to use a 555 timer in astable mode as described in the schematics shown above. Frequency and duty cycle can be adjusted using the RV1 and RV2 potentiometers, 15 to 50khz frequency range worked best in my case. The rectangular wave output of 555 drives an npn transistor which then drives a n-channel power MOSFET connected to the primary windings of the flyback. Primary windings with 8-10 turns are mounted on the ferrite core of the transformer. You can see them in the setup images below. Don’t wind more than 10 turns as it might damage the flyback.
I tried IRF540N and IRFP260N MOSFETS. The later one can handle more current through the primary windings. A heat sink is required for the MOSFET as it will get terribly hot. I used a pn2222 but any general purpose npn transistor would work. To generate plasma we have to bring the output terminals of the flyback closer to each other. Or we can also build a Jacob’s ladder as shown in the image below. The whole setup is powered by a 12v and 5 amps power supply. The purple arc in the image is the plasma formed after the dielectric breakdown of air.
To generate music we have to vary the frequency of the voltage to the primary windings. This can be done by applying a modulating signal to the pin 5 of 555 timer which will generate a corresponding frequency modulated pulsed waveform. Here is a snippet from page 15 of the datasheet of NE555 timer explaining the output waveform. If you connect your audio jack output to pin 5 and power this circuit with 12 volts and 5 amps power source and play music, you would hear the music coming out of plasma. But the sound would be very low. That’s because our laptops and phones output a weak signal which has to be amplified.
The second part of the circuit is an audio amplifier to amplify the weak music signal from the audio jack. Basically it’s an op-amp which amplifies the signal at the non inverting input. I used LM358, you can use any other op-amp. The circuit amplifies the weak signal by 100 times. If you connect the the audio jack to the amplifier, build the circuit as shown and play music, you will hear it much louder. Here is how my setup looked like.
Playing my favourite song on the plasma speaker.