What is it?
In technical terms, a plasma speaker is an audio modulated plasma arc. What that means is that you have a gap between two electrodes, and when the voltage between the electrodes is enough to break down the air between them, a plasma arc is formed. That plasma arc is then switched on and off at a frequency determnined by an audio signal. Because of this, we hear sound.
I originally did this as a science fair project, so I'm going to post it in the same format.
Optimization of a High Voltage Plasma Arc Speaker
Frequency vs. Spark Gap Distance
Using a 555 Integrated Circuit and a Flyback Transformer
Many people have built plasma speakers, and there are various designs. They all have one thing in common: a flyback transformer. A flyback transformer is a device that converts a pulsed DC signal at a low voltage to a pulsed DC signal at a very high voltage. In this experiment, the flyback was tested at different frequencies using a 555 IC to control the signal, and the resulting voltage was determined by measuring the farthest distance it could arc. The duty cycle, or percentage of the signal that is high, was also changed as a result of changing the frequency, because they were both dependent on the values of the resistor in the circuit, which was the only way to change the frequency. Because there is a fixed relationship between frequency and duty cycle, all results and conclusions are only for this frequency-duty cycle relationship.
My hypothesis was that as the frequency was increased, the voltage would increase because the transformer would have more electrical energy delivered in a shorter amount of time. The experimental results show that the voltage does increase with frequency, but only to a certain maximum point, at which the voltage begins to drop sharply. Further research showed this to be the saturation point of the transformer, or the point at which the magnetic field fails to fully collapse between pulses, and instead remains unused. Because of this, there is an optimum point at which the transformer should be operated to ensure the loudest plasma speaker. This was determined to be approximately 35.7 kHz for the transformer tested.
Table of Contents
What frequency provides the highest output voltage from a flyback transformer?
Independent Vatiable - the value of the potentiometer R2 in the circuit
Dependent Variables - Frequency, Duty Cycle, Output Voltage
Controlled Variables – Frequency-Duty Cycle Relationship (R1 value of the circuit), Temperature of the MOSFET, the voltage being applied to the primary, the gauge of the wire on the transformer, the number of turns on the transformer, the 555 chip being used, the MOSFET being used, the oscilloscope used to measure the frequency and duty cycle, the physical properties of the circuit (wire gauge, wire length, location of circuit and flyback, ect.), properties of the air (density, humidity, airflow, temperature, ect.)
*properties of the air were controlled as best as possible, to ensure no large variation all tests were performed within a small timespan indoors to prevent excessive airflow, with time given between tests to allow ozone dissipation
As the frequency is increased, the output voltage of the flyback transformer will also increase.
A plasma speaker is a device that creates a plasma arc using a flyback transformer, and then audio-modulates this plasma arc using pulse width modulation (PWM). The plasma speaker used in this experiment was built using a 555 timer Integrated circuit in a simple astable configuration, as shown in the included schematic. This signal is somewhat weak and is sent to a Metal Oxide Semiconducting Field Effect Transistor (MOSFET), which switches the power from a modified computer power supply through the custom wound primary on the flyback transformer consisting of 5 turns of 18ga magnet wire. This turns the ultrasonic base frequency of the arc on and off at the same frequency as the music. This, In turn, creates small shockwaves at the same frequency as the music. The small shockwaves are caused by the ionization of the air. When it ionizes, it produces a great amount of heat. This causes the air to rapidly expand and become less dense, just like the thunder you hear after a lightning strike. The expansion causes a compression wave to form, which we interpreted as sound.
A plasma arc forms between the two electrodes of the transformer, and is created when the Electric Field between the two points becomes higher than the ionization potential of the material between them, at which point the molecules to break down into ions and become conductive. The Ionization potential of air is approximately 75 volts per .001 inch, or 3 million volts per meter at sea level. This scales to approximately 30kV per centimeter or 75kV per 2.5 cm, which equals 75kV per inch (http://www.angelfire.com/ak5/energy21/grayreproduction2.htm)( http://northbrevardarc.org/lightning.htm).
This ionized gas is a plasma, which is where it gets the name of plasma arc. Once created, the plasma arc is easier to maintain because the heat produced by the ionization keeps the air ionized, or if it cools slightly is remains easier to ionize at the next pulse. Because the voltage is directly linearly related to the distance the arc can jump, the relationship between distance and frequency is analyzed because of the difficulty of measuring voltages that high.
An electric field is the field force of charge created by the voltage at the contacts. It can be considered the “electrical pressure” exerted by the concentration of electrons.
A flyback transformer is a DC-DC converter. It takes a small voltage at a small number of windings on the primary coil and creates a large voltage on a large number of windings on the secondary coil. If this charge is not enough to ionize the air in the gap it is stored by the large diode inside the flyback which prevents the voltage from equalizing. This works because when the low voltage is applied at the primary coil, it is at a fairly high current, and creates a magnetic field. This magnetic field stores electrical energy and this energy is known as magnetic flux. When the current is removed, the magnetic field collapses in on the transformer core and all the energy is dumped back into the windings of the transformer. This electrical energy is transferred to each winding, and since the secondary coil has so many windings, the voltage created is very high. When the magnetic field is collapsing and the energy is being dumped into the coil, the transformer is said to be in a state of flyback, hence the name.
If one of these pulses is not enough energy to ionize the gap, the energy in the secondary is stored because of a large diode which prevents the electrons from flowing back into the coil and equalizing. Some flybacks have a capacitor in them, which assists in storing the charge (http://dos4ever.com/flyback/flyback.html). Following this, as the frequency is increased, the voltage that is outputted each second increases, because the magnetic field is created and collapsed more times each second.
However, this relationship does not continue forever. At a high enough frequency the transformer goes into saturation. This means that the magnetic field does not have time to fully collapse after each pulse. Since the magnetic field is not collapsing as far, less energy is being dumped into the windings of the secondary coil, and less voltage is generated. The magnetic field does not fully collapse due to the imperfect nature of the materials it exists in. If all materials were ideal, the magnetic field would be able to form and collapse at the speed of light, allowing for frequencies which are almost infinite. (http://focus.ti.com/lit/ml/slup124/slup124.pdf)( http://focus.ti.com/lit/ml/slup124/slup124.pdf)
A capacitor is an electrical component which stores electrical charge. It does this with two surfaces, or plates, that are parallel, with a dielectric between them. A dielectric is a material that does not allow electrons to flow through it. This prevents any current from flowing, but allows for charge to build up on one plate, and an opposite charge to form on the opposite plate. It is called a capacitor because of its capacity to store charge. Capacitors were placed at certain places within the circuit to prevent the electrical noise and voltage spikes caused by the transformer from interfering with the operation of the circuit.
Pulse Width Modulation and Audio Modulation
PWM, or Pulse Width Modulation, is the modulation (changing) of the width (or length) of pulses of electricity. In this experiment, the frequency is changed through pulse width modulation. This will be explained further in the portion discussing the astable operation of the circuit. Audio modulation is very similar to PWM. It is the changing of a signal by using an audio signal, after which the modulated signal is carrying the audio signal.
An integrated circuit is a circuit that is completely contained, or integrated, into a small plastic and silicon chip. Access to the circuit is provided by pins along the side of the chip.
A 555 integrated circuit is an integrated circuit that makes its output (pin 3) high when its trigger (pin 2) is less than 1/3 of the input voltage, and makes its output low when its threshold (pin 6) is greater than 2/3 of the input voltage. In addition, it also has a control pin, which can be used to modify the threshold voltage, and a discharge pin, which can be used to discharge a capacitor. All of these features allow it to be built in a simple astable configuration.
An astable circuit is one in which the output is not stable in either a high or a low state, but is constantly switching between the two. The simplest astable configuration of a 555 timer is shown below.In this configuration, the capacitor is charged through both R1 and R2. The resistors act both to protect the chip from voltages shorting through it during the discharge portion, and to also set the frequency of the output signal. They slow the time which it takes to charge the capacitor by limiting the current that reaches its plate. Charging a capacitor through a resistance creates what is known as the time constant. The time constant is the resistance (in Ohms) multiplied by the capacitance (in Farads), which gives you the amount of time in seconds that it takes to charge the capacitor to 63% (http://www.electronics-tutorials.ws/rc/rc_1.html). After the capacitor charges to above the threshold, it is discharged through the discharge pin (#7). By combining all of these portions of the operation of the circuit, the resulting equation for frequency is:
The duty cycle is the percentage of the time that the signal is high, and can be determined by:
Based on these equations, the R2 value can be changed to change the frequency, and the duty cycle must also change with it. This creates a PWM setup, which allows the frequency to be changed by varying the resistance. Changing the R1 value will change the relationship between frequency and duty cycle, so for the purposes of this experiment, it was kept at a constant value. In order to ensure that the threshold voltage remained constant, the control pin was connected to ground via a small capacitor and no audio was played.
MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor. It allows large voltages and currents to be switched using rather low input currents because it switches based on the electrical field created by the electrons rather than the electrons physically entering a semiconducting zone and allowing current to flow. How MOSFETs operate is not relevant to this experiment and will not be discussed further. More information can be found at (http://www.electronics-tutorials.ws/transistor/tran_6.html).
Magnet wire is wire with a special, thin insulation that allows it to be wrapped closer to the core of the transformer which allows more of the energy to be transferred.
Ultrasonic Base Frequency
The ultrasonic base frequency is the frequency that the circuit oscillates at without any audio input. It is ultrasonic, or out of the human hearing range (>20kHz), so that it seems to be silent. It is needed because the 555 does not allow true modulation, only control of the existing frequency. This is the frequency that was controlled in the experiment.
Modified Computer Power Supply
A modified PSU was used for 3 reasons. Firstly, it will provide a constant 12V source of power. It is also designed to shut off is a short is detected in the circuit. Finally, it uses a true ground line running to the earth, which allows for a much greater absorption of the voltage spikes created by the transformer.
- 1 555ic
- 1 breadboard
- 1 IRFS654 MOSFET
- 1 1000uF cap
- 1 470nF cap
- 1 100nF cap
- 1 10nF cap
- 1 1nF cap
- 2 50K trimpots
- Socket for MOSFET
- 1 modified computer PSU
- 1 2 axis vise
- 1 12V sealed lead-acid battery
- 1 flyback transformer
- 20ga Hookup wire
- Large stranded wire
- Wires with alligator clamps
- 2 banana plugs
- Assorted wood
- 2 electrodes
- 1 tub for icewater
- 1 heatsink
- Thermal paste (sunscreen works too)
- 2 metal plates (for mounting)
- Electrical tape
- Zip ties
- Assorted bolts, nuts, and washers (for mounting)
- Hot glue (for insulating transformer pins)
- Solder and soldering iron (for assembling)
- 1 multimeter
- 1 audio jack, cable, and audio source (for speaker)
- 1 oscilloscope
- 1 set of calipers
- Construct all necessary equipment, apparatuses, and circuits
- Set the R1 value using the multimeter.
- Set the R2 value using the multimeter.
- Attach positive oscilloscope lead for channel one to pin 3 of the 555 IC.
- Attach negative oscilloscope lead for channel one to ground.
- Apply power from the 12V battery to the 555 IC.
- Observe the output signal trace on the oscilloscope
- Record the frequency and duty cycle.
- Disconnect power from the 12V battery from the 555 IC.
- Turn the computer power supply to the transformer on.
- Set the terminals far apart.
- Check to make sure the MOSFET is adequately cooled.
- Apply power from the 12V battery to the 555IC
- Move the terminals together slowly until a blue arc is initiated.
- Turn off both power supplies.
- Measure and record the distance between the terminals using the calipers.
- Repeat steps 3-15 for each of the desired R2 values.
Data and Graphs (and a couple 'scope traces)
The best way to look at the optimum point is by looking at the Frequency vs. Maximum Spark Gap graph, since distance is directly proportional to the voltage. This graph clearly shows a curving trend, peaking at a distance of 24.76 mm with a frequency of 35.7 kHz. The data points become more spread out towards the top of the curve, but this is slightly exaggerated as each division correlates to only half of a millimeter. The R2 vs. Maximum Spark Gap was also plotted, and it also showed the curving trend.
It is believed that the distance jumped increased up to the maximum value at the frequency of 35.7 kHz because as the frequency increased, the magnetic field collapsed more times each second, and more energy was dumped into the secondary coil over the same amount of time. It is believed that the distance decreased above this frequency because the transformer went into saturation, and the magnetic field did not have time to fully collapse and dump its energy into the secondary coil with each pulse.
Across the data, the duty cycle did not change in a perfectly straight line as it should have, but had some up and down fluctuations. This is believed to be the fault of either the oscilloscope, which could only measure to the nearest full percent, or due to complex inconsistencies within the chip itself.
The frequency and duty cycle data was collected with the power to the flyback disconnected because the operation of the flyback and the strong electrical fields created caused interference picked up by the lines of the oscilloscope probes.
The optimum frequency was 35.7 kHz for this transformer with this frequency-Duty cycle relationship which produced a maximum spark gap distance of 24.76 mm. This is the point at which the maximum output voltage is being created by the flyback transformer.
My Hypothesis was that the voltage, and maximum distance arced, would increase as the frequency increased. My results do support my hypothesis. However, my hypothesis did not account for the drop in voltage and distance arced that was created past a certain point when the transformer began to go into saturation. Because of this effect I was able to determine that there is a maximum output voltage point which correlates to the optimum point at which to operate a plasma speaker for maximum volume. For this circuit that value is 35.7 kHz which is obtained by setting R2 to a value of 13 KOhms.