Introduction: Class D Audio Modulated Kit for High Voltage Generation and RF Power Demonstration



This kit is a high voltage device. It is intended for individuals over 18-years old with proper knowledge of electronic. Before using the kit, make sure you have thoroughly read the safety information.

This device will generate electrical and magnetic fields. Do not use this device if you have an implanted pacemaker or other biomedical device. The RF fields and noise generated by the device can potentially damage the audio source. The use of portable audio device such as MP3 players, IPODs, Iphones, etc… is NOT RECOMMENDED.

Fire hazard. This device creates a high temperature arc across the electrodes. Make sure the device is not near any flammable material. Do not touch the electrodes when it is hot.

This device is used for demonstration or proof of theory only. Always work with more than one people when testing this device. Wear eye protection and do not work on bare feet. ALWAYS allow proper time for any large electrolytic to discharge after removing power prior to working or touching any circuit. Use the metal pin to short the electrodes to discharge the coil before touching the electrodes.

Step 1: Theory of Operation



Class-D amplifier is one in which the output transistors are operated as switches. When a transistor M1 is off, the current through it is zero. When it is on, the voltage across it is small, ideally zero. In each case, the power dissipation is very low. This increases the efficiency, thus requiring less power from the power supply and smaller heat sinks for the amplifier (Figure 1).

Figure. 2 shows the circuit waveforms for the case where vS is a sine wave. For purposes of illustration, the sine wave frequency is fS = 1 kHz and the triangle wave frequency is fT = 40 kHz. For vS > 0, the duty cycle of the square wave changes so that vo’ spends more time at its positive level than at its negative level. This causes vo’ to have a positive average value. Similarly, for vS < 0, vo’ has a negative average value. The waveform for vo’ is said to be pulse-width-modulated. When Vs is zero volt, the duty cycle of Vo’ is set to 50%. The duty cycle of Vo’ is changing depending on Vs.

Step 2: Circuit Design

IC XR2209 is configured as triangle wave generator. The output frequency is set to 40KHz. The frequency is adjust by changing the value of C6 and Rset (Check XR2209 datasheet for detail). Part A of IC LMC6482 is configured as inverting amplifier. It amplifiers the audio signal about 10 times. The gain is set by R5/R1. Since the triangle wave is centered about 5.1V, the output of part A of LMC6484(pin 1) need to shift the same level in order to get 50% duty cycle when audio signal is zero. The offset level is set by voltage divider R6/R5. Part B of LMC6484 is used as comparator. When input voltage at pin 5 is higher than at pin 6, the opamp will output its maximum value, which is 9Volt in this design; when input voltage at pin 5 is lower than at pin 6, the opamp will output its minimum value, which is zero in this design. The output signal at pin 7 is a PWM signal, and the PWM signal turns the power mosfet Q2 on and off. The load of the circuit is connected between power source Vcc and the drain of Q2.

Output Connection:

For easy and fast switching between the plasma speaker function and RF demonstration function, I use a DPDT switch to switch the connection between flyback transformer and the RF coil.

Check the circuit diagram for coil configuration.

Step 3: Plasma Speaker and Jacob's Ladder Configuration:

A plasma arc speaker or plasma tweeter is a speaker that creates sound by varying air pressure through a corona discharge or electric arc. As the ionized air in the electric arc thermally expands and contracts (i.e. Heats and Cools) during the audio modulation, it vibrates the air in and around the arc creating sound. These Plasma Speakers require a larger amount of power than conventional speakers, and generate more heat. This is an inherent disadvantage to the plasma arc systems because of the requirements to produce electrical arcing. This device uses a monitor flyback transformer to generate a high voltage arc. It needs about 20 Kv/cm to create the arc. The flyback transformer is driven by a Class-D audio amplifier. For best result and sound quality, set the electrodes about 5mm apart.

Caution: Always turn off the system and then short the electrodes to discharge the secondary coil before touching or exchanging the electrodes. Also, the electrodes are VERY HOT!!

Step 4: Wireless Power and Radio Configuration:

The RF receiving circuit is shown below. Receiving coil and C1 together is the resonant circuit that resonates at 40 KHz. The frequency is given by

f=1/(2*3.14*sqrt(L*C))

L is the inductance of the receiving coil; C is the capacitance of C1.

The inductance of the receiving coil is given by:

L= (d^2 * n^2)/(18d+40l)

where:

L is inductance in micro Henrys, d is coil diameter in inches, l is coil length in inches, and n is number of turns.

For the transmitting coil, 20 turns on a 2-inch PVC pipe using 22AWG wire. For the receiving coil, 35 turns on 2-inch PVC pipe. D1 rectifies the signal to drive the speaker. D1 should be fast recovery and low voltage drop Shockley diode. In the current design I use 1N5819. C2 filters out the high frequency component of the signal, but it is optional. You can try different value and hear the sound difference. I recommend you can try from 0.1uF to 1uF. For the speaker I use an 8ohm 3W speaker. If you plug in the audio source to the system and place the receiving coil near the transmitting coil, you can hear sound from the speaker.

When the switch is connected to D2, D2 rectifies the signal, and C3 make the signal to a smooth DC signal. I place the transmitting coil and receiving coil about 5mm apart and measure the power output of the receiving circuit. I replace the lamp with a variable resistor and measure the voltage across it. The power output is calculated as P=V*V/R. The maximum output power is about 4W at 150 Ohm load. The output voltage is about 45Volt when I drive the system using a 19Volt power supply.

Step 5: Putting Things Together

I use some acrylic boards to mount everything together.

Plasma Speaker



1. Make sure the power is OFF!!
2. Loose the two screws as circled in the picture above to change the electrodes. I use some 12AWG wire as electrodes.
3. Adjust the gap between electrodes to be about 5mm apart.
4. Turn the toggle switch to the flyback transformer side.
5. Turn on the power.
6. Plug the 3.5mm audio input cable into CD player or radio audio output. Adjust the volume of the player output. If the output of the player is too large, it will cause the Class-D amplifier saturate, over drive the circuit, and burn the fuse.
7. Adjust the electrode gap for best sound quality.


Note:

The electrodes are very HOT. Let it cold down before touch.

When changing electrodes, always turn off the power first. Use the short metal pin to short the electrodes to discharge the energy stored in the coil.

Jacobs Ladder

This device also can be converted into Jacobs Ladder. Beware that the right electrode will be very hot, so always make the right electrode long enough to avoid over heat. Also, do not operate the circuit for a long time in this configeration since the electrode might melt the connector!!

Ring Electrode

I also use a 0.5inch ID brass washer from Home depot (item number 204596441) as one of the electrodes, and put another electrode inside the washer. You can see the arc is turning around inside the washer. You can try different size of the washer to get best effect.

RF Power and Audio Transmission

I mount the RF receiving coil, light bulb, speaker, and the circuit board on another plactic board.



1. Turn off the power.
2. Turn the toggle switch to RF coil side.
3. Turn on the power.
4. Place the RF receiving coil close the RF coil, and turn the switch to the left side on the RF receiving coil board. You should be able to see the light bulb will glow, and the light intensity changes as the distance changes.
5. For audio transmission, turn the switch to the right side on the RF receiving coil board. Plug the 3.5mm audio input cable into CD player or radio audio output, and you should be able to hear sound from the speaker.
6. Adjust the volume of the player output. If the output of the player is too large, it will cause the Class-D amplifier saturate, over drive the circuit, and burn the fuse.

Step 6: More Videos

Step 7: Have Fun With Caution

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Unlike the ordinary speakers, the plasma speaker are dangerous high voltage device, do NOT attempt to build this device unless you know what you are doing... And do NOT even attempt to build one if you have heart problems or weak heart or wearing a pacemaker, because one little shock from this thing can put you out... I am not responsible for any injuries or deaths caused by this device. Since the plasma speaker generates high voltages, there is a chance there will be high voltage spikes on the low voltage side of the device, which can get onto the audio line and damage (or destroy) the player. They are some safety features to prevent the spikes damaging the player, but they can fail... So I am not responsible if your player gets damaged. This device is intended for educational and demonstration purposes only. It is not intended for use in commercial applications. If it is used in such applications, the user assumes all responsibility for ensuring compliance with local laws.

Comments

author
bluenoiseMF made it!(author)2015-12-05

Great write-up! I especially appreciate the clear explanation of the circuit's operation and the use of PWM in this application.

author
bbsbb made it!(author)2015-12-05

thank you

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