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Control Board Theory
- 555 timer oscillation circuit tuned to generate high frequency (22.5KHz ultrasonic) square wave [variable with trimpot]
- Low-power square wave drives high-power switching MOSFET through a gate driver transistor
- High-power square wave from MOSFET drives LC resonator between primary coil (L) and on-board capacitor bank (C)
- Primary coil builds magnetic field in ferrite core of flyback transformer when on, and dumps the energy from that magnetic field into the transformer's secondary coil when off
- Secondary coil creates a high voltage across spark gap to maintain current flow, ionizing the air and producing a continuous plasma arc
- We want to trip the active-low reset of the 555 timer oscillator with a square wave at sound frequency to pulse the plasma arc at that frequency and produce a tone
- Putting 12V at the reset pin activates the arc oscillation, and 0V at the reset pin turns off the oscillation and the arc
- Comparator measures an audio signal relative to signal ground, and pulls the 555 reset pin low when the audio signal crosses a given threshold (just above signal ground), otherwise pulling it high [MAYBE WE SHOULD REVERSE?]
- Comparator essentially converts an audio signal of variable frequency and amplitude to a fixed-amplitude square wave matching the input frequency
- Filters on the input clean up signal noise
This section explains each part of the control circuit in detail. The reader should reference the schematic (EasyFlybackRev2.sch) while reading this section.
The Power Supply provides power to the rest of the circuit. Power (24V, ~15A) is supplied to the screw terminal DC_IN. This feeds a 24V rail that primarily goes to the Power Stage. The 24V is also fed into a voltage regulator (IC3) with smoothing capacitors on both the input and output. The 12V output from the regulator drives the 12V rail used by the Comparator and Oscillator stages. It also feeds a power indicator LED (LED1) and outputs to a screw terminal (12V_OUT). This screw terminal may be used to drive a cooling fan.
The Power Stage drives the primary coil of the flyback transformer. It is fed by the 24V rail and has a bank of smoothing capacitors (C9-14) to prevent voltage sags when current draw is high. The primary coil is controlled by a large MOSFET (Q2). When the MOSFET is open, the capacitor bank charges. When the MOSFET closes, that energy is dumped into the primary coil. The large MOSFET requires a significant amount of current to switch states. This current is supplied using a gate driver made with a BJT (Q1). The signal comes from the Oscillator stage.
Note: this part of the circuit was copied directly from Franzoli. We haven't had time to fully reverse engineer it yet.
This stage of the circuit filters the audio input. An audio signal comes into the audio jack. It then passes through a low-pass filter made by R2 and C3. This filter is designed to remove noise that is outside of the hearing range. It is tuned to have a corner frequency of 20kHz (Fc = 1/(2piRC)). The output from the low-pass filter is fed into the Comparator stage.
This stage outputs a square wave matching the input audio signal. Both inputs of the comparator are DC biased to 6V (although the bias on the positive input can be adjusted using the potentiometer VR2). The capacitor C4 blocks the DC from flowing into the audio source, while allowing the AC signal from the audio source to pass through. This capacitor (C4) in combination with the biasing resistors (VR2) create a high-pass filter with a corner frequency of a few Hertz. The audio signal is an AC wave with no DC bias. With the positive input of the comparator biased just slightly below that of the negative input, the comparator will pull the output high whenever the audio signal is positive. It will pull low when the audio signal is negative or zero. Normal operation can be overridden by the test button (S1) which will force the audio signal to be above zero. The output from the comparator is fed into the Oscillator stage.
This stage creates a high-frequency square wave to operate the flyback transformer. To create a continuous arc, the primary coil of the transformer must be turned on and off at a high frequency. Each time the primary coil is turned off, the magnetic field inside the transformer collapses on the secondary coil creating a high voltage. The square wave that feeds the primary coil is generated by the 555 timer (IC2). Our circuit works best at 22.5kHz, and that frequency is controlled by the potentiometer VR1. To create audible sound, the 555 timer is turned on and off by the audio signal. This is done by feeding the output of the comparator into the reset pin of the 555 timer. The output of the 555 timer is fed into the Power Stage.
Note: this part of the circuit was copied directly from Franzoli. We haven't had time to fully reverse engineer it yet.