WARNING!: Working with electricity is dangerous, all information found on my site is for educational purpose and I accept no responsibility for others actions using the information found on this site.
Read this document about safety! http://www.pupman.com/safety.htm
This is a audio modulated arc generator designed for simplicity rather than reliability, its made with very few and common components. There is however some serious trade offs described below in considerations.
WARNING: sensitive audio players might get damaged by this circuit. I bricked my iPod shuffle, seems that the controller chip for the mini jack got wasted as it could no longer detect charger, PC connection or play music as it could not detect headphones.
The arc have to be very short in order to limit the distortions of an unstable arc. The sound quality is low due to the way the audio modulation is implemented. If the distance between the elctrodes is too large, the high open loop potential of the high voltage transformer can generate some high transient voltages that through inductive kickback can destroy the MOSFET.
The 555 IC supplied by 12 VDC can not source much more than 140 mA before the voltage drop on the output gets very high. At 140 mA it is already 1,82 VDC. At 200 mA the voltage drop is at 2,5 VDC. The low output will affect MOSFET switching speed and result in higher losses. This graph shows the voltage drop vs. output current of the 555 IC.
In order to optimize the switching of the MOSFET, a small intermediate driver stage can be introduced with two transistors, a NPN and PNP. As illustrated in the red and green graph is the difference between running a MOSFET with proper switching and the other always in linear mode, where losses are very high. This is a future improvement and is not a part of this little project, but it is recommended to add this if you want reliability.
If the circuit can not produce a arc try to reverse the polarity of the primary coil on the flyback transformer.
There are basically 2 kinds of modern flybacks, television flybacks are driven near 15kHz and monitor flybacks are driven between 30-150Khz. Depending on which type we use, we have to adjust the frequency of the 555 timer to match the resonance of the flyback for maximum performance.
Choosing a MOSFET
There are some basic rules of thumb that I will just list here to start with, I will come with an explanation later on.
The voltage rating of the MOSFET (VDSS) needs to be 6 to 10 times higher than the supply voltage, reverse voltage spikes and EMF can be high enough to destroy the MOSFET if its too small. But we still need to use MOSFETs with a reasonable low on resistance (RDS(on)). Try to find a MOSFET with a RDS(on) value not much higher than 0.1 ohm, if you have problems try one with a lower RDS(on) value.
The gate resistor R3 is there to
- Limit parasitic oscillations that could kill the MOSFET.
- Limit the current that is needed from the driver stage, in this case our 555 timer.
- Protect against surge voltages on the MOSFET gate, effectively this would require a much higher resistance, a high gate resistance would lower the operation speed significantly.
- The values of a gate resistor could be anything between 10 ohm to 200 ohm, it all depends on the MOSFET, experimentation is needed. The alternative is complicated calculations involving data that is usually not available in standard data sheets.
How does the audio modulation work?
Pin 5 on the 555 timer is a direct access to the 2/3 voltage divider point of the upper voltage comparator in the 555 timer. This allows us to pulse width modulate the output on pin 3 of the 555 timer. By applying a voltage to this pin, it is possible to vary the timing of the chip independently of the RC network. When used in the astable mode, as we do with this circuit, the control voltage can be varied from 1,7 VDC to the full Vcc. Varying the voltage in the astable mode will produce a frequency modulated (FM) output.
If the control-voltage pin is not used, it should be bypassed to ground, with a 10n capacitor to prevent noise entering the chip
Both R1 and R2 can be 10K potentiometers.
13th November 2008
I wanted to do a audio modulated flyback arc with few components and a small form factor. I installed the MOSFET on a old CPU heat sink with fan, the 555 timer circuit is also installed underneath this heat sink, its then all put on the side of the flyback transformer with wire strips.
The primary coil is 8-9 windings of 0,75 mm² isolated wire. More windings will stress the MOSFET less but also output voltage will be lower.
The frequency output from the 555 timer is 26,7 kHz at 59,3% duty cycle. This is in the low end for a monitor flyback so further improvements will be adding a potentiometer to adjust frequency to match the resonant frequency of the flyback.
2nd February 2009
Its time to improve the driver with a variable frequency control so the driver can be used with most conventional monitor flyback transformers without changing any parts, but merely turn the potentiometer.
I installed a 9K potentiometer as R1 and a 10K potentiometer as R2, I adjusted the potentiometers till I had a nice silent thin arc at about 15 mm length. 10K potentiometers can be used for both R1 and R2, I just used what I had at hand.
Using a 555 calculator with the measured values of the potentiometers. R1 at 1K3 and R2 at 1K. Duty cycle is 69.7% and frequency is 43700 Hz. Very reasonable for a monitor flyback. Compared to the old frequency I now have a longer and more silent arc.
A quick and very rewarding little project, its fun to play music without conventional speakers. This was also known in the 1970’s as a plasma tweeter and could be found in special hi-fi speakers.
The arc is very very hot and I had to extend the copper wires where it is drawn between to avoid the heat being transferred far enough to start melting the flyback transformers casing.
The 555 IC is not able to supply enough output current to drive a IRFP250N MOSFET at a high duty cycle, so the MOSFET will at times still be in linear mode and this causes excessive heating, which is why the heat sink is necessary. So more notes under considerations about this.