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PLL SSTC 1

PLL SSTC 1


Thick streamers from break-out point
Streamers with current transformer feedback, half wave rectified.

I thought it was about time I made a SSTC. This is actually the product of several weeks of trying different designs and secondaries. Instead of going with a fail-proof concept first I tried various discrete designs with little success, which I later see was most likely due to the uber high fres secondary I was testing with. Anyway I decided I just wanted it to work, so I made a low frequency secondary, and settled on Steve Conner's PLL. It has the advantage of easy startup thanks to a constantly running oscillator, and with an antenna providing feedback it's (nearly) always in tune. The best of both worlds- in one chip.

schematic

The capacitor on pins 6 & 7 and the resistors on pin 11 and 12 determine the frequency that the coil runs at. The ratio between R1 and 2 determine how far the oscillator can wander. The 10k potentiometer by pin 9 will adjust the voltage bias on the VCO input and alter the frequency, or if running with feedback adjust the phase angle between input and output. I suggest setting R1 and 2 for a 1.5 ratio during testing, and reduce the ratio as you determine the exact resonance frequency. This makes it much easier to tune for the perfect phase angle. The best way to set up the 4046 is by setting the resistor range first without the gate drivers, half-bridge or secondary. Basically find some ballpark resistor and capacitor values, and then use the potentiometer to see what frequency range you get. The 4046 is rated for an operating frequency of up to 2.7 MHz, so it'll work for practically any coil you may want to make. Audio modulation is also possible by further biasing of the VCO voltage, but I had to run my coil from half-wave rectified mains to keep it from burning up, so it was never implemented. Check Steve Conner's page to see how. Once the circuit is built the potentiometer must be used to tune for resonance.

The phase locking itself works by using an XOR gate to detect the phase angle between the two inputs, pin 3 and 14. The output from the XOR (pin 2) is a PWM signal, and the duty cycle will vary from 0 to 100% as the phase difference between two 50% duty square waves moves from 0 to 180 degrees. A low pass filter is used to get a DC voltage proportional to the PWM signal's duty cycle. This signal is fed into the VCO, which then oscillates at some frequency set by the timing components R1, R2 and C1. (12k, 15k and 330pF in this case) A constant DC bias is also be placed on the VCO input by the 10k potentiometer. Adjusting this bias allows you to roughly set the phase angle.

Finnished SSTC mounted on board
Mounted on a board for quick and easy use.

SSTCs function much like SGTCs. A large secondary coil with much inductance also has capacitance in the form of parasitic capacitance between turns and capacitance from to the topload ground. These reactive components form a resonant circuit, and if a waveform is applied at the right frequency the AC impedance of the setup is reduced to zero. By then coupling energy into this setup through transformer action we step-up the voltage by a ratio at the base, and this voltage is stepped up further by the resonant rise. The result is an incredible increase in voltage. For a much more thorough explanation see Richie Burnette's page on SSTC driving. Coupling is important for getting power into the secondary system, also explained in Richie's page. Quite simply put though, try to get the coupling factor as high as possible as this seems to work best for SSTCs. (K=1 implies that the primary occupies the same space as the secondary, which is impossible. High K means that the primary coil is wrapped tightly and over as much of the secondary as possible.) The limiting factor is flashover, when sparks occur between the primary and secondary. Due to insulating pipe size restrictions I was unable to tune for optimal coupling, and thus the mosfets heat quite a bit. Ideally I would have the primary reaching half-way up the secondary and with a few more turns. This would reduce magnetizing current and couple more current into the secondary.


Left: Nice thick streamers. This was before I changed the logic power supply to an auxiliary smps which altered the streamer appearance. The other three picture show the result, much longer, but thinner streamers. The pic on the top right shows how salt on the breakout point colors the streamer. This works well with other salts too, just look up flametests on wikipedia for other colors.


Videos! Poor quality and in .3gp format. halfwave input and borax breakout
Update: Youtube video!



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This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.

Disclaimer: I do not take responsibility for any injury, death, hurt ego, or other forms of personal damage which may result from recreating these experiments. Projects are merely presented as a source of inspiration, and should only be conducted by responsible individuals, or under the supervision of responsible individuals. It is your own life, so proceed at your own risk! All projects are for noncommercial use only.