Tesla coil 18 inch projects on this page include:
Well, here comes the big one. Time to give it my best shot. The big cheese... Now at first light!
First light sparks are from a hasty lash up. The primary is thin 19G wire taped onto a flattened cardboard box on a wooden pallet. The topload is a toroid with a sphere, with a breakout point, sitting on top. The coupling was all wrong, the tuning very brief (11 turns) and everything was wet with dew. My ARSG gap was not optimised. The current draw was over 20 A (5KVA). Still the 5 foot sparks were gratifying and limited by the proximity to the basketball ring. Neither the Tesla coil or basketball ring were easily moved to get a peak spark length. There were some problems and the current draw did not 'feel' like a high inductance coil. This may simply be a reflection of the larger capacitor used. The power would tend to fade at intermediate power but not at high power. The spherical electrode makes this look like a scaled down Electrum.
Progress and plans so far:
Secondary former. The former is made of 3 mm (0.12 inch) polypropylene sheet welded around end caps and one centre cap. This is light (weighs 9 Kg, 20 lbs) and the polypropylene is a suitable plastic in regard to dielectric losses. It measures 45 x 150 cm (18 by 59 inches) and cost AUD$250 to be made up by my local plastics shop.
Secondary coil 1330 turns of 0.9 mm(19 AWG 0.037 inch) wire on 45 x 137 cm (18 x 54 inches) elevated 15 cm (6 inches). I have obtained 11 kg (24 lbs) of C 220 (220 deg C) polyimide coated enameled wire. Wire with 200 deg C rating was suggested by Dr Resonance. See other comparative wire ratings. Cost was AUD$ 165. I have used a geared down motor and a latex belt to drive the rotation for winding. That's 7 seconds per turn at 1400 turns = 3 hours in theory.
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Unfortunately, I have been beset by Murphy's Law. Winding was interrupted when only 1/3 complete, by rain and gale force winds. The following day the windings had loosened and overlapped, possibly due to compression of the rather thin polypropylene former. I unwound the few hundred meters and started again. This time the winding belt broke so family were called out to provide the rotation. Then the wire ran out about 4 inches short of the end. Either my calculations were out or the wire was less than the specified 11 kg. I am not happy about the smoothness of some of the turns as well but will have to wait and see. I gave several coats of polyurethane (total 1 litre) to fix the windings.
Toroid I have used a truck inner tube of 9 x 33 inches covered with aluminium tape. Of course in keeping with Murphy's Law (above) I found a puncture half way through taping the tube requiring hasty patching. Cross fingers and hope.
Synchronous rotary spark gap (SRSG) 100 breaks per second (below). I have modified a 1/8 HP 2850 RPM 50 Hz motor for true synchronous operation at 3000 RPM = 100 bps for 2 electrodes. This was done by milling two flats in the rotor to a total of 30 % of the circumference with an angle grinder. For motors of half that speed you will need to grind 4 flats to a total of 40% of the circumference.
A synchronous motor is the most efficient way to discharge the tank capacitor as it discharges when it reaches full charge at each peak in the mains 50 Hz cycle which is 100 beats per second (BPS). The synchronous rotation is visible under fluorescent or incandescent light if there is a white mark on the disk. More accurately I use my high intensity Luxeon Star white LED run off half wave rectified DC adjusted so it only just turns on at the peak voltage. I will also use the ARSG as well to compare. This motor is a bit small but probably is OK for my current disc size but might not be for a larger one.
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The middle photo above shows the spark gap connected to the MOT supply without a capacitor. The power arc extends almost around to the other contact. The last shot is with the normal Tesla coil setup with the capacitor discharge giving an intense spark.
I plan to use a phase controller by John Freau.
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4 MOT supply as with previous coils. Maybe a pole transformer if I upgrade the Scitech coil.
I have obtained 3 Powerex IGBT Modules (half-bridge config) CM300DY-24H. (Thanks Dave Trimmel). These are rated at 1200V 600A and are suitable for a DRSSTC (double resonant solid state Tesla coil)
I have acquired a large 60 kW 30 Mhz transmitting triode that runs on 12 kV and has a filament current of 89 A. Perhaps a little large for my first attempt at a VTTC (vacuum tube Tesla coil).
This page was last updated August 28, 2005