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Doing it the traditional way

It is a sad truth that no affordable semiconductors can switch as much power as good old nineteenth century technology. Yes, bin the mosfets, scrs, triacs and all, and make way for the spark gap.

This coil was very much designed around what I could get my hands on - mostly one-off purchases and salvage. In particular I used the 4" diameter coil that I had already wound for my first solid state experiments.

The power supply.

At first the power supply was a 5500V transformer plus a 2000v transformer in series. The current was limited by putting a 2.5k electric heater in series with the primaries; but resistance causes a loss of voltage and power. I replaced the heater with some large inductors. This greatly improved the output. Instead of voltage loss, all the loose induction together with the main capacitor gave a resonance voltage rise. I was able to dispense with the 2000v transformer.

The main transformer, tr1 is a 5500v, 350mA transformer. Tr2 and tr3 are microwave transformers with current limiting shunts, the secondaries are deliberately shorted out, making them current limited inductors. Var 2 is a 110v 10A variac which is being used as a variable inductor - the current is fairly controllable between 5A and 10A. Var 1 is a 240v 8A variac, it is tapped to give a maximum output of about 250V. The fuse is a 10A slow blow type. The initial magnetising current of the variac tended to blow fuses so a surge suppressor r1 was added; this can be shorted out by a switch to avoid it over heating. There is also a filter capacitor c1, 30uf at 440v (intended as a motor run cap) to suppress any RF interference to the mains.

This is the power supply at the lash up stage: top left 5500v transformer, top centre microwave transformers, right 240v variac, bottom left microwave transformer (which was used in series), bottom centre 110v variac and ammeter. These components have now been cased which the addition, in series, of a second 110v variac used as a fixed inductor, and two switches so that this variac and one of the microwave transformers can be removed from the circuit (shorted out).

The Coil.

The coil is about 1500 turns of 0.4mm wire on a 4inch diameter drain pipe. The primary is a flat spiral of 10 turns of 3/8 diameter copper pipe. The pipe is mounted on a sheet of polystyrene glazing, over a plywood base and is held in place using cable fasteners through holes drilled in the base..

The Main Circuit

The chokes and the safety gaps are to protect the main transformer and the capacitor from overvoltage. The main spark gap is 5 pieces of 2inch diameter copper pipe about 3" long.

The capacitors are six 0.1uf 10kV DC capacitors; these are arranged in two sets in parallel of 3 caps in series giving 0.067uf at 30kV DC. The box under these caps contains another 2 caps 0.01uf, 28kV DC in parallel to give a total of 0.087uf.

Still at the lash up stage!

Performance

The performance is great if its the only conventional coil you've seen! 30 inch hot arcs in all directions. But I know from the list that this performance is below average for the amount of power I'm putting in, so it's back to the drawing board. I'm planning a new secondary - 6" or 12" diameter maybe 1000 turns of 0.56mm wire, a conical primary made from 1/4" copper pipe, slightly bigger toroid using 4" Aluminium ducting, a little more primary capacitance and a couple more gaps with a fan behind them. The only difficulty is going to be pushing out the garage walls to make room for the sparks!

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