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DRSSTC experiments



Original poster: "Antonio Carlos M. de Queiroz" <acmdq@xxxxxxxxxx>

Some time ago I started experiments with a DRSSTC, described here:
http://www.coe.ufrj.br/~acmq/tesla/drsstc1.pdf
I didn't update the documentation, but last week I and a student finally tested the system at full power. We made a driver using components from discarded PC power supplies. Input filter, rectifier, half bridge transistors, fast diodes, and even the transformer driving the bridge were all taken from them. A 12 V power supply to power the driver was also made with the components of a 5 V standby switched power supply that these power supplies contain. We used four transistors in parallel, with equalizing base resistors, in each leg of the bridge, what is enough to about 30 A of current. The components proved adequate to switch at 294 kHz, using the technique of driving the system between the two resonances of the system. A CMOS controller was built, designed to produce a burst of an adjustable number of cycles to the driver, with a break rate also adjustable from a few Hz to about 2 kHz. There are provisions to cut the burst if the current exceeds a limit, but the current meter was used only for measurements. The meter is just a toroidal coil with the return wire of the primary coil trough it, followed by an RC integrator, producing 100 mV/A. This picture shows the driver output voltage and the primary current, with the system adjusted
for 10 cycles bursts:
http://www.coe.ufrj.br/~acmq/tesla/finalcurrent.jpg
And this picture shows the driver output voltage and the output voltage without breakout, captured with a probe suspended about a meter away from the coil. The peak value obtained was about 60 kV.
http://www.coe.ufrj.br/~acmq/tesla/finalvoltage.jpg
The coils and primary capacitor were the same of my test Tesla coil:
http://www.coe.ufrj.br/~acmq/tesla/tefparcg.jpg
What I can comment is:
- The power (probably the voltage) was insufficient for great results. The same system powered by
a 5 kV NST and spark gap performs much better. Faint arcs to a grounded target
could be obtained with up to 15 cm of length, without a breakout point at the terminal. These arcs don't discharge the terminal significantly, unless reduced to less than 10 cm, when they
become clearly visible.
- The peak voltage was evidently greater with excitation between the resonances. Excitation at the resonances only increased the input current, quickly exceeding the limits of the transistors. - The output power could be easily monitored by the noise of the arcs, even when they were not
visible.
- Nothing was burnt :-) The tests were made with a 100 W lamp in series with the power supply, to prevent disasters. The lamp was dull red at the maximum power. Once I accidentally turned the power on while an expensive signal generator was connected across the secondary coil. The generator survived, because the 50 Ohm output short-circuited the coil and limited the output current to a safe level...We stopped the tests before installing a breakout point to see if streamers develop, because
the computers in the lab started to do strange things...
- Something important about systems with interrupted operation: We found that it is necessary to guarantee that the bridge transistors are cut off, by inverse biasing their base-emitter junctions, after the burst ends. Otherwise the very high dv/dt when the energy returns trough the free-wheeling diodes turns then partially on with enormous power dissipation. This was simply achieved by short-circuiting
the primary side of the driver transformer.

Antonio Carlos M. de Queiroz