DRSSTC experiments

["Tesla list" <tesla@xxxxxxxxxx>]

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