For
someone who dabbles in solid state Tesla coils, it was high time I
built a DRSSTC. The difference between a conventional SSTC and the
DR-SSTC is the use of "dual resonance". What this means is both the
secondary circuit (the topload and secondary coil) AND the primary
circuit (now consisting of both a primary winding and a tank
capacitor) are tuned to form a coupled resonant circuit. Just like the
old spark-gap Tesla coils, which were popular before solid state
coiling gained steam. The use of a rather high-Q resonant primary
circuit results in massive tank currents, often in the range of
200-500A, which requires a low average duty cycle to keep power
dissipation down. Typically a DRSSTC is run at 1-5% duty cycle using an interrupter,
and yet is able to produce impressive streamers despite low average power.
There
is a wealth of information on DRSSTCs freely available on the internet,
which I advise you to read. Check the provided links further down for
starters. The driver design I used for this coil is one of Steve Ward's
designs.
I designed my own PCB based on his schematic, and had a bunch of
PCBs fabricated by Seeedstudio. Gerber files and a mouser order list
for the driver can be downloaded here. The interrupter I used was
something I
had completed a long time ago, the universal Timer/interrupter module.
With that done, I measured the resonance frequency of the coil with
topload, which was used to determine the tank capacitor/ primary
winding dimensions. In order to made tuning of the primary circuit
easy, many turns were desired on the primary winding. This would
lessen the inductive contribution of a single turn to the entire
circuit, meaning I could set tap points at larger intervals. It also
lessens the effect of undesired inductance in leads and internal wiring.
Construction details.
Specs: IGBTs: FGH60N60SMD Tank Capacitor: 3x 942C20P15K in series for 50nF Resonant Frequency: 200kHz Primary Diameter: 11cm Secondary Diameter: 8cm Interrupter
Settings: 250µs on time, 150Hz repetition rate. Have taken up to 360µs,
170Hz, although this doesn't increase streamer length, only arc power.
Streamers! Exposure time is 1/2 second, f2.8.
Some
minor gotchas I encountered were phasing of the CT transformer used for
feedback, in relation to the primary winding and GDT. Also, coupling
needs to be fairly low to avoid racing sparks. It's not immediately
clear in the images above, but the secondary is raised enough that the
bottom turn is roughly level with the top turn of the primary. The
primary tank capacitors had to be separated from the driver box due to
corona forming at their contact point. 3mm of plexiglas placed between
the capacitors and the box was enough to prevent this.
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.
This work is licensed under a
Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.