Big Coil (DRSSTC v2)

Big Coil has been an ongoing long term project, with a bit of recent research into the optimal ways to get phase lead compensated feedback working within a half cycle at the start of each pulse. It is trivial to get a phase leading signal a cycle or two from start, but it has proven relatively difficult to get it immediately.

The project has also gone through a number of gate drive revisions. I’m currently using optically signaled drivers on each brick with a 60hz iron isolation transformer powering the board. It pulls the gate +20V/-5V, capable of about 30nS switch time with minimal gate resistance, although I put an ohm or two in to help limit any ringing that could occur both on the gate, and more importantly voltage spikes that could form from emitter to collector.

Machining bus-plates

Bridge assembly

Resonant tank capacitor (CDE942 array)

Logic controller

Realtime tuning inductor with stepper motor

Primary current waveform with secondary in place at tuned resonance.

  1. Did you ever get further with your v2 of this coil? The use of a motorized variable capacitor is not a new idea, but very few have tried it, so was looking forward to hear about your results :)

    Your DC bus filtering capacitors should be mounted directly to the very sound made busbar to get a short high current path. The ripple current ability of 4 capacitors in series, and their ESR is not very good. If the series connection is needed for the 1000VDC supply, you might have to consider exchanging it to all DC link polypropylene capacitors.

    Kind regards

    • This coil worked fantastic… I ultimately did add a bunch of film caps because as you mentioned, the electrolytics weren’t doing much except making a big boom when something went wrong.

      The motorized inductor was perfect. I put a scope on the primary current when running, had a remote motor control, and turned it until the current envelope had a zero-crossing indicating maximum coupling and efficiency. Sparks got bigger and everything ran cooler… of course you don’t need a motor inductor to get the system optimized but it was vastly easier.

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