The 40,000 Watt Experimental Coil

Originally constructed in 1990 by G. Leyh for the Seattle SRL show, this 40,000 watt Tesla coil has served as a test bed for several design improvements to the traditional Tesla coil topology.  This coil was the first ever to feature a DC resonant charging power supply, thus allowing operation with three-phase power and with improved load balancing and power factor. A variable-speed rotary gap provides control of the output power, eliminating the need for large Variac transformers on the mains. The coil is now located at the Lightning Lab's San Francisco Facility, and operates as a design study platform.

Design Upgrades to the 40kW Experimental Coil

Primary characteristic impedance. Live measurements of the 4-stage rotary gap have revealed that its effective operating impedance is actually several times higher than values expected from original design extrapolations. Since the overall efficiency of the coil highly dependent on the primary Q (primary surge impedance divided by the rotary gap impedance), we increased the surge impedance in order to improve overall efficiency. The large rolled copper bars of the primary coilform were bisected laterally, effectively doubling the number of turns and increasing the primary impedance by a factor of 4. This reduced the losses in the primary tank by about a factor of 3.

Closeup view of the bisected primary coil

New Technical Specifications

Secondary Toroidal Capacitance. Live measurements performed in 1998 of the sphere-to-arc currents on the Electrum coil demonstrated the abundant presence of dart leaders in the advancing streamers. In order to enhance the dart leader performance we increased the capacitance of the toroidal top electrode by nearly doubling its physical size. Even though the larger toroid reduces the output voltage, the improved effectiveness of the advancing dart leaders more than compensates by providing the leaders a low-impedance local reservoir of charge.

Photos of the toroidal electrode construction