Dual Resonant Tesla coil II
To mimic the bigger coils out there dual resonant primary/secondary is the way forward. This complicates the design and, as always with Tesla Coils stresses the electronics. This design is based on the medium sized coil with a 150mm diameter secondary as before but with a larger number of turns and finer gauge wire. The overall length is also up to 660mm. This arrangement reduced the resonant frequency to around 100kHz for the bare coil. The top load reducing this to 80kHz. Lower frequencies seem to create longer arcs. To allow higher operating powers IGBT block modules are used. Not only are these designed to handle higher currents they also don't produce the hot shrapnel on failure that individual IGBTs tend to. However, they still fail and cost a lot more. At the moment, the coil is fitted with a 10A breaker so limiting the consumption to about 2kW. This energy should be good for 1m length sparks.
The main driver has evolved during the building of this coil. It started as a half bridge SSTC and quickly grew into a full bridge. This was worked well and reliably but would only generate 50cm arcs. MMC were added to the primary circuit to convert it to dual resonance. This caused some issues. An IGBT module blew during tests which due to the fail was likely caused by a primary coil strike. The coil developed a lot of 'racing-arc' and a few modifications were required. The driver had been modified to incorporate a over current sense, but after the fail it was decided to use an off the shelf driver design from 'Loneocean'. Unfortunately, the previous drive would run from 20V batteries but the new driver requires 24V minimum.
Other modifications were a redesigned flat primary coil to reduce strikes and a plastic shield with a grounded graphite edge cover over it. The
secondary was also lifted from the coil by 30mm. This reduced the ground rail strikes and although reduced the coupling between the coils didn't
reduce the performance.
The whole coil has been built in a modular fashion, not only to allow easy packing for storage and transportation, but for ease of maintenance. All components are easily accessed for test or repair.
The coil has also been designed to run directly from the mains without the need for a Variac. This means an inrush protection circuit required to be added. A neon lamp is added to show when operating.
The original driver, taken from the 'Onetesla' design is shown on the right with its Over Current sense modification. There are a number of videos on the making of this coil. This link takes you to the most recent.