Wednesday, November 2, 2011


 The North Island College, Solid State Tesla Coils are almost Done! Some adjustments and cosmetic things remain before they are finished. There is currently one bug left to iron out involving once-again gate driver IC shutdown! Grr they shutoff at random but no damage is done, it just becomes annoying to reset the power switch each time they latch-up! Other than that both units have been tested and do work, with the internal interrupter mode and audio modulation mode both functioning. Programming of the controller was done with the Arduino IDE and GCC AVR compiler. The spark-length is a little shorter than expected due to primary turns and placement of the primary coil form height. This will be adjusted when the other cosmetics are finished. This design turned out to be rather robust and despite numerous stress testing not a single MOSFET has died yet! I did blow one fuse by accidentally putting in the wrong value. Compare this to the last version of this coil which so far has "consumed" over 20 MOSFETS and 8 Fuses.

The details made the difference in this design! The top of the metal enclosure was slit to break circulating currents induced in the metal by the primary. In old designs this was a source of huge losses and heating, essentially the enclosure became induction heated. The diameter of the primary was increased and a PVC pipe was used as an insulator. The air gap combined with the PVC meant less heating issues and no arc-over from secondary to primary. The primary was also raised up to couple more to the coil and less to the base this further reduced losses and improved coupling despite the increased diameter. The redesign fixed allot of space and cooling issues. The H-Bridge no-longer gets hot only warm thanks to the cooling fan included and this is only under CW mode. In BPS everything is cold to the touch. The design changes significantly reduced losses! The coils are much more efficient than previous versions, they use about half the power to produce the same spark-length and most notably no heating issues in secondary or bridge.

Here's a quick view of the anterior driving circuitry in its finished form. Top is the H-Bridge Consisting of x4 IRFP460 MOSFETS and additional Blocking Diodes, Recovery Diodes and capacitors for DC blocking and bus smoothing. The Blue-Yellow transformer is the Gate Drive Transformer. To the right of that is the main power fuse in an inline holder at 10A 120V mains. Below the fuse is 2 large 1000uF 400V capacitors used in the voltage doubler circuit which has its power rectifiers tucked under right lip of the enclosure and beside the mains connection and power switch. Also tucked under the lip of the enclosure below the large smoothing capacitors is a small 5A 12V switch mode power supply for the driver and controller electronics. Below the H-bridge we see first the driver board with the yellow capacitor and below that is the controller which is a programmable microprocessor connected to knobs on control panel below and front of the enclosure. The one thing I am most proud of is incorporating a reprogrammable chip in this design. It has been very useful for troubleshooting and making changes without any physical changes. With the chip I could see on my computer - supply voltages, faults that have occurred in the driver, line noise etc and allowed me to tune the coil or operate it remotely. It was also very handy in producing precision pulses to control the coils output and modulate the power to limit it for overloads during high duty cycle operation. Currently noise is tripping off my fault detection routine to easily so some software changes are still needed before I release.

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