Halloween Musical SSTC

So I have a working Solid State Tesla Coil that accepts audio input. Halloween is coming around and what a better way to scare people then to pipe audio out of arcs and sparks. But since I have no working audio source to modulate the coil I thought it was about time to build one! I have no need to buy expensive fiber optic cable isolation as there is almost no risk to the operator with the coil I have. The coils driver board is internally isolated by optocouplers, the board only runs at 12VDC and has a clamping circuit for anything above that. The driver is physically isolated from the high voltage bridge. Further more additional optocouplers can be used in the circuit I will be building. The purpose of the optocouplers is mostly to seperate small transients which may effect sensitive microcontrollers. Input and output clamping (protection) will be designed into this board, similar to the design used in the current sensing portion of digital multimeter.

The output protection portion clamps down any induced high voltage transients on the output buffer to protect it. The input protection may not be needed but it is to protect the mp3 player or other device from anything induced onto the circuit itself protecting the input device. Adjustable gain to set the volume and adjustable threshold control for the buffer allows the duty cycle to be adjusted.

This is not the final design I plan to use a zero voltage crossing detector after the preamplifier with an adjustable pulse width for better results. This circuit would only be suitable for specially formatted music input such as one instrument or frequency at a time. Missing here is the channel select jumper or switch to allow left or right channel to be used separately.

NIC Coil Project Finished

Finished the Twin Solid State Tesla Coils just a few minutes ago. Periodically working on the coils for over 8 months now I finally spent a good few hours to get the primary coils done. Everything is wired up and I have tested both coils both separately as well as together to see how they handle feedback.

H-Bridge went through two hardware revisions mostly due to cooling and space issues. The first design had an entirely different bridge which was level to the floor of the case. It used a linear regulator off a toriodial step down transformer. The second and final design used a Switch Mode Power Supply providing 12.7V which was broken up into other voltages with linear regulators to keep noise low. The control knobs also changed sides to bring them in reach of the power switch for operator safety. The driver circuit went through two hardware revisions. The first being a Duemilanove at 16MHz mounted beside a driver board. The second and final one being two stacked boards with the ATMEGA 328P running on the same PCB as the VCO at 20MHz.

The software went through five revisions in total. Rev 4 used a pulse timer system which worked but proved unreliable for audio input at frequencies above 1KHz and had a number of harmonics which it could not respond to. The final revision Rev 5 used a hybrid Zero Crossing Detector and Pulse Timing system which allowed up to 2.5KHz audio modulation while remaining completely stable and with no harmonic problems. Rev 5 also included reduced power consumption and higher noise immunity. The control system draws 30mA idle (mostly LED power consumption) to 50mA when fully active in Audio Input Mode.

Now all that is left is some tuning to get longer sparks, some cosmetic clean-up perhaps a washup and paint. To get a bit bigger sparks I plan to raise the primary coil up which should be easy as it was originally designed to do this. I reduced the primary winding to 20T from 21T which did almost nothing. The current draw of these things is 0.3A to 3.4A on the interrupter mode and 0.3A to 6.8A on the audio mode (0-2.5KHz).

That is all for now, documentation, schematics, and operators manual are next.

NIC SSTC NOV Update

 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.

Variac Modification

A while back I purchased a Variac (Auto-Transformer) on eBay; for a rather good low price at the time. I had planned to replace or at-least supliment the cheap inbuilt voltage meter with something more useful and "cool looking". So I decided today was a good opportunity to do just that! and mounted my homemade volt/amp panel meter box to the Variacs output. The project turned out to not be that hard but did take a few hours due to finicky wiring, drilling holes, and hard to get to bolts.

Inside the variac you can see the core/case ground wire (Yellow/Green), Neutral Wire (White), Hot (Brown) and Tap Wire (Red). In the typical variable auto-transformer configuration.



Inside the panel a 20A Breaker / Switch, Analog volt-meter and outlet plug. The 50A shunt was added to accommodate the Amp-meter. The old volt-meter was originally wired to the output of the Variac but is now rewired to the input so I could see the mains voltage which is useful for troubleshooting other issues.

Here you see the panel meter box. It contains the two digital meters on the front (not shown) and 2 isolated switch-mode power supply units in the back (visible). The volt-meter runs off 12VDC and the current-meter runs of 5VDC, both power supplies must be isolated to measure both voltage and current without explosions! I like switch-mode supplies because they can run as low as 50VAC and up to 250VAC so the meters will work under different input supply voltages over a wide range. This variac was originally a 240V input 0-280VAC output so this allows me to still operate it in both European countries and in North-America without any issues (other than input meter being pinned to max).

Here are the results! Nothing high power to test out the output yet but the current meter is rated for continuous 50A I have noticed it can measure several hundred Amps for a short period of time however ;) . The volt-meter is a 700VAC true RMS panel meter both are from Sure Electronics on eBay. (Running a ~23W Compact Fluorescent Light seen below)