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SGTC 1

Project description

In this project, I wanted to “wrap up” the knowledge I gained about SGTC’s into a compact project that can be easily transported and reliably demonstrated. There is nothing extraordinary about it’s design nor performance, but it is still a fairly complex project which I thought would be good to share.

This is how I drilled my primary coil supports. Pieces of acrilyc were first filed while glued together to make them as similar as possible. Then, they were glued again (with double-sided sticky tape) with and holes were drilled through all of them to attain required helical hole offset.

This is how I drilled my primary coil supports. Pieces of acrilyc were first filed while glued together to make them as similar as possible. Then, they were glued again (with double-sided sticky tape) with and holes were drilled through all of them to attain required helical hole offset.

I had to painstakingly strip the insulation from many meters of solid core wire, in order to prepare it for being wound into primary coil

I had to painstakingly strip the insulation from many meters of solid core wire, in order to prepare it for being wound into primary coil

Supports were bent using a hot-wire bender so that they can be mounted onto wooden frame

Supports were bent using a hot-wire bender so that they can be mounted onto wooden frame

3mm thick copper wire was used for primary. I calculated that I'd need a fairly large number of turns, hence the double layer coil.

3mm thick copper wire was used for primary. I calculated that I’d need a fairly large number of turns, hence the double layer coil.

After many hours of painstakingly pulling the wire through, the primary is nearing completion

After many hours of painstakingly pulling the wire through, the primary is nearing completion

This is how I first designed my spark gap: short pieces of copper pipe were soldered onto appropriately prepared PCB. The clearances between pipes were too large and I only ended up needing 3 or 4 gaps

This is how I first designed my spark gap: short pieces of copper pipe were soldered onto appropriately prepared PCB. The clearances between pipes were too large and I only ended up needing 3 or 4 gaps

My initial filter board. The power resistors went open after a short while due to huge pulse currents being drawn through them every time the spark gap fired. The safety gaps were constructed the same wayas the main gap, from carefully spaced bits of copper tubing

My initial filter board. The power resistors went open after a short while due to huge pulse currents being drawn through them every time the spark gap fired. The safety gaps were constructed the same wayas the main gap, from carefully spaced bits of copper tubing

A 250VA oil burner ignition transformer (OBIT) being used as power supply, connected to the low pass filter. I made the filter capacitors out of pieces of PVC tube, with aluminum tape glued from inside and outside to provide about 100pF of capacitance. The resistors which failed were replaced by strings of tougher carbon resistors

A 250VA oil burner ignition transformer (OBIT) being used as power supply, connected to the low pass filter. I made the filter capacitors out of pieces of PVC tube, with aluminum tape glued from inside and outside to provide about 100pF of capacitance. The resistors which failed were replaced by strings of tougher carbon resistors

The whole test setup. The MMC was highly improvised here, and the spark gap was cooled and quenched by a microwave oven fan

The whole test setup. The MMC was highly improvised here, and the spark gap was cooled and quenched by a microwave oven fan

First light! After a little tuning, I was pleasantly surprised by some 40cm long sparks!

First light! After a little tuning, I was pleasantly surprised by some 40cm long sparks!

Assembly

After a successful test run, I decided to rectify a few problems I’ve ran into and start putting the coil together into a transportable package. I needed to fix my spark gap and find a way to mount the MMC. I’ve also found another OBIT with indenical ratings as the first one, and decided to use them both in parallel.

Low pass filter was improved by two 20kV 680pF doorknob capacitors

Low pass filter was improved by two 20kV 680pF doorknob capacitors

The new spark gap, now built following the circular "TCBOR" fashion. A high power 12V fan is used to blow air through the tubes for cooling and quenching. There is a fan grill in between the gap and the fan to minimize chances of radio frequency interference affecting the fan

The new spark gap, now built following the circular “TCBOR” fashion. A high power 12V fan is used to blow air through the tubes for cooling and quenching. There is a fan grill in between the gap and the fan to minimize chances of radio frequency interference affecting the fan

This is how I arranged my MMC. The number of capacitors in series is now lower than before, to increase capacitance (I determined that I have more than enough voltage clearance). Small pieces of PCB are glued from behind the acrylic panel, and caps are soldered to them

This is how I arranged my MMC. The number of capacitors in series is now lower than before, to increase capacitance (I determined that I have more than enough voltage clearance). Small pieces of PCB are glued from behind the acrylic panel, and caps are soldered to them

Rectifier and filter cap for the fan

Rectifier and filter cap for the fan

I decided to make control of this coil as straightforward as possible. There is no switching of any kind on it, instead I use a switch integrated into the sole mains cable that goes to it. This powers the HV transformers over a line filter and a fuse, as well as the small transformer used to power the fan.

I decided to make control of this coil as straightforward as possible. There is no switching of any kind on it, instead I use a switch integrated into the sole mains cable that goes to it. This powers the HV transformers over a line filter and a fuse, as well as the small transformer used to power the fan.

This is how I made my HV feedthroughs. I drilled through middle of some plastic rod I had laying around, and then threaded it so it can act as a feedthrough through wood. Never have your high voltage conductors in direct contact with wood!

This is how I made my HV feedthroughs. I drilled through middle of some plastic rod I had laying around, and then threaded it so it can act as a feedthrough through wood. Never have your high voltage conductors in direct contact with wood!

Results

While I was more than satisfied with the appearance of the finished project, I didn’t quite get as much improvement with addition of an extra transformer as I hoped – the spark length increased only by about 15cm or so. The most probable suspect I can think of could be the spark gap, which I fear might have became more lossy after I split it into more segments. But then, my primary wire was also quite thin and could be contributing to losses. My toroid could also be seen sparking from various points other than the breakout point, likely reducing the arc length I could get from the breakout point itself.

In overall, I’m quite happy with having a neatly wrapped up SGTC that I can demonstrate to anyone, and spark length is impressive enough for it’s size. Perhaps I’ll investigate the possible spark gap and other improvements one day, but until then I have lots of other projects to work on. Thanks for reading!

Links and references

[1] 4hv thread http://4hv.org/e107_plugins/forum/forum_viewtopic.php?53234.post

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