Kaizer SGTC I


This Tesla coil is my first and was build without any expenses worth mentioning, its the prototype from which I learned a lot about Spark Gap Tesla Coils, high voltage and where to find components in household items and trash.

In the development the first version was more of a proof-of-concept model build only from old microwave ovens, televisions and cable.

Mathematics and theory was not the leading part of this project in the start, but as optimizing went on I learned about tuning the Tesla coil for a better output, resulting in longer sparks.



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There is no protection against RF spikes going back into the High voltage supply and destroying it. 

The secondary coil is limited by the amount of copper wire I had available from the cooling fan motor from a microwave oven. The diameter / height ratio of the secondary is far from optimal.

I used my flyback transformer as the power supply, it have a very limited current at about 1mA at 20kV, roughly estimated.

In the start I used home made salt water capacitors which are far from optimal.



High voltage supply20 kV from a flyback transformer
Primary capacitor
Primary coilinner 45 mm, outer 90 mm diameter, 1.78 mm diameter isolated copper wire, 6.6 turns.
Secondary coil50 mm diameter, 113 mm long, 800 windings, 0.127 mm enamelled copper wire.
Resonant frequencyTuned at around 655 kHz.
Topload60 mm diameter sphere, tennis ball wrapped in aluminium foil.
Input powerAround 20 – 30 Watt
Spark lengthup to 105 mm long sparks.





10th May 2008

The secondary coil was wound on a plastic tube, 50mm in diameter and 160mm in height. Its bottom was conical so practically there could only be wound wire on 110mm of the height.

The copper wire had a diameter of 0.127mm, varnished its diameter is 0.14mm, this very thin wire made it hard to wound nicely without overlaps.

It took me 2 days to wound the secondary coil, it was hard on the eye, arms and hand to do in one stretch. The plastic tube was mounted on a gear motor controlled by a frequency inverter so I could control the speed by a potentiometer.

11th May 2008

To keep the windings in place and insulate the coil it was varnished with common ship varnish, it was given a layer in the morning and one more in the evening.

12th May 2008

As the power supply I used my 20kV flyback transformer driver at 12Vdc input, the capacitor is a home made salt water capacitor with a capacitance of 2.7nF. The primary coil was wound of ordinary 2.5mm2 wire around a cut up soda bottle.

The topload is a sphere made of bobble wrap, tape and aluminium foil.

Spark gap is just 2 copper wires.

In the first test I could get 4-5mm sparks to a grounded object, the biggest problem was the too tight coupling between primary and secondary. Too tight coupling resulted in alot of racing sparks on the secondary coil, these are dangerous as they can destroy the thin wire on the secondary coil.

Here is a picture of some racing sparks I provoked to get a picture of it.

The primary coil was wound in a bundle and by adjusting the height of it, the coupling could be changed. It was now possible for 40mm sparks to jump to a fluorescent light hold in my hand.

15th May 2008

varnish, varnish, varnish, varnish, glue, glue and more capacitance…

The flyback transformer driver runs at 12Vdc input.

I made another salt water capacitor and installed it in parallel with the other, the total capacitance was now 5.9nF.

As it can be seen in the pictures there is sparks or just “violet light” coming from other parts than the topload of the Tesla coil, its corona loss and decreases the spark length.

To isolate the secondary coil further it was given 4 more layers of varnish and the top of it was glued all over with hot glue. The metal cap was the new topload and 54mm sparks could be achieved.

28th May 2008

The flyback transformer driver runs at 17Vdc input.

85mm sparks can now jump to my fluorescent light.

The bottom from a beer can is the new topload, it results in some spectacular pictures, the higher voltage on the flyback transformer driver is the best improvement towards longer sparks, but also racing sparks on the secondary starts reappearing.

As it can be seen in the picture taken in the dark, there is still corona losses, and with this particular coil it will be impossible to avoid it at these driver input voltages. It is not easy to insulate 100kV. The following picture is taken with long exposure to show the corona around the coil itself, unfortunately its not as clear in the picture as seeing it live. The violet field around the coil is faint, but can be seen in the picture.

This picture is taken directly above the Tesla coils topload.

21st June 2008

I bought 10 old high voltage capacitors at 150 dkr (25$) for the lot, a real bargain in Denmark compared to the joy it has brought me. I only use one of them instead of the 2 salt water capacitors. Its a Fribourg Condensateurs from 1961, 8nF rated for 20kV pulse driving at maximum 2MHz.

A new topload was made from a tennis ball wrapped in aluminium foil, its not as smooth a surface as it should be, but it works.

The spark gap now consists of 2x WT20 tungsten welding electrodes, these are able of withstanding high temperatures without vaporizing as the copper wires were likely to do over time. There is still room for improvement on the spark gap as its still just a single jump static spark gap.

By calculating the Tesla coil in JAVATC I tuned the circuits to theoretically be in resonance, but its only approximating as the construction is far from precise.

With the new improvements 105mm sparks will jump to a grounded wire.

I took a series of pictures with different exposure times. Sparks are about 90mm long.

In the next picture racing sparks can be seen at the top of the secondary windings, without a breakout point or something to jump to, the energy build up is too large for the coil and its under a huge stress.



This small project started as a proof-of-concept model, to see if the theory I had learned would work in practice. It has come a long way since I started on it almost 2 months ago.

I learned a lot more about the theory of the Spark Gap Tesla Coil, the maths behind tuning the circuits and the importance of planning the design before building. This is no surprise. So there have been spend a lot of time trying to optimize a Tesla coil build around a badly designed secondary coil, so the result will never be near optimal.

Despite all this, I am very satisfied with the results of 105mm sparks. There are several things to optimize in a final version of the Kaizer SGTC I, it would be a better spark gap, shorter and more suitable wires, shape of the primary coil and its coupling to the secondary and a topload with a smoother surface.


8 thoughts on “Kaizer SGTC I”

  1. Hey Jasshopper

    I would say a couple of minutes, then the static spark gap would heat up too much, the high voltage power supply itself had a large heat sink and could run for hours.

    Kind regards

  2. Firstly, this is really great work. Keep it up. Secondly i built a TC using similar design, but there are a few probs. Im not getting any breakout on a tennisball covered in foil. My spark gaps makes an unbearable noise and flash. Im running it on 12V at this stage for simplicity purposes. I can draw about a 20-30mm spark from the topload. Ive got 6 turns of the TC primary and about 800 turns on the secondary. The flyback transformer has about 5+5 windings on its primary though so im only getting about 1-2kV out of it. Otherwise, everything else is basically the same as your design. Do you think i should reduce the turns to 3+3 and increase the HV for better results?

  3. Hey Joshua

    I think it is merely a question about tuning it better, it is not as simple as just adding or removing a turn on the primary coil. The sweet spot is within a few centimeters on moving the tap point.

    Are you familiar with the Tesla coil calculator JavaTC? You should simulate your coil to get the “correct” tune point calculated and try to lower the primary frequency bit by bit to increase performance. But only very little on such a small coil where the streamer load will not pull the secondary frequency that much down.

    Kind regards

  4. Hi,

    The spark gap forces the frequency of the circuit at the same frequency as it is switching or it has another frequency?

    What happens when the secondary starts to resonate, cause if it makes to decrease the impedance of the secondary the current on the primary increases a lot, supousing a relation 1:2, it makes to double the current from the secondary to the primary. My question is, if the current increases it is possible that the house magnetotermic switch will be switched off??

    Sorry for the english, I tried my best.

    Kind regards ans wishing to know your opinion.

  5. Hi Jesús

    The high voltage supply charges up the tank capacitor until it can break down the air in the spark gap, so the switching frequency of the primary circuit on the flyback transformer is not the same as the one in the Tesla coil primary circuit.

    This circuit is powered from a DC source, which could shut down at over-current situations before your house breaker does. But the flyback transformer itself limits the current greatly, if the current rises too much, the voltage will drop enough for the coil not to operate properly. You should not fear that this trips your fuses.

    Kind regards

  6. Hi Aaditya

    This is a old and by todays standards not a proper way or grounding and terminating a secondary coil base, it should have its own RF ground.

    This properly only works because it is a low power circuit, since there is galvanic isolation from the driver input voltage in the flyback transformer, the secondary coil will just act as a step-up transformer in regard to the primary:secondary turns ratio.

    Kind regards

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