The idea was to build a very small and compact Tesla coil as a gift for my mother that works in various science classes for the lower grades in public school.
This driver circuit is very similar to the one used in Kaizer SSTC I. This time I have made a PCB containing both driver circuit and bridge.
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I knew this would get claustrophobic with so little space for a complete interrupter, driver and bridge.
Using the enclosure as the heat sink is the reason why a low break rate is chosen, to avoid excessive heating.
|Bridge||2x IRFP460 MOSFETs in a half bridge configuration|
|Bridge supply||230VAC directly from the wall, 4A rectifier bridge and 330uF smoothing capacitor|
|Primary coil||Rev 1: 55 mm diameter, 1.38 mm diameter isolated copper wire, 10 windings.Rev 2: 80 mm diameter, 1.38 mm diameter isolated copper wire, 10 windings.|
|Secondary coil||Rev 1: 50 mm diameter, 200 mm long, 1430 windings, 0.127 mm enamelled copper wire.Rev 2: 75 mm diameter, 165 mm long, 1500 windings, 0.1 mm enamelled copper wire.|
|Resonant frequency||Rev 1: Self tuning at around 470kHz.Rev 2: Self tuning at around 180 kHz.|
|Topload||Rev 1: Made of two bottoms from beer cans, 65mm diameter and 30mm in height.Rev 2: 45 x 152 mm turned aluminium toroid.|
|Input power||Interrupted mode: ?W at 230VAC input voltage.|
|Spark length||Rev 1: up to 140 mm long sparks.Rev 2: up to 250 mm long sparks.|
Schematic and PCB files
21st July 2009
I designed a compact single sided PCB that contains both driver and bridge section on a mere 65 x 75mm board. Here is newly etched board, traces are a bit shaky as I have drawn them all by hand.
The MOSFETs uses the enclosure as a heat sink, I sanded down the paint for metal contact and use pads to isolate between MOSFETs and enclosure.
BPS is kept low, but can be varied from 4 to 20 BPS, to avoid excessive heating as the enclosure is not an optimal heat sink.
In the bottom of the following picture you can see the bridge rectifier mounted to the enclosure and the input filter for 230VAC in. The red wires lead to the 330uF/400V smoothing capacitor and the 100nF/1600V Rifa capacitor is the DC blocking capacitor in the primary circuit.
The coil is connected directly to 230VAC without any kind of voltage regulation and also requires a external 12VDC supply for the driver.
Antenna and primary coil connections are temporary solutions for the sake of demonstrating the Tesla coil in working order. A fold out antenna from a small radio or such will be added later. Some kind of support with banana jacks with a secondary and primary coil mounted on will be added, to avoid wrong phasing of the primary coil.
Here the complete setup is size compared to a 330ml beer can
Here is one of the more spectacular spark pictures I have taken, in my eyes it looks like a demon waving its arms over the head which also have a distinct face with glowing eyes and a open mouth, or maybe I am just seeing things from inhaling too much ozone 😀
24th July 2009
I borrowed a expensive macro lens for my Canon 350D camera and took some pictures with great details of the sparks, very sharp pictures!
1st August 2009
Doing a short demonstration I adjusted the antenna with my hand while the coil was running, this resulted in unstable oscillations and the bridge was short circuited. I am now replacing the destroyed MOSFETs and here I can feel the disadvantage of servicing on a compact design.
A new secondary coil is in the making, it is wider, shorter and have half the resonant frequency of the first. It will be fitted nicely on a piece of acrylic for a complete look.
19th August 2009
The new secondary is finished, it took me about 8 days to do the winding as it is very intensive to wind with such a thin wire. Keeping the wire tight, windings close to each other, not pulling the wire too hard from the spool, watch for jams and overlaps and it all have to be done with a bright light very close to get a good view.
It uses the topload from my VTTC I, a 45 x 152 mm aluminium toroid, with this it have a new resonant frequency around 180 kHz.
Top of secondary was filled with epoxy to insulate the brass bolt from the inside of the secondary and the bottom earth connection is fastened with a nylon bolt.
It is all fitted onto a piece of acrylic with additional protection around the primary connections so it no longer possible to touch any conducting part of the primary circuit.
I use a audio modulator made by the user Reaching (Martin Ebbefeld) from 4hv.org.
For sound input I use a cheap children’s keyboard from a toy store, its far from perfect for the job, especially because its waveform is highly distorted and its not clean tones but seems to involve a lot of modulation inside it to simulate different instruments. But its cheap and expendable.
Watch the film and look at the schematics for more about the audio modulation.
I am very satisfied with the final result, that I got to fit everything and use the enclosure as a heat sink turned out real good. Heating is not a problem with run times at about 2-3 minutes which is also the durations its been built to be demonstrated for.
Enclosure dimensions are 125W x 80D x 50H mm.
Revision 2 looks even better, performs better but was also a lot of work to wind the new secondary with such a thin wire.