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I built this is a modified version of mt first SSTC, the Kaizer SSTC I. It uses the same secondary, topload and driver board. New things is a full bridge of IRFP460 MOSFETs, audio modulation, shielded drivers and a new casing.
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In the quest for longer sparks I decided to use a full bridge to take advantage of the full voltage on the bridge.
Firstly the MOSFETs will be mounted on top of a heat sink so its easy to change them by only removing the secondary platform and solder them off.
Secondly the drivers will be shielded in order to avoid the EM field generated by the Tesla coil itself to inject noise into the drivers.
The UCCX are MOSFET driver ICs, one non-inverted output and the other inverted, in order to get a push-pull drive of the gate drive transformer. A gate driver IC can deliver the high peak currents needed to drive MOSFETs efficiently.
The 74HC14 is a inverting hex schmitt trigger, it is used to get a proper solid 0-5V square wave signal from signals that are not perfectly square, the antenna feedback can vary a lot in waveform and amplitude, the 74HC14 converts this to a clean drive signal for the MOSFET drivers.
I tied all unused input pins of a 74HC14 to ground, floating inputs and a noisy environment is a recipe for trouble. The noise can couple between the gates internally and make the whole IC not work properly.
The music modulator works by amplifying the audio signal in the LM741 and at the BC547 transistors. The 555 timer ensures that the signal length of the generated square wave is much shorter than the audio signal, in order to not have too long on-time and thus damage the MOSFETs / IGBTs from over-current.
15th March
I took apart a 19&#; LCD monitor and a 24&#; CRT monitor, from these respective computer parts I salvaged a good piece of acrylic from the LCD monitor and a fairly sized heat sink from the CRT. I cut the acrylic in half for a 2 level platform and the heat sink was cut in 4. It is necessary to isolate the MOSFETs from each other as their housing is also a conductor.
19th March
I installed both driver electronics and audio modulator under a metal casing from the CRT monitor. I wanted to shield it from the heavy EM field surrounding the Tesla coil. This is to avoid problems with the driver being interrupted by its own EM field.
I made the bridge out of four IRFP460 MOSFETs, four MUR diodes, four 5R resistors. The power supply is a 8 A rectifier bridge with a BHC uF/450 V smoothing capacitor, a 27K 7W bleeder resistor is added in the final build.
I later removed the audio input jack, due to it making a short through its metal housing to the ground rail, I had overlooked that the audio in negative was not common with the ground rail, but there is a capacitor in between.
The secondary is held in place by a crate for ventilation on houses, its an easy and quick way of taking the coil apart for transport or storage, and it holds the secondary firm and tight.
A acrylic tube is added to support the antenna, in this way it is possible to adjust the coupling of the antenna to the secondary simply by pulling the wire.
The new shielding of the audio in signal is made from a piece of shielding from a industrial cable pulled over it and grounded.
The secondary with terminations. 110 mm diameter, 275 mm long, windings, 0.25 mm enamelled copper wire.
The complete coil looks, except maybe the electrical tape used to hold the topload together.
Interrupted mode
At 250 VAC input voltage, 350 VDC on the bridge, it was possible to reach 475 mm long sparks. This was in interrupted mode, to a grounded object.
More pictures of sparks in interrupted mode, it is running at about 4 &#; 5 BPS.
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3rd May
Continues Wave mode
At 200 VAC input voltage, 280 VDC on the bridge and a power consumption around 10 A, peaking at 20 A, the coil was drawing somewhere in between to Watt. Surprisingly resultung in a very hot, thick white arcs punishing the dead iPod shuffle which remarkably left the player relatively unharmed considered what had just taken place.
These flame like sparks are 250 mm in length.
18th August
I constructed a new topload from two cheap aluminium frying pans from Ikea. Firstly I removed the handles and the screw from handles was grinded away, in order to have a smooth surface. Secondly I fixed the two parts together with a long screw through both of them.
6th September
During a run of CW at full input voltage, the full bridge blew apart completely, with a loud bang.
I suspect it was something with the new topload, it was smoother, but also a bit smaller. The resonant frequency did maybe go up too much. The higher losses caused the MOSFETs to exceed their maximum die temperature.
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. It is far from perfect for the job, especially because its waveform is highly distorted. It is not clean tones, but seems to involve a lot of modulation inside it to simulate different instruments. But it is cheap and expendable.
Watch the film and look at the schematics for more about the audio modulation.
Upgrading the SSTC I with a full bridge was a absolute must. It is small changes compared to the better performance and the driver have no problems at all driving four MOSFETs instead of just two.
Getting sparks at 475 mm length in interrupted mode and white power arcs at 250 mm length is truly satisfying for this little coil, the secondary winding itself is only 275 mm in height in comparison.
Enjoy the demonstration.
Demonstration of different modes.
Interrupted mode with the new topload.
New topload, running in CW mode.
New topload, running in interrupted mode and closeup of sparks.