Kaizer SSTC II

Introduction

This is a modified version of the first SSTC I built, 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.

 

Safety

WARNING!: Working with electricity is dangerous, all information found on my site is for educational purpose and I accept no responsibility for others actions using the information found on this site.

Read this document about safety! http://www.pupman.com/safety.htm

 

Considerations

In the quest for longer sparks I decided to use a full bridge to take advantage of the full voltage on the bridge.

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.

The drivers will be shielded in order to avoid the EM field generated by the Tesla coil itself to inject noise into the drivers.

 

Specifications

Bridge 4x IRFP460 MOSFETs in a full bridge configuration.
Bridge supply 0 – 260 VAC from a variac, 8 A rectifier bridge and 1500 uF smoothing capacitor.0 – 365 VDC on the bridge.
Primary coil 115 mm diameter, 1.78 mm diameter isolated copper wire, 8 windings.
Secondary coil 110 mm diameter, 275 mm long, 1000 windings, 0.25 mm enamelled copper wire.
Resonant frequency Self tuning at around 250 kHz.
Topload 100 mm small diameter, 240 mm large diameter, toroid.
Input power Continues Wave mode: 2000 – 4000 Watt at 200 VAC input voltage.
Interrupted mode: 100 – 2000 Watt at 260 VAC input voltage.
Audio modulated mode: 300 – 400 Watt at 150 VAC input voltage.
Spark length up to 475  mm long sparks.

 

Schematic

The UCC3732X 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.

All unused input pins of a 74HC14 has to be tied 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.

 

Construction

15th March 2009

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, its necessary to isolate the MOSFETs from each other as their housing is also a conductor.

19th March 2009

Driver electronics and audio modulator are installed under a metal casing from the CRT monitor 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.

The bridge is made out of four IRFP460 MOSFETs, four MUR1560 diodes, four 5R resistors. The power supply is a 8 A rectifier bridge with a BHC 1500 uF/450 V smoothing capacitor, a 27K 7W bleeder resistor is added in the final build.

The audio in jack was later removed 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 inbetween.

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, 1000 windings, 0.25 mm enamelled copper wire.

The complete coil looks, except maybe the electrical tape used to hold the topload together.

 

Sparks

Interrupted mode

At 250 VAC input voltage, 350 VDC on the bridge, it was possible to reach 475 mm long sparks, in interrupted mode, to a grounded object.

More pictures of sparks in interrupted mode, it is running at about 4 – 5 BPS.

3rd May 2009

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 2000 to 4000 Watt. This resulted in 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 2009

I constructed a new topload from two cheap aluminium frying pans from Ikea. With handles cut off and screw from it grinded away it had a smooth surface and was fixed with a long screw through both of them.

6th September 2009

During a run of CW at full input voltage, the full bridge blew apart completely, with a loud bang.

 

Audio modulation

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.

 

Conclusion

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

Demonstration of different modes.

New topload, running in interrupted mode.

New topload, running in CW mode.

New topload, running in interrupted mode and closeup of sparks.

555 Audio modulated flyback

Introduction

This is a audio modulated arc generator designed for simplicity rather than reliability, its made with very few and common components. There is however some serious trade offs described below in considerations.

WARNING: sensitive audio players might get damaged by this circuit. I bricked my iPod shuffle, seems that the controller chip for the mini jack got wasted as it could no longer detect charger, PC connection or play music as it could not detect headphones.

Safety

WARNING!: Working with electricity is dangerous, all information found on my site is for educational purpose and I accept no responsibility for others actions using the information found on this site.

Read this document about safety! http://www.pupman.com/safety.htm

Considerations

The arc have to be very short in order to limit the distortions of an unstable arc. The sound quality is low due to the way the audio modulation is implemented. If the distance between the elctrodes is too large, the high open loop potential of the high voltage transformer can generate some high transient voltages that through inductive kickback can destroy the MOSFET.

The 555 IC supplied by 12 VDC can not source much more than 140 mA before the voltage drop on the output gets very high. At 140 mA it is already 1,82 VDC. At 200 mA the voltage drop is at 2,5 VDC. The low output will affect MOSFET switching speed and result in higher losses. This graph shows the voltage drop vs. output current of the 555 IC.

In order to optimize the switching of the MOSFET, a small intermediate driver stage can be introduced with two transistors, a NPN and PNP. As illustrated in the red and green graph is the difference between running a MOSFET with proper switching and the other always in linear mode, where losses are very high. This is a future improvement and is not a part of this little project, but it is recommended to add this if you want reliability.

If the circuit can not produce a arc try to reverse the polarity of the primary coil on the flyback transformer.

There are basically 2 kinds of modern flybacks, television flybacks are driven near 15kHz and monitor flybacks are driven between 30-150Khz. Depending on which type we use, we have to adjust the frequency of the 555 timer to match the resonance of the flyback for maximum performance.

 

Choosing a MOSFET

There are some basic rules of thumb that I will just list here to start with, I will come with an explanation later on.

The voltage rating of the MOSFET (VDSS) needs to be 6 to 10 times higher than the supply voltage, reverse voltage spikes and EMF can be high enough to destroy the MOSFET if its too small. But we still need to use MOSFETs with a reasonable low on resistance (RDS(on)). Try to find a MOSFET with a RDS(on) value not much higher than 0.1 ohm, if you have problems try one with a lower RDS(on) value.

The gate resistor R3 is there to

  • Limit parasitic oscillations that could kill the MOSFET.
  • Limit the current that is needed from the driver stage, in this case our 555 timer.
  • Protect against surge voltages on the MOSFET gate, effectively this would require a much higher resistance, a high gate resistance would lower the operation speed significantly.
  • The values of a gate resistor could be anything between 10 ohm to 200 ohm, it all depends on the MOSFET, experimentation is needed. The alternative is complicated calculations involving data that is usually not available in standard data sheets.

How does the audio modulation work?

Pin 5 on the 555 timer is a direct access to the 2/3 voltage divider point of the upper voltage comparator in the 555 timer. This allows us to pulse width modulate the output on pin 3 of the 555 timer. By applying a voltage to this pin, it is possible to vary the timing of the chip independently of the RC network. When used in the astable mode, as we do with this circuit, the control voltage can be varied from 1,7 VDC to the full Vcc. Varying the voltage in the astable mode will produce a frequency modulated (FM) output.
If the control-voltage pin is not used, it should be bypassed to ground, with a 10n capacitor to prevent noise entering the chip

Schematic

Both R1 and R2 can be 10K potentiometers.

Construction

13th November 2008

I wanted to do a audio modulated flyback arc with few components and a small form factor. I installed the MOSFET on a old CPU heat sink with fan, the 555 timer circuit is also installed underneath this heat sink, its then all put on the side of the flyback transformer with wire strips.

The primary coil is 8-9 windings of 0,75 mm² isolated wire. More windings will stress the MOSFET less but also output voltage will be lower.

The frequency output from the 555 timer is 26,7 kHz at 59,3% duty cycle. This is in the low end for a monitor flyback so further improvements will be adding a potentiometer to adjust frequency to match the resonant frequency of the flyback.

2nd February 2009

Its time to improve the driver with a variable frequency control so the driver can be used with most conventional monitor flyback transformers without changing any parts, but merely turn the potentiometer.

I installed a 9K potentiometer as R1 and a 10K potentiometer as R2, I adjusted the potentiometers till I had a nice silent thin arc at about 15 mm length. 10K potentiometers can be used for both R1 and R2, I just used what I had at hand.

Using a 555 calculator with the measured values of the potentiometers. R1 at 1K3 and R2 at 1K. Duty cycle is 69.7% and frequency is 43700 Hz. Very reasonable for a monitor flyback. Compared to the old frequency I now have a longer and more silent arc.

 

Conclusion

A quick and very rewarding little project, its fun to play music without conventional speakers. This was also known in the 1970’s as a plasma tweeter and could be found in special hi-fi speakers.

The arc is very very hot and I had to extend the copper wires where it is drawn between to avoid the heat being transferred far enough to start melting the flyback transformers casing.

The 555 IC is not able to supply enough output current to drive a IRFP250N MOSFET at a high duty cycle, so the MOSFET will at times still be in linear mode and this causes excessive heating, which is why the heat sink is necessary. So more notes under considerations about this.

 

Demonstration