24 kV Marx generator

Introduction

This was my first high voltage circuit that eventually led me into building other high voltage generators, supplies and Tesla coils.

A Marx generator works by the principle of charging up a number of capacitors in parallel and when the voltage is high enough to break down the spark gaps, the capacitors will be discharged in series. When a Marx generator fires, it is said to be erected.

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.

Considerations

A Marx generator can work as a electromagnetic pulse generator, at high energy levels, if it is not shielded and grounded correctly. You can risk damaging electronic equipment if this circuit is not constructed or operated in a safe manner with these precautions in mind.

In the construction of the Marx generator it is very important to give great thought to distances that works as insulation between stages and components. An advantage against spark gap jitter can be achieved easily and for free by designing all the spark gap in one line where they can see each other. The UV light from the first spark gap will help break down the following spark gap and so forth.

The ideal Marx generator circuit will deliver n times the input voltage with n stages. So with a 4 kV supply and 6 stages, ideally I should have 24 kV output.

Each stage of 1 nF is charged to 4 kV and gives me 0.008 Joule energy per stage. With all 6 stages in series the capacitance is now 1/6 and the voltage is 6 times higher. So the erected capacitance is now 167 pF with 24 kV across it resulting in 0.048 Joule discharge energy.

Specifications

 Voltage supply 4 kV at 20 mA / 20 kHz from a solid state neon sign transformer. Stage design 6 stages of 2x 1M5 3500 V charging resistors and 1 nF 7500V capacitors Discharge 24 kV at 0.048 Joule Longest arc 20 mm sparks

Construction

14th April 2008

The 4 kV supply used is a solid state neon sign transformer that delivers 20 mA. The output comes at 20 kHz from the switching and normally is no problem for a neon tube, but I need to rectify it to use it in the Marx generator as DC supply is needed. A string of 10 1N4007 diodes are used for a 10 kV rectifying diode.

3500V 1M5 metal-oxide resistors were used with two in series for each stage and 1 nF 7500V ceramic capacitors, this gave a good head room for a 4 kV stage voltage.

The long leads on the capacitors were used as the spark gap by soldering them to the string of resistors as far up the legs as possible and then bending them into forming spark gaps.

Conclusion

A Marx generator in this size and level of supply voltage is a very forgiving and hard to break circuit. This makes it perfect as a entry level circuit for high voltage experiments.

The few number of components makes it cheap and it is easy to source the high voltage capacitors on Ebay.

The sparks generated from the Marx generator is very loud and it is easy to gain higher voltages and thus longer sparks by just adding more stages. A circuit that is easy to upscale just by adding more stages and where output is quickly calculated with number of stages.

Definitely a must-build circuit for everyone with a interest in high voltage generation and experimentation.

28th April 2008

TL494 flyback driver

Published on: Jun 14, 2013. Updated on: Nov 28, 2017.

Introduction

I wanted to design a versatile driver circuit that could drive a half- or full-bridge of MOSFETs or IGBTs through a gate drive transformer (GDT). This should make a driver that is able to run flyback transformers found in CRT TV sets and computer monitors.

The TL494 IC is designed for maintaining all the functions needed in a switching mode power supply using pulse width modulation (PWM). The output transistors can be run in either single ended mode or push-pull. The pulse width is normally controlled through a feedback signal in the power supply, but for this project we want to control it manually, this is done differently in almost all schematics found.

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.

Considerations

Flyback transformers from a CRT TV are typically driven at 15 kHz and flyback transformers from computer monitors are typically driven between 30 to 150 kHz.

The TL494 IC uses a 5% dead time to insure proper switching and at frequencies over 150 kHz this minimum dead time is higher.

The design goals for this project will be a driver with a variable duty cycle from 0% to 45% and a variable frequency from 50 kHz to 150 kHz.

This should make for a efficient driver and one that works out of the audible spectrum. In order to design with components at hand, the frequency span is not going so low as 15 kHz.

Specifications

 Voltage supply IRFP250N: 0 VAC to 120 VAC Frequency span 38 kHz to 150 kHz. Duty cycle span 0% to 43%

Construction

25th May 2009

The breadboard prototype is ready to be tested, the tape is to hold the timing capacitor in place since the legs on it was too short.

In the first oscilloscope shot we see the output waveform without pull up resistors, it is about 38 kHz at 43% duty cycle.

In the second oscilloscope shot we see the output waveform without pull up resistors, it is about 38 kHz at 5-7% duty cycle.

In the third oscilloscope shot we see the output waveform without pull up resistors, it is about 150 kHz at 43% duty cycle.

27th May 2009

PCBs was made for both the driver and half-bridge section. The full bridge rectifier used here in the pictures is only rated for a mere 4 A. This is not enough for running a flyback with low input voltage and high duty cycle. A 25 A bridge with heat sink should be used to ensure some overhead.

Test

29th May 2009

In the oscilloscope shot we see the waveform of the primary side of the GDT driving a MOSFET half-bridge. To test the circuit I first used a old half-bridge I had from an earlier project.

The sturdiness of this new driver shines through when I killed a flyback transformer due to over-voltage on the secondary side. Corona glow can be seen in the center towards the ferrite core.

Conclusion

This universal inverter makes it possible to adjust the output voltage and current exactly to ones needs. It makes a great and much more sturdy flyback driver than many simple drivers with just a single transistor, which is of course no surprise as it implements its own control IC, MOSFET driver ICs and a half-bridge of MOSFETs.

For a final constant voltage or current power supply it will not work, as there is no feedback adjusting the pulse width to a certain load.

Demonstration of Thrige power station

Introduction I visited one of the monthly openings at an old power station from 1916 that is still operational, it is only maintained and started …