My first (working) 555 transformer driver circuit

I set out to see if I could build an astable 555 circuit to drive a MOSFET or transistor, in turn driving a TV flyback, car ignition coil or a monitor CCFL transformer with the goal of producing beautiful high-voltage arcs and corona. The video below shows an arc from a CCFL transformer driven by the 555 circuit! This was before adding the MOSFET gate drive circuitry which results in better arcs.

Typical 555 astable circuit

The circuit above is a typical astable 555 timer circuit to turn on and off the MOSFET at high speed. I have been experimenting with different resistor and capacitor values to change the frequency and duty cycle, which can significantly affect the quality of the transformer arc. I believe both my flyback and CCFL transformer work well at around 20Khz, while an automotive ignition coil runs at a much lower frequency.

Letting out the magic smoke

My first attempt was fairly disastrous, I managed to fry an IRFZ44N MOSFET and the 555 timer itself by attempting to run the circuit straight from an 18v power adapter (even though I was sure 18v was okay for the 555, I believe it tried to source too much current and fried).

First Lesson learned, no more magic smoke!

Splitting the circuit into two different voltages meant no more dead 555s. I used an L7805CV voltage regulator to seperate the 555 into a 5v, ‘low voltage’ section, while the MOSFET (now an IRFP260N) having the full 12v from my ATX power supply. I also read the 555 timer should have a 10nF capacitor across its ground and VCC, I think this prevents transient voltages as the 555 sources current. The voltage regulator should be mounted on a heatsink, although I haven’t noticed mine getting even close to warm during operation.

MOSFET gate driver - Major improvement!

Rather than having the output of the 555 going straight to the MOSFET’s gate, I discovered that using a ‘driver’ arrangement of two transistors (NPN and PNP) resulted in a larger, thicker arc from the CCFL transformer. I believe outputting straight to the gate is very inefficient as the 555 cannot source current fast enough, again I may be wrong, please always double check, I’m still very much a newbie to electronics.

MOSFET gate driver example

In the schematic above, the 555 output (Sig in) goes through a 10kΩ resistor to the bases of the NPN and PNP transistors. 5v goes into the collector of the NPN transistor, while the PNP’s goes to ground. The emitters connect to the gate of the MOSFET which handles the switching of the 12v from the transformer primary.

Project idea - Building the circuit on a protoboard

I thought of building a permanent driver circuit onto a protoboard, this should be better than using flimsy jumper wires and dodgy breadboard connections. Before I do that, I will need some DIP-8 sockets, but also to work out how I could layout the circuit, I feel I’m a little out of my depth and risk messing up. I will see if I can do a mockup layout in Fritzing, first.

Major parts

The following are the major components used in the circuit:

  • NE555 timer (DIP-8)
  • IRFP260N MOSFET (TO-247) - requires heatsink, gets very HOT!
  • L7805CV linear voltage regulator (TO-220)
  • MUR820 fast switching diode (TO-220) - also requires a heatsink, gets pretty toasty
  • PN2222A NPN transistor (TO-92)
  • 2N3906 PNP transistor (TO-92)

Miscellaneous parts

Capacitor (ceramic)100nF3
Capacitor (electrolytic)100μF2
LED (Green)3mm1

Going further

I’m going to build circuits with the TL494 chip next.