Flyback (LOPT) Transformer Driver

This is a basic flyback (line output) transformer based on the information on Mike Harrison's "Quick & Dirty" Marx generator page (2/3 of the way down the page. The output voltage is variable by altering the duty cycle of the 555 timer.

I used this to power my small Marx generator which produces 2" sparks.

Circuit Diagram

Mike's original circuit:

My version of the circuit:

Circuit Operation

The diodes and the potentiometer split the timing capacitor charging and discharging currents. Since the output of the 555 is high when the capacitor is being charged, turning the pot varies the amount of on time.

However, since increasing the charging time decreases the discharging time, the frequency remains fairly constant whilst the duty cycle changes. The series resistors can be used to limit the range of duty cycles from a full adjutment of the pot. Drive frequency varies slightly with duty cycle changing

This alters the on time of the transformer, affecting the amount of energy stored in the core and then released when the flyback action occurs. Output voltage range from the flyback depends on supply voltage and duty cycle.

The capacitor across the MOSFET resonates with the primary inductance of the flyback. The resonant peak measured from a 12V supply at 95% duty cycle was 300V, so the MOSFET should have a Vds-max of 500V or more. The 150V zener across the capacitor in Mike's schematic isn't required - in fact mine overheated from trying to clamp the resonant spikes.

Performance is fine, 540V to 4kV ish output from a 12V input. It was difficult to measure the output accurately due to the lack of any decent measuring equipment.

Implementation

I stuck the driver in a cardboard box to make it easier to transport. There's also a connection for a PC power supply to provide the 12V supply.

[ pictures ]

See the Marx generator page for details on how it was used.