DISCLAIMER! High voltage experiments are dangerous. I refuse to take ANY responsibility for any possible injuries, legal problems, property damage or deaths, anything you find here is provided WITHOUT ANY WARRANTY and you do everything AT YOUR VERY OWN RISK!
This is a simple high voltage flyback transformer driver. It was designed for a small spark gap Tesla coil.
The circuit is based on the NE555 IC operating in astable mode. Diode D1 allows independent adjustment of on/off times. The frequency range is set by RV1, RV2, R1, R2, C1. Power for the 555 IC is filtered using D3, C3, C4, L1. Parts R3, R4, D2, Q1, D2 drive the output MOSFET, which switches the transformer. This driver circuit ensures fairly slow turn-on and fast turn-off. A capacitor (C6) is placed between the MOSFET's drain and source. When the FET turns off, a damped oscillation will occur (C6 and the transformer primary will form a resonant circuit). During the first half cycle, this limits the drain voltage. During the second half cycle, the energy from this resonant circuit gets discharged back into the filtering capacitor through the MOSFET's internal diode. During following oscillations, the drain-source voltage shouldn't exceed approximately 2 times the input voltage. Then the FET turns back on and the cycle repeats. Slower turn-on limits the maximum current when C6 discharges through the FET. For optimum operation, the waveforms can be tuned by watching the drain voltage with an oscilloscope (use a good 1:10 or 1:100 probe!) and adjusting RV1, RV2. Turning the MOSFET on when the drain-source voltage is high increases power dissipation. Before turning the circuit on and tuning it, it is advised to set maximum possible off-time and minimum on-time (set RV1 to max. resistance, set RV2 to min. resistance).
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