Abstract: A power supply circuit for any kind of loads or power consuming devices which are connected to an electrical power supply; especially lamps, such as at least one glow or gas discharge lamp, wherein a pulse width modulator controls electronic switches of a push-pull oscillator, and wherein the pulse frequency of the pulse width modulator is tuned to the resonant frequency of a resonant transformer of the push-pull oscillator which has the secondary side thereof connected to the gas discharge lamp. A control circuit monitors a current and/or voltage value and/or time value which is characteristic for the presence of the resonance and which, upon a change in the resonant frequency, will change the pulse frequency of the pulse width modulator to ensure operation at the new resonant frequency.