Abstract: An electronic ballast or energizing circuit for a fluorescent lamp is disclosed. A rectified supply voltage is applied through an electronic switch to a fluorescent lamp through a current sensing resistor. A switching regulator produces a pulse width modulated pulse train which is applied to the electronic switch for supplying a pulsed voltage across a smoothing filter to the fluorescent lamp. The pulse width output of the switching regulator is modulated in accordance with the voltage across the sensing resistor for maintaining the current drawn by the lamp at a substantially constant value.