The present invention is to provide an electronic ballast connected in parallel to two filaments of a fluorescent light tube, which comprises a controller for generating two output signals, two power switches being switched according to the output signals, a resonant circuit oscillating according to switching of the power switches and thereby causing a resonant capacitor thereof to generate a voltage, a voltage sensor connected in series to the resonant capacitor and connected in parallel between the two filaments so as to sense an AC voltage proportional to a voltage across the fluorescent light tube, and a regulator connected separately to the voltage sensor and controller so as to convert the AC voltage into a DC voltage and deliver the DC voltage to the controller. Upon determining that the DC voltage exceeds a preset voltage level, the controller generates an interruption control signal for immediately stopping operation of the controller.