Abstract

A synchronized ballast for a discharge lamp having an increased dimming range. The ballast includes semiconductor switches coupled to a generator to receive a high frequency signal. A variable pulse width modulator includes a one-shot multivibrator for generating a pulsed signal for interrupting conduction of the semiconductor switches and thereby controlling the intensity of the discharge lamp. A delay means is electrically coupled to the variable pulse width modulator for delaying the generation of the interrupting pulsed signal whereby the voltage across the discharge lamp is zero for a predetermined amount of time after power is applied to the ballast and prior to lamp starting. The ballast may further include a power factor correcting means in the form of an inductor shunting the secondary winding of an arc transformer. The power factor inductor has a predetermined inductance whereby the inductor and the lamp's ballasting capacitor resonate at the frequency of the signal from the high frequency generator. In a preferrred embodiment, the ballast further includes a harmonic filter in the form of an inductor coupling the secondary winding of the arc transformer to the lamp. The inductor of the harmonic filter has a predetermined inductance whereby the filter inductor and the lamp's ballasting capacitor resonate at the second harmonic frequency of the high frequency signal. In accordance with another aspect of the present invention, a combined overvoltage and reverse voltage protection device for an electronic circuit operable from a direct current supply is disclosed.