Abstract: A low cost, high efficiency, dimmable electronic ballast for gas discharge lamps which can be controlled by a traditional phase controlled dimmer is disclosed. This ballast includes a push-pull, parallel resonant, self-oscillating inverter which is comprised of a DC base drive circuit and an adjustable, loosely coupled (between the primary and the secondary) transformer with a pair of magnetic shunts, and a LC filter which has both common and differential mode filter functions to reduce the high frequency interference and improve the power factor. The Q factor of the LC filter will maintain a nearly constant lamp current at low light levels. Wide range dimming (100% to less than 1%) is achieved. A delay start circuit and an overvoltage (open circuit) protection circuit are connected to the LC filter. The overvoltage protection circuit can be connected to either the AC side or DC side of the ballast.