Abstract

Circuits, and methods of using the same, are disclosed for controlling the power supplied to a discharge lamp of the type having a closed inductive loop, such as the resonant ballast circuit for a fluorescent lamp or the inductive ballast loop of a high pressure sodium lamp, wherein the closed inductive loop is operated by an electrical power supply having a d-c input stage and an output power controlled by the switching frequency of a switch means within the power supply itself whereby current flows to the closed inductive loop when the switch means is conductive and no current flows from the power supply to the closed loop when the switch means is non-conductive. This power control circuit comprises means for sensing the instantaneous current flowing through the switch means itself, means controlled by this instantaneous current for creating a first signal with a value that is proportional to the actual power being supplied by the power supply to the closed loop, means for creating a second signal with a value proportional to the desired set point power for the lamp, means for creating an error signal having a value indicative of the difference between the first and second signals, and means for adjusting the switching frequency of the switch means in accordance with the value of the error signal, whereby the output power of the power supply is continuously adjusted toward the set point power for controlling the power actually supplied to the lamp circuit irrespective of the parameters of the lamp circuit itself. The disclosed circuits provide for constant power to a high pressure discharge lamp to yield a constant color temperature. Further, the disclosed circuits provide for dimming of the discharge lamp to selective power levels.