A
ballast (20) for powering one or more gas
discharge lamps (70,72,74,76) comprises an
inverter (200) and a lamp fault protection circuit (400).
Inverter (200) has an
operating frequency that is load-dependent. Lamp fault protection circuit (400) monitors an electrical
signal within
inverter (200). In response to a change in the
fundamental frequency of the electrical
signal, such as what occurs when a lamp is removed, when a lamp approaches the end of its
operating life, or when an arcing condition occurs at one or more of the
ballast output connections (302,304,306,308,310), lamp fault protection circuit (400) disables the
inverter (200) for a predetermined shutdown period. Lamp fault protection circuit (400) also provides a restart function for periodically attempting to ignite and operate the lamps. Additionally, in response to a sustained fault condition, lamp fault protection circuit (400) increases the predetermined shutdown period so as to minimize any undesirable effects due to the restart function. Lamp fault protection circuit (400) is preferably realized using a suitable
microcontroller integrated circuit (420) with associated discrete circuitry, and is especially well-suited for use in ballasts for powering multiple lamps via a current-fed self-oscillating inverter and an isolated parallel resonant output circuit.