Ballast with frequency-diagnostic lamp fault protection circuit

Abstract

A ballast () for powering one or more gas discharge lamps () comprises an inverter () and a lamp fault protection circuit (). Inverter () has an operating frequency that is load-dependent. Lamp fault protection circuit () monitors an electrical signal within inverter (). 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 (), lamp fault protection circuit () disables the inverter () for a predetermined shutdown period. Lamp fault protection circuit () 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 () increases the predetermined shutdown period so as to minimize any undesirable effects due to the restart function. Lamp fault protection circuit () is preferably realized using a suitable microcontroller integrated circuit () 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.