Patent ID: 6741043
Filing Date: 2004-05-25
Classification: H05B

Abstract:
A ballast for powering at least one gas discharge lamp, comprising:a pair of input connections adapted to receive a source of alternating current; first and second output connections adapted for connection to the gas discharge lamp; an inverter operably coupled between the input connections and the first output connection, the inverter including an inverter drive circuit for providing inverter switching at a predetermined operating frequency, the inverter drive circuit having a protection input and being operable, in response to application of a fault signal at the protection input, to take protective action; a direct current (DC) blocking capacitor coupled between the second output connection and circuit ground; a control circuit having a control input operably coupled to the DC blocking capacitor, and a control output coupled to the protection input of the inverter drive circuit, wherein the control circuit is operable: (i) following initial application of power to the ballast, to measure the voltage across The DC blocking capacitor and to store that voltage as a reference value; and (ii) following each subsequent application of power to the ballast: (a) to monitor the voltage across the DC blocking capacitor; and (b) in response to the voltage across the DC blocking capacitor departing from the reference value by more than a predetermined threshold amount, to provide the fault signal at the control output.