Patent ID: 8310160

Claim:
A protection circuit for an electronic ballast, the circuit comprising: a lamp signal sensing circuit coupled across one or more lamps and designed to detect a signal through the lamps; a shutdown circuit coupled to the sensing circuit and operable to disable the ballast in response to a disturbance in the detected signal, at least a portion of the shutdown circuit defining a first time delay from detection of the disturbance in the signal during which the ballast operates normally, and after which the ballast may be disabled in response to the disturbance; an automatic restart circuit coupled to the shutdown circuit and operable to enable restarting of the ballast, at least a portion of the restart circuit defining a second time delay during which the ballast remains disabled, after which the ballast may be restarted; the shutdown circuit further comprising a first switching element having a threshold, shutdown circuit coupled between the lamp signal sensing circuit and the first switching element, the circuit effective to transfer the signal detected by the sensing circuit to a DC signal driving the switching element, a DC voltage source coupled to the first switching element, and a second switching element having a threshold and coupled to the DC voltage source, the DC voltage source operable to drive the second switching element when the first switching element is on, the second switching element when turned on arranged to disable the ballast; the shutdown circuit further comprising a first capacitor, the charging time of the first capacitor selectable to define the first time delay; and the automatic restart circuit further comprising a third switching element coupled to the DC voltage source and arranged to be turned on in association with the second switching element, a second capacitor coupled to the third switching element, the charging time of the second capacitor effective to prevent startup while the third switching element is on, and a third capacitor and a resistor coupled in parallel and selectable to define the second time delay.