Method for operating at least one fluorescent lamp with an electronic ballast, as well as ballast therefor

A method for operating at least one fluorescent lamp using an electronic ballast which has a rectifier bridge having AC mains voltage across the bridge, a charging inductor which is connected to an output side of said rectifier bridge and said ballast having a connected charging diode, which is fed back to the rectifier bridge via a shunt capacitor, a half-bridge circuit connected to said changing diode and fed back to the rectifier bridge, said half-bridge circuit having two power transistors which are in series with one another and are activated alternatively, a control loop having a monitoring circuit for continuously monitoring lamp current of said at least one fluorescent lamp and having a high-frequency controlled drive circuit, derived from said monitoring circuit, for the power transistors, said ballast being connected to a load circuit which is arranged at an output of the half-bridge circuit and which has the at least one fluorescent lamp, an ignition inductor and an ignition capacitor, comprising the steps of:

• starting a timer in a defined manner each time the lamp is started and each time a disturbance occurs during lit operation of the at least one fluorescent lamp, which timer generates a time base for subsequent control operations and respectively emits time control signals at predetermined time instants;
• using said time control signals, setting respectively predetermined, different reference levels for lamp current to be detected are set in the monitoring circuit or preparing automatic disconnection of the electronic ballast for a predetermined, limited period of time
• comparing via the monitoring circuit an instantaneous value of the lamp current with a respectively activated reference level and emitting a respective control pulse when said activated reference level has been reached;
• and
• controlling the lamp current as a function of time using said control pulses, which reproduce normal or alternatively faulty states in the load circuit as a function of their occurrence or failure to occur during predetermined periods of time defined by the timer, wherein the control pulses act on the controlled drive circuit, in the event of an undisturbed operating state or wherein said control pulses trigger the prepared automatic disconnection of the electronic ballast when a fault occurs.