A ballast circuit for operating a discharge lamp equipped with a control loop for controlling the power consumed by the lamp. A first circuit part is coupled to the discharge lamp and to input terminals for connection to a source of supply voltage for the ballast circuit. The first circuit part is coupled to the input terminals to supply lamp current from the supply voltage. The control loop includes a second circuit part coupled to the first circuit to generate a first signal that represents a desired value of lamp power. A third circuit part is coupled to the input terminals for generating a second signal that depends on the amplitude of the supply voltage. A fourth circuit part has an input coupled to the third circuit part and an output coupled to an input of the second circuit part for adjusting the value of the first signal dependent upon the second signal. The power consumed by the lamp is decreased when the amplitude of the supply voltage decreases. Too high currents in the ballast circuit are thereby prevented.