The invention is based on an apparatus for operating a light generator (2) with a voltage source (4). The invention provides: • a device (5; 13) for generating a measurement voltage proportional to the voltage present at the light generator (2), • device (6, 9) for generating a first and a second comparison voltage, each proportional to the current flowing through the light generator (2), • a device (9, 8) for forming a difference voltage from the measurement voltage and one comparison voltage and a summation voltage from the measurement voltage and the other comparison voltage, • two capacitive rectifiers (7) for rectifying the difference voltage and the summation voltage, and • a controller (3), by which the rectified difference voltage and the rectified summation voltage can be evaluated.