The invention relates to a method of determining a zero point (V0) of a current sensor in a circuit arrangement for operating a gas discharge lamp. The invention is characterized by the following process steps: the current (11) through the sensor is switched off for a short period during a first half wave (13) and a first test value (V6) is determined, then the current (11) through the sensor is switched off for a short period during a second half wave (14) having a different polarity and a second test value (−V7) is determined, whereupon an average value is formed of the two test values (V6, −V7), and the zero point (Vx, V0) is determined by means of said average value. It is prevented thereby that the zero point drifts during lamp operation, for example owing to heating, and that amplitudes of the positive and negative half waves (13, 14) of the lamp current (11) are formed differently. Impairment of lamp life and visible artefacts in a presented projection image are prevented.