A control algorithm for operating a fluid disinfecting system by means of UV radiation, wherein the UV radiation is generated by at least one UV lamp comprising a pair of heating cathodes having a discharge voltage (U D), said UV lamp is operated by an electronic ballast unit, which is equipped with the control algorithm, which allows to adjust the UV power of the UV lamp by pulse-width-modulation, to reduce UV power said control algorithm includes the following steps: .cndot. Decreasing the current to a level (I kmin)； .cndot. Increasing the voltage amplitude (U) above the discharge voltage (U D) until a desired UV power level is reached； .cndot. With increasing voltage amplitude (U) decreasing the pulse width (PW), until PW min is reached； .cndot. Wherein the decrease in current and the increase in voltage are carried out in such a way, that an ineffective current-voltage-ratio is generated, whereas the too high current is used for cathode heating.
