A phase gating controller includes a thyristor/triac having a control terminal and two power terminals, a sampling device for sampling a voltage present across the power terminals of the thyristor/triac and a control device configured to provide a control voltage at the control terminal in order to trigger the thyristor/triac. The control device is further configured to switch off the control voltage at the triggered thyristor and to detect an unexpected turning-off of the thyristor/triac if the sampled voltage exceeds a predetermined threshold value.