A control circuit is proposed, which can be used for the phase-shift control of an electric motor in an a.c. voltage supply line. The ground connector of the control circuit, together with the cathode of a semiconductor switch (triac), is directly connected with load current circuit. Because of this, it is possible to do without potential-separating means when it is intended to feed signals at the load current circuit directly into the control circuit. An auxiliary voltage source is connected between the synchronization input and the ground connector to maintain the function of the synchronization input.