a system for controlling the outer diameter of a strand , particularly of a cable or the like including a measuring device for measuring the outer diameter of the strand of plastic material emerging from the extruder . the signal of the measuring device is fed to a comparator which compares it with a reference diameter and generates a difference signal . a control unit receives the difference signal to control the rotational speed of the extruder and / or the speed by which the strand emerges from the extruder . a computing stage is connected to the control unit which generates a comparison signal modulated by the actual diameter signal and fed to the control unit .