The device comprises a closed-loop control system in which a voltage proportional to the consumed power is continuously compared with a voltage representing the desired power consumption; the difference between the two voltages then actuates a servo-motor, which rotates the brush arm of an auto-transformer so as to minimize the deviation between the desired and actual power values. The system is not linear, but by varying the stabilizing feed-back according to the position of the brush-arm of the auto-transformer, an almost uniform transient response is obtained over the whole working range. Control of power from 50 to 2 000 W to an accuracy of ± ¼% has been achieved.