DC motor control circuit for cine appts. - maintains precise rotation and has end stage coupled to comparator for exponential control

摘要

The control circuit has an end stage controlled by a comparator (14) receiving a voltage from an RC element (12,13) proportional to the actual motor rotation and a reference voltage representing the required rotation. The output of the end stage is fed back via a further RC element (30,31) to the comparator (14) with the end stage only switched in when the motor (10) is switched on, to obtain exponential control of the end stage. Pref. the comparator (14) uses two transistors (19,20) in cascade with the motor (10) connected in parallel with the collector resistance of the second transistor (20) via a pair of switch contacts (27,27a). These also allow completion of a feedback path to the base of the first transistor (19) with the feedback RC element (30,31) coupled to the collector of the second transistor (20).