Control circuit responsive to a component's varying resistance

摘要

An electronic control circuit for controlling the power applied to an electronic component such as a heater element or an electric winding for a motor. This is effected by measuring variations of the resistance of the component caused by the power flow therethrough. In a preferred form, the circuit has two operational amplifiers, one of which is a voltage comparator and summing amplifier to sum various signals, one being representative of the resistance of the component, the other amplifier operating as a monostable multivibrator. The component, which may be an electric heater for energizing through a switch controlled by the second amplifier. The resistance measurement is effected during the inflection point of the applied AC voltage. The heater is then energized for a full wave of the AC input. During every inflection point the resistance measurement is updated. A modification of the circuit includes a second heater disposed closely adjacent to the object to be heated to serve as an anticipatory circuit. Only one of the heaters is controlled. Alternatively the second heater may be represented by a thermocouple. It is also feasible to utilize two heaters, each of which is controlled by a separate set of two operational amplifiers. Finally, the circuit may be modified to provide a digital readout.