Patent GB-640709-A (GB)
Title: Improvements relating to electronic control circuits
Inventor: BRAMSON SOLOMON MICHAEL
IPC: G05B11/01
Filed: 1947-02-17
Abstract: 640,709. Automatic control systems. HIRST ELECTRONIC DEVELOPMENT, Ltd. Feb. 17, 1947, No. 4525. [Class 38 (iv)] [Also in Group XL (c)] An electronic control circuit comprises a cold-cathode tube in which the potential applied to the control electrode corresponds to the difference between an A.C. potential and a negative D.C. potential, and the control electrode is prevented by means of a rectifier from attaining a negative potential. One of the potentials corresponds to a controlling quantity and the other to a predetermined value, the firing of the tube causing the value of the controlling quantity to be varied. Preferably the tube anode is fed with alternating current approximately in phase with the alternating component of the control electrode potential so that if the control electrode receives a resultant positive pulse, causing it to fire, the anode receives an overlapping positive pulse. Fig. 1 shows a control circuit for example for controlling the speed of a motor. Normally the cold cathode tube A is non-conducting, the relay C is released and its associated contacts are closed. The D.C. voltage across resistors H<SP>1</SP>, H<SP>2</SP> tends to bias the control electrode of tube A negatively but is prevented from doing so by the rectifier K. When an alternating potential proportional to the motor speed is applied across terminals L of transformer J, the positive peaks of the signal exceed the standing bias, rectifier K conducts, tube A fires and relay C operates. Contact C<SP>2</SP> disconnects the A.C. supply to the motor terminals M and contact C<SP>1</SP> removes the short from resistor H<SP>1</SP>, thus reducing the bias voltage at rectifier K. Consequently the tube continues to conduct until the motor speed is reduced to a value which is lower than the maximum value at which the tube first .fired. The difference between these two values may be varied by the adjustment of the resistor H<SP>1</SP>. In a modification, Fig. 2 (not shown) two control circuits are employed, one serving to raise the motor speed when it falls below a certain minimum value and the other to reduce the speed when it rises above a certain maximum value. Each circuit may control a separate auxiliary motor for shifting the speed controller of the main motor or the two circuits may control separate windings of a common auxiliary motor which may drive the controller in either of two directions. The anodes of the two cold-cathode valves are energized from the A.C. supply. In the control circuit for reducing the motor speed when it rises above a certain maximum value the potential applied to the control electrode of the controlling tube corresponds to the difference between an alternating voltage (proportional to the motor speed) and a direct voltage (proportional to the predetermined maximum value) as described with reference to Fig. 1. In the control circuit for raising the speed when it falls below a certain maximum value the corresponding alternating voltage is proportional to the predetermined minimum value and the direct voltage is proportional to the motor speed.