GB1119418A - Condition-responsive control apparatus - Google Patents

Abstract

1,119,418. Electric protective systems. TEXAS INSTRUMENTS Inc. 17 Aug., 1965 [17 Aug., 1964; 3 Feb., 1965], No. 35205/65. Heading H2K. [Also in Division H3] An arrangement responsive to a variable condition, e.g. temperature, to control a load device, e.g. an electric motor, comprises a voltage divider for connection across an A.C. source 11. The divider including a variable impedance, e.g. a thermistor T 1 embedded in the windings of the motor, and a reference impedance R11. A thyristor SCR1 is effective to control energization of the load device and is controlled by a switch, eg. a transistor Q1 connected to the junction 21 between the impedances T1, 31. The arrangement functions as a regenerative circuit to provide an adequate differential in the response of the thyristor. In the arrangement shown, the thyristor SCR1 is normally conducting so that a relay RY1 is energized with its contacts RY1B, RY1C open and its contacts RY1A changed over to complete a circuit L6, L7 for the load device. Three voltage dividers with positive temperature coefficient thermistors T1, T2, T3 are provided and when overheating occurs, the potential at a junction 21, 23 or 25 becomes positive and causes the associated transistor Q1, Q3 or Q5 to conduct rendering the thyristor SCR1 non-conductive. The relay RY1 releases to open the load circuit and its contacts short circuit differential resistors R21, R23, R25 to increase the forward bias on the transistor so that a substantial fall in the resistance of the affected thermistor is necessary before the thyristor can conduct. Further voltage dividers with negative temperature coefficient thermistors may be provided in conjunction with PNP transistors, Fig. 4 (not shown). In this case the differential resistors cease to be shunted when the relay releases. By using isolating diodes, two such arrangements with a common thyristor are operative on alternate half cycles to control two load devices, Fig. 5 (not shown). In a further arrangement, the transistors are connected in series and normally conduct to render the thyristor conducting, Fig. 6 (not shown). Fig. 7 (not shown) illustrates a further arrangement in which a transistor trigger circuit controls the operation of the thyristor and provides the operating differential.