GB666502A - - Google Patents

Abstract

666,502. Automatic temperature control systems. BRITISH THOMSON-HOUSTON CO., Ltd. Dec. 20, 1948 [Dec. 20, 1947], No. 32867/48. Class 38 (iv). In apparatus for controlling the temperature of an electric blanket or the like, the heating winding 2 has a high temperature coefficient of resistance and is corrected in series with a low resistance 9 and a large resistance 8 acting as a current limiter for the heater when the temperature exceeds a predetermined value. The resistance 8 is short-circuited during heating periods by means of a relay 12 controlled by the anode current of a thyrator 11, to the control grid of which is fed a control voltage dependent on the resistance of the heater 2. When the heater is initially energized the voltage across the resistance 9 is fed via the primary of a transformer 26 and potentiometer 30 to the grid of the thyratron 11, thereby causing the valve to conduct, energizing the relay 12 and short-circuiting the resistance 8. Full current then flows through the heater 2 until at a predetermined temperature the resistance of the heater 2 has so increased that the voltage across the resistance 9 is no longer sufficient to maintain the thyratron conducting. The relay 12 is then de-energized placing the resistance 8 in series with the heater, thereby limiting the current flow through the heater 2. De-energization of relay 12 also places a second primary 28 of the transformer 26 and the secondary 17 of a mains transformer 15 in series across the heater 2. The windings 17 and 28 are so connected that the voltage of the secondary 17 is greater than the voltage across primary 28 when the resistance of the heater is high. and the thyratron is maintained non- conducting. As the heater cools, however, the voltage across primary 28 increases until the resultant voltage applied to the grid of the thyratron is sufficient to cause it to conduct, whereupon the heater is again fully energized. In a modification, Fig. 3 (not shown), the thyratron may be replaced by a " hard " double triode, the cathode of one section being under the control of a second temperature variable resistance responsive to ambient temperatures. The relay may include additional contacts for disconnecting the primary 27 during cooling periods to achieve a closer control.