| 摘要 |
1299382 Automatic control of temperature PENNWALT CORP 3 Dec 1969 [30 Dec 1968] 59082/69 Heading G3R [Also in Divisions G4, H4] In a system for maintaining constant the temperature of an enclosure containing a mercury manometer whose two meniscus heights are compared by an ultrasonic pulse-echo device (see Division H4), heating elements 538, 540 past which air is blown by a fan are disposed one at the top and one at the bottom of the enclosure and are provided with both on/off and proportional control systems. The on/off control comprises a mercury-in-glass thermostat switch 541 (inside the enclosure) which closes if the temperature rises above 95‹F, thereby causing transistors 1232, 1233 to conduct and provide a shunt path which prevents the development of trigger pulses for the gate of a triac 1234 and so prevents energization of the heating elements by a mains supply 1203. Below 95‹F switch 541 remains open and the proportional control takes over as follows. An A.C. signal derived from the mains is fed over network 1208, 1224, 1209 to integrated circuit 1206 which discharges capacitor 1226 rapidly as the mains voltage goes through zero. This discharge induces a pulse in transformer 1236 for triggering triac 1234. The capacitor 1226 recharges slowly via the integrated circuit and with a time constant dependent on the impedance of a thermistor 543 in a bridge at the input of the integrated circuit. Thermistor 543 senses the ambient temperature of the enclosure. The heating element duty cycle is illustrated in Fig.13 for different ambient temperatures: thus at 94‹F ambient, the elements are on only during occasional mains cycles and at 60‹F are on during most mains cycles. A thermal cut-out (not shown) disconnects the mains supply if the enclosure internal temperature exceeds 120‹F.
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