Patent GB-1423095-A (GB)
Title: Thermostat
IPC: H01H37/10
Filed: 1974-02-21
Abstract: 1423095 Thermal switches INVENTRONICS Ltd 21 Feb 1974 [27 March 1973] 7912/74 Heading H1N A thermostat comprises a primary temperature sensor 20 actuating a switch 13 through a transmission 50, 54, 56 including a secondary temperature sensor 54 located adjacent switch 13 and responsive to its temperature when it is on and to a heater 57 when it is off to vary the effective length of the transmission, and mechanism 60 for applying to primary sensor 20 an amount of heat proportional to the load current through switch 13 when switch 13 is on. As shown, sensor 20 is a bimetal pivoted at 23 and having its other end raised or lowered by pin 25 adjusted by knob 38 to adjust the operating temperature. Knob 38 rotates pin 25 which has a thread 28 which runs in bobbin 30, engaging a pin 29 in bobbin 30, which is fixed in hole 33 in casing 10 by projection 32 and spring 34. Bobbin 30 can be adjusted within limits by having teeth 39 on its periphery engaging a calibration gear 37 on member 40 which fits into recess 41 in casing 10 and has a screwdriver slot 42. Stop cam 35 on pin 25 limits the rotation of knob 38. Sensor 54 is a U-shaped bimetal, the legs of which move towards each other when heated, thus reducing the length of the transmission 50, 54, 56. Mechanism 60 is a heater in series with switch 13. The operation is as follows. For thermostatic action, bimetal 20 opens and closes switch 13 through transmission 50, 54, 56 with heater 60 acting as an anticipatory heater. When switch 13 is open, most of the voltage drop occurs across resistor 57, with resistor 60 generating little power, with switch 13 closed and resistor 57 shorted out, more power is generated by resistor 60. The main anticipating effect is produced by sensor 54 and its heater 57. Heater 57 is energized in the off condition of switch 13, at which time heater 60 is inoperative. When heater 57 is energized, sensor 54 is flexed which shortens the transmission which allows pin 56 to move outwards of switch 13 and turn on switch 13 before the fall in temperature of the controlled space causes sensor 20 to turn on switch 13. Conversely, when switch 13 is on, lack of heat from heater 57 allows sensor 54 to cool, thus spreading its legs and tending to move switch 13 to the off condition. To compensate for the effect of heat generated by switch 13 when closed on sensor 54, when switch 13 is closed heater 60 is energized which heats sensor 20 which is flexed towards switch 13 to move pin 50 towards switch 13. If this motion was transmitted through pin 56 it would quickly turn switch 13 off, but the transmission length is shortened by the movement together of the legs of the heated sensor 54. The movements of the two sensors thus concel each other out. In alternative embodiments, switch 13 is a change-over switch, Fig. 8a (not shown), switch 13 is a double-pole switch, one pole change-over, which it is stated may also be manually operated, Fig. 8b (not .shown), sensor 20 may be replaced by a hydraulic bulb and bellows with heater 60 adjacent the bulb, Figs. 9, 10 (not shown), and in Fig. 11 (not shown) the bulb may be at a distance from the switch with the heater near an auxiliary bulb also connected to the bellows.