Patent GB-628304-A (GB)
Title: Improvements in or relating to thermal-responsive electric control devices
IPC: G05D23/30
Filed: 1947-06-10
Abstract: 628,304. Thermal switches. PLESSEY CO., Ltd. June 10, 1947, No. 15216. Convention date, June 15, 1946. [Class 38(v)] A thermally-operated device, unaffected by variations in ambient temperature, gives infinitely-variable control over a range of 5 per cent to 100 per cent of the maximum possible energy supplied to a cooker heater element 34. The device comprises Figs. 1, 2, an air-filled chamber 10 enclosing a heating coil 14 and having snap-action and breather diaphragms 12, 13 respectively, which actuate the control rods 28, 42 of their respective snap-action switches 26, 40. When, the device is cold, the diaphragms 12, 13 are collapsed, Fig. 2 and current flows through the element 34, heating coil, 14, contacts 30, switch lever 29, and lead 35. As the air in the chamber 10 expands, the breather diaphragm 13 moves a lever 44 to close the contacts 45 of the switch 40, and the snapaction diaphragm 12 extends to, move the lever 29 thereby opening the contacts 30 to de-energize the heater 14 and closing contacts 31. Current now flows through the element 34, switch lever 44, contacts 45 and 31, and lead 35. The air then cools and the diaphragm 13 opens the contacts 45 to break the circuit through the element 34; finally, the diaphragm 12 opens the contacts 31 and closes the contacts 30 to re-energize the heating coil 14 and repeat the cycle. The proportion of time in each cooling part of the cycle that the element 34 is energized, is controlled by varying the distance between the switch 40 and the diaphragm 13, the switch 40 being mounted for this purpose on a resilient bracket 41 and movable by a knob 47. When the element 34 is to be energized for a minimum time, the switch 40 is moved away from the diaphragm 13 so that the lever 44 is not moved sufficiently to close the contacts 45. For maximum energization of the element 34 the switch 40 is moved toward the diaphragm 13 so that the lever 44 is not moved sufficiently to open the contacts 45. The chamber 10 is so constructed that the proportion of time in each cycle that the coil 14 is energized, i.e., until the moment that the diaphragm 12 extends, is only 5 per cent, the element 34 being energized throughout this period; however, during the remaining 95 per cent of the cycle time, i.e., when the contacts 30 are open and the contacts 31 closed, the period that the element 34 is energized can be varied as described above from zero to the maximum. Variations in ambient temperature affect the cycle time but not the proportion of the cycle time that the element 34 is energized, and hence over a number of cycles the energy supplied to the element 34 is substantially unaffected by these variations. In alternative constructions, only the switch 26 or the diaphragm 12 has a snap-action. In a modified construction, Fig. 3, the chamber 10 is mounted on a resilient bracket 66 and is adjustable by a shaft 67 engaging the breather diaphragm 13. The chamber 10 supports a contact stack 69 comprising strips 70, 71, 72 and carrying a snap-action spring 79 which actuates a contact arm 76 co-operating with a stop 78. Contacts 73, 74, and 80 correspond respectively with contacts 30, 31 and 45 of Figs. 1, 2. As the air in the chamber 10 expands, the chamber moves toward the stop 78 and the spring 79 closes the contacts 80 with a snap-action; also the snapaction diaphragm 12 opens the contacts 73 and closes the contacts 74, thereby initiating a cycle identical to that described with reference to Figs. 1, 2.