Patent GB-661242-A (GB)
Title: Improvements in electric thermostats
Inventor: FLIGHT THEODORE NICHOLAS
IPC: G05D23/12
Filed: 1949-05-03
Abstract: 661,242. Thermal switches. RHEOS<SP>T</SP>ATIC CO., Ltd. :May:3, 1950. [11av3, lJ4Py No.llSW3/M). Class 38 (v). A thermostat primarily for the control of an electric oven has one or more of the magnetic snap-action switch units disclosed in Specification 625,492 actuated by a bellows or expansible capsule through the medium. of an adjusting screw. Fig. 2 shows one arrangement in which a capsule 17 connected by a capillary tube 21 to a liquid-filled phial 22, operates a switch unit having contacts 7, 9 for connection in series with "bake" oven elements and a similar switch unit having contacts 45, 47 for connection in series with a "pre-heat" element. The lower switch unit comprises a spring switch lever 6 carrying the moving contact 7 and a twin armature unit 8, a plate 10 carrying adjustably the fixed contact 9, and a plate 5 carrying a permanent magnet 4. The lever 6 and plates 5, 10, together with the equivalent members 44, 43, 48 of the upper switch and insulating bushes 11, 12 are assembled as a pack on a pair of insulated tie rods or bolts 2, 3. Fig. 3. The adjusting screw 28 has nuts 29, 50 held against rotation by insulating blocks 30, 49 which engage the levers 6, 44, the blocks being also prevented from rotation by their engagement in apertures in the plates 10, 48. Thus when the adjusting screw 28 is turned by the knob 25, the nuts 29, 50 are moved equally along the screw to adjust the operating temperature, the difference between the settings of the two switches being maintained. The blocks 30, 49 are formed so as to have line engagement with the switch levers 6, 44 as indicated at 31, 51. To prevent overstressing of the spring levers 6, 44, relative movement between the capsule and the adjusting screw 28 is permitted by a plunger 32 loaded by a spring 33. This plunger engaging a disc 20 in a cup 40 containing a bi-metal disc 19, the cup being fixed to the lower diaphragm of the capsule, and the disc providing compensation for the variation of ambient temperature in the thermostat head. In use, as the temperature of the oven approaches the desired level, the contacts 45, 47 open first to disconnect the pre-heat element. These contacts remain open whilst the contacts 7, 9 open and close the "bake" element circuit to maintain the oven temperature. To enable the application of the thermostat to an electric cooker in which the pre-heat element also serves as a griller, a further switch in series with the lower switch controlling the "bake" elements, Fig. 3, has contacts 36, 37 which are opened by a cam 38 lifting a right-angled lever 39 when the adjusting knob 25 is turned to the "grill" position. Modifications are referred to in which the "pre-heat" or upper switch when opened may be held open, possibly by a magnet, until reset by the manual operation of a, cam by the knob. Alternatively the pre-heat switch may be normally biassed to its open position, be closed by a cam on the operating knob spindle, held closed magnetically, and opened to remain open when the oven temperature approaches the desired level.