Patent GB-936801-A (GB)
Title: Thermostatic switches
IPC: H01H81/02
Filed: 1959-10-22
Abstract: 936,801. Thermal switches. TEXAS INSTRUMENTS Inc. Oct. 22, 1959 [May 11, 1959], No. 35882/59. Class 38 (5). A switch adapted for use in a magnetic location comprises an electrically conducting support for a thermostatic element carrying a movable contact at one end, the other end being supported as a cantilever between, and in electrical contact with, two portions of the conducting support. A movable silver or silver alloy contact 38 is riveted to a temperature and current responsive bi-metal or trimetal blade 30 which has a dished portion 32 to provide snap-action. The blade is welded at 44 between two arms 20, 22 of an electrically conducting terminal which may be made of " Monel " metal (Registered Trade Mark), stainless steel or tin-plated cold-rolled steel and is clinched to a conducting lead 46. The terminal has shoulders 50, 58 which assist the snap-action and prevent slow opening or closing of the contact 38. An excessive current will cause the rivet 40 to strike the metallic terminal at 62 and become fused thereto to maintain the contacts open. The fixed contact 70 comprises a serrated layer 72 of silver or silver alloy bonded to a cold rolled steel base 74 mounted on a block 86 of insulating silicone or steatite which is of smaller area than the contact 70 to prevent particles of silver produced during arcing forming a leakage path between the contact 70 and the metallic case 12 which, being made of steel, aluminium or copper is in electrical contact with the movable contact 38 via the terminal 20, 22. The case 12 being metallic minimizes the effect of the magnetic field in which the switch may be placed, provides good heat conductivity, and is comparatively unaffected by arcing. The terminal portion 60 is at the same potential as the moving contact 38 and tends to draw arcing away from the thermal element 30 in order to minimize damage thereto. The contact 70 and terminal portion 62 are each notched as at 49 (Fig. 6) in order to provide a narrow section which will rupture when carrying an excess current, these notches and a similar pair in the insulation 86 co-operating with a shaped ceramic steatite header 104 to form a complete assembly which is then slid into the case 12 and sealed with epoxy resin 120. The header 104 is slotted at 108 so that if it is sprayed with silver particles because of arcing, there will remain a " dead " area 114 which breaks any possible leakage path across its face. The switch is calibrated by bending the portion BA and thus altering the contact pressure, and the whole may be enclosed in an insulating sheath 10, e.g. of polyethylene terephthalate which may be cylindrical or shaped like the case 12. In a modification (Figs. 23, 24, not shown), the thermal element 30 is not dished or welded and so acts slowly. In order to increase sensitivity, the terminal 20, 22 may be formed of high resistance material and insulated from the casing 12 in order to act as a heater for the thermal element. This may be further modified (Fig. 25, not shown) by making the terminal 20, 22 bi-metallic, the bi-metal thermal element being arranged to increase contact pressure when heated and the contacts being opened by the differential effect of the terminal 20 and the thermal element 30. In another modification (Fig. 26, not shown), the upper terminal is bi-metallic and the movable contact is mounted directly thereon, being insulated from the. metallic casing. In a further modification (Fig. 27) a snap-acting thermal element 406 is clamped between limbs of upper terminal 404 in a casing 402, but the terminal is sealed within the casing by a glass seal 434, carrying lower terminal 422 and fixed contact 420, within a metallic collar 432 welded to the casing. An external upper terminal 410 is also welded to the casing. In yet another modification (Fig. 22, not shown), a snap-action thermal element is clamped directly between the sides of the casing, which is shaped in the same fashion as terminal 20, 22 of Fig. 2, this terminal now being omitted. Specification 178,103 is referred to.