Electrical Switches for Control Devices Such as Temperature Regulators

This invention relates to electrical switches for control devices, particularly for temperature regulators.

The purpose of a temperature regulator in a heating appliance is to switch off the appliance when a predetermined temperature has been reached and to switch the appliance on again whenever the temperature of the appliance falls below a certain pre-determined temperature. There is, however, a demand for small heavy duty switches for use with temperature regulators of small size. Heating appliances usually have a high current rating, which imposes a considerable strain on the switch contacts of the associated temperature regulator. If the separation of the switch contacts upon opening of a switch is observed in slow motion erosion by arcing becomes apparent. It follows, therefore, that the switch contacts should be separated as rapidly as possible, which in turn necessitates the use of a strong spring for moving the contacts apart. The material of the spring would not in general be sufficiently conductive to enable the switch to pass strong currents.

The present invention provides a new and improved snap acting electrical switch which is durable and has a long working life (that is, is capable of a large number of switching cycles) despite a relatively high contact current rating and a relatively small overall size and length of contact travel.

An electrical snap switch unit embodying the invention comprises at least one substantially stationary switch contact, a movable switch contact, a contact supporting member carrying the movable contact for motion into and away from engagement with the stationary contact, a stressed blade spring member extending toward the contacts having a portion effective to snap move in a direction transverse to the direction of extent of the blade spring and structure for interconnecting the blade spring portion and the contact supporting member so that snap movement of the blade member portion abruptly moves the contact supporting member to open or close the contacts.

In accordance with one preferred embodiment of the invention an electrical switch unit is provided for use in a control device such as a temperature regulator. The switch unit includes a housing having two stops at least one of which is a switch contact and a movable switch contact assembly movable between the stops and normally abutting one of them. The contact assembly includes a contact carrying member constructed of a material having a relatively great electrical conductivity and a snap action stressed blade spring member engaged with the contact supporting member for snap moving the contact supporting member into and away from a position in which the contacts are engaged.

The spring member is Q-shaped with two legs drawn toward each other to stress the blade and having lugs projecting into engagement with the contact supporting member to transmit snap motion to the contact supporting member.

The blade spring and contact supporting members are cantilevered in the housing and disposed substantially parallel to each other.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a longitudinal cross section of a switch according to one embodiment of the invention, taken along line I--I in FIG. 2, and

FIG. 2 is a plan view of the switch in the direction of the arrow II in FIG. 1.

The illustrated switch has a housing 1 made of an insulating material and having the appearance of an open box of rectangular shape typically measuring 33 (37).times.21 mm and a depth of 18.5 mm, with two short sides 2,3 two long sides 4, 5 and a base 6. The side walls 2, 3, 5 project below the base 6, forming thereby a peripheral flange. The two shorter sides 2, 3 are provided with external projections 7 which cooperate with fixing stakes on the casing of a temperature regulator (not shown) to anchor the housing 1 to the regulator in a manner known per se.

The base 6 is provided near its four corners with respective rectangular slots 6a, 6b, 6c and 6d in which respective lugs are inserted and anchored. The base 6 is also provided with two further slots 6e and 6f near its centre. Respective flat terminal lugs 10, 11 and 12 are inserted and anchored in the slots 6e, 6a and 6b respectively. The terminal lug 11 is formed with an integral bridge portion 11a within the housing 1, extending parallel to the base 6 and supported by a cleat 13 which is anchored in the slot 6d. The bridge portion 11a has a laterally cantilevered support plate 11b with a central through hole 11c. A cantilevered switch contact-carrying arm 15 is riveted to the bridge portion 11a together with one end of a snap action spring 16.

The snap action spring 16 is Q-shaped, having two lateral legs 16a, 16b interconnected at one end by a base portion 16d and at the other end by a connecting portion 16c. A trip tongue 16e projects from the base portion 16d towards the connecting portion 16c, and a mounting foot 16f projects from the base portion 16d in the opposite direction to the tongue 16e. The legs 16a, 16b, the base portion 16d, the connecting portion 16c, the tongue 16e and the foot 16f are formed integrally with each other from a single piece of sheet metal--spring strip was originally used, but stainless steel strip is now preferred.

The two legs 16a, 16b are originally parallel to each other; the connecting portion 16c is then deformed by the formation of a double-curvature arch therein to draw the two legs 16a, 16b towards each other, thereby stressing the base portion 16d. This stressing results in a dish deformation of the snap action spring 16, whereby the connecting portion 16d is resiliently urged upwards or downwards. The direction of the dish deformation of the base portion 16d is determined by the trip tongue 16e: when the tongue 16e (FIG. 1) is moved downwards, the base portion 16d is dished in such a manner that the legs 16a, 16b are urged upwards as viewed in FIG. 1. The snap action spring 16 can therefore assume two stable positions according to the relative position of the trip tongue 16e.

The contact carrying arm 15 of the switch has essentially an isosceles triangular shape with a rounded vertex (FIG. 2) which a movable contact 17 is fastened. Adjacent its fixed end 15a the cantilevered arm 15 is formed with an angled step 15b connecting the fixed end to a flat portion 15c which imparts a resilient flexibility to the arm 15. The arm 15 is reinforced by means of a longitudinally extending stiffening corrugation 15d. The arm 15 is manufactured from copper plate and is substantially planar with the exception of two upstanding tabs 15e, 15f which are perpendicular to the general plane of the arm 15. The tabs 15e, 15f are inclined at an angle to each other, converging towards the contact 17. Each tab 15e, 15f is provided with a slot 15g which is diabolo shaped, with greater width at the ends than in the centre. Respective rounded lugs 16h, 16i at the ends of the legs 16a, 16b of the snap action spring 16 engage in the slots 15g so that the snap action spring 16 and the contact-carrying arm 15 are coupled to one another, constituting a movable switch contact assembly. The position of the switch contact 17 is determined by the snap action spring 16 the position of which is in turn determined by the trip tongue 16e. The switch contact assembly is effectively bistable but is prevented from occupying its end positions by engagement with restraining stops.

The terminal lug 12 is formed with an integral bridge-like contact-carrying portion 12a which extends transversely within the housing 1 parallel to the base 6, terminating in a bifurcated end in the vicinity of the slot 6c. This bifurcated end is supported by a cleat 18 inserted in the slot 6c, similar to the cleat 13 inserted in the slot 6d. In the centre of the bridge-like portion 12a a fixed contact 19 is fixed by rivetting, the contact 19 cooperating with the movable contact 17.

A further fixed contact 20 is fastened by rivetting to an arm 10a formed integrally with the terminal lug 10 and extending longitudinally within the housing 1 to a position opposite the movable contact 17. The two fixed contacts 19, 20 act effectively as stops limiting the angular movement of the movable contact assembly 15, 16 to an angular range which is less than half of the angular separation between the two stable positions of the bistable contact assembly 15, 16, that is, the angular movement which the said contact assembly 15, 16 would make if its movement were unobstructed. The position of the contacts 19, 20 is such that the contact assembly 15, 16 in moving between the fixed contacts 19, 20 does not pass through its overcentre or planar-symmetrical configuration, so that the contact assembly 15, 16 is always urged by the resilience of the snap action spring 16 in the same direction, in this example, towards the position shown in FIG. 1, in which the contact 17 engages the fixed contact 20. The switch therefore has a monostable action, even though the snap action spring 16 itself is a bistable element.

The base 6 has a hole 6g in which a tubular support 21 is inserted. The end of the tubular support 21 within the housing 1 supports the fixed contact 20. The distance of the projection of the support 21 into the interior of the housing 1 determines the position of the fixed contact 20 relative to the base 6 of the housing 1. As can be seen in FIG. 1, the contact 20 is accessible through the tubular support 21 for the purpose of, for example, triggering a switching cycle.

Instead of the tubular support 21, a screw-threaded socket may be screwed into the base 6 to support the fixed contact 20. The position of the fixed contact 20 would then be determined by screwing the socket into or out of the housing 1, deforming the arm 10a as shown in FIG. 1.

The electrical switch is operated by a plunger 25, shown in ghost outline in FIG. 1, forming part of the temperature regulator (not shown). When the temperature-sensing means of the temperature regulator senses a predetermined temperature, the plunger 25 is displaced downwards, as viewed in FIG. 1, to such a position that the trip tongue 16e is deflected sufficiently to trip the switch contact assembly 15, 16 upwards, away from the contact 20 and its engagement with the fixed contact 19. The circuit between the terminal lugs 10 and 11 is therefore interrupted, while a circuit is established between the terminal lugs 11 and 12.

The plunger 25 has a shoulder 25a which abuts the support plate 11b upon depression of the plunger to limit the stroke of the plunger 25. This avoids `overtravel` of the switch, preventing excessive bending of the trip tongue 16e which could damage it.

According to the circuit in which the switch is connected, it can be employed as a normally-closed or normally-open switch, or as a selector switch. In any case the current passes predominantly through the contact carrying arm 15 which consists of a highly conductive material (copper or copper alloy), very little current in practice passing through the snap action spring 16, which may be made of a material of poor conducting properties such as stainless steel.

Although two elements 15, 16 form the movable contact assembly, they occupy in practice a space no bigger than that occupied by a single element, since both elements 15, 16 are substantially flat and lie substantially parallel to one another. This enables an electrical switch of very small overall size to have a high current rating.