Electricity: electrothermally or thermally actuated switches – Thermally actuated switches – With bimetallic element
Patent
1993-10-12
1995-06-27
Donovan, Lincoln
Electricity: electrothermally or thermally actuated switches
Thermally actuated switches
With bimetallic element
337380, 337 89, H01H 3754
Patent
active
054283361
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention concerns improvements relating to electric switches and more particularly concerns thermally responsive electrical switches employing bimetallic elements as thermal actuators.
BACKGROUND OF THE INVENTION
Many kinds of electrical switches employing bimetallic actuators are known and likewise many different forms of bimetallic switch actuators are known. Early bimetallic switches simply employed a plain bimetal blade which moved relatively slowly in response to temperature changes and gave rise to arcing problems in the switch, and the development of the snap-acting bimetallic actuator, constructed as a dished bimetallic element capable of moving between oppositely curved configurations with a snap-action, provided a major advance in the art. Various forms of snap-acting bimetallic actuators are known, such as those disclosed in GB 600055, GB 657434, GB 1064643, GB 1542252 and GB 2124429 for example, and various forms of electric switches employing such bimetallic actuators are likewise known; GB 2124429 abovementioned for example discloses the utilization of a pear-shaped snap-acting bimetallic actuator in a current-sensitive switch where the heating of the bimetal by electric current flow therethrough is designed to trip the switch in a current overload situation.
OBJECTS AND SUMMARY OF THE INVENTION
The object of the present invention is to provide a thermally responsive switch incorporating a bimetallic switch actuator and capable of response to very small through currents, the switch being of inexpensive and uncomplicated construction.
According to the present invention there is provided a thermally-responsive switch comprising a moulded plastics body portion capturing therein first and second terminal conductors, and a snap-acting bimetallic actuator secured to one of said conductors and carrying a contact which constitutes the moving contact of the switch and is arranged for co-operation in switching operations with the other of the two conductors, characterized in that the moulded plastics body portion of the switch defines a closed chamber which accommodates the bimetallic actuator, the first and second terminal conductors are moulded into the body portion of the switch at spaced-apart locations so as to have exposed portions spaced-apart from each other within said chamber and externally of the body portion, the bimetallic actuator is directly welded to the exposed portion of said one of said conductors within the chamber, and the contact carried by the bimetallic actuator co-operates directly with the exposed portion of the other of the two conductors within the chamber.
The bimetallic actuator of the switch is advantageously of such low thickness as to be responsive to through current as low as 2 amps or less without requiring provision of a supplementary heat source, for example a bimetallic element of the order of 0,076 mm (0.003 inch) thickness, and the switch body part preferably provides physical support for the bimetallic element during switching operations.
In an exemplary embodiment which will be described in detail hereinafter the bimetallic actuator is of a kind having a generally U-shaped cut-out defining a tongue between spaced apart leg portions which are bridged adjacent the free end of the tongue. The terminal conductors are formed as simple wires and the tongue of the bimetal is secured to one of the terminal conductors by welding, and the bridging portion carries the contact which co-operates with the other conductor. No discrete contact is provided on the other conductor which however comprises a silver or silver alloy coating, for example a silver antimony coating as described in PCT/GB 92/00185. The moulded plastics body portion of the switch defines an enclosure for the bimetallic actuator and, as will hereinafter be described, also incorporates portions which provide structural support for the bimetal. The resultant switch is of simple and easily manufactured construction which enables small size to be achieved for enhanced sensit
REFERENCES:
patent: 4088976 (1978-05-01), Bell
patent: 4507642 (1985-03-01), Blackburn
patent: 4902999 (1990-02-01), Ball, Jr. et al.
patent: 5014034 (1991-05-01), Wehl
Florentine Andrew V.
Smith David A.
Donovan Lincoln
Otter Controls Limited
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