Electricity: electrothermally or thermally actuated switches – Electrothermally actuated switches – With longitudinally expansible solid element
Reexamination Certificate
1998-11-02
2001-02-20
Picard, Leo P. (Department: 2835)
Electricity: electrothermally or thermally actuated switches
Electrothermally actuated switches
With longitudinally expansible solid element
C337S014000, C337S298000, C337S333000, C429S061000
Reexamination Certificate
active
06191679
ABSTRACT:
BACKGROUND OF THE INVENTION
This application relates to the art of thermal switches and, more particularly, to thermal switches that use a contact member of temperature sensitive shape memory alloy for making and breaking a circuit. The thermal switch of the present invention is particularly applicable for use in batteries and will be described with specific reference thereto. However, it will be appreciated that the invention has broader aspects and can be used in other devices.
Shape memory alloys can be used to replace bimetals in thermal switches. A switch contact arm of shape memory alloy has a deformed shape at normal temperatures and changes to a recovered shape at elevated temperatures. These changes in shape of a shape memory alloy contact arm provide different contact arm positions that can be used to open and close an electrical circuit.
Electrical switch components of shape memory alloy have been incorporated in batteries to open the battery circuit when the battery overheats. It is difficult to incorporate a switch component of shape memory alloy into the battery construction in a manner that provides reliable operation and protects the shape memory alloy from the corrosive battery environment. Therefore, it would be desirable to have a self-contained unitary sealed thermal switch assembly that can be used with batteries and other devices in a convenient and reliable manner.
Thermal switches for use in preventing thermal runaway in batteries must have a very low resistance so they will not be too wasteful of energy during normal charging and discharging of the battery. It is difficult to achieve low resistance with a current carrying actuator of shape memory metal. For protecting batteries against thermal runaway, it also is desirable that the protective switch function as a one-shot fuse that will not reclose once it has opened.
SUMMARY OF THE INVENTION
A self-contained sealed thermal switch assembly includes a contact member of shape memory alloy sandwiched between a pair of electrically conductive metal outer members. The contact member has a contact arm that completes or opens a current path between the pair of outer members by changing between a deformed shape at normal temperatures and a recovered shape at an elevated temperature.
In accordance with one important aspect of the present application, a switch with a current carrying actuator of shape memory metal that responds to an overcurrent or overtemperature condition has a resistance less than 20 milliohms, more preferably less than 12 milliohms and most preferably less than about 5-7 milliohms.
In accordance with another aspect of the application, a current carrying actuator of shape memory metal has a closed position engaging a fixed contact with a high contact force of at least 150 grams.
In accordance with another aspect of the application, a switch with the above characteristics has a continuous current rating of at least three amps at a temperature of 15-32° C.
In a preferred arrangement, the current carrying actuator of shape memory metal has an end edge that makes line contact with a fixed contact surface in the closed position of the switch.
In accordance with another aspect of the application, a switch having a current carrying actuator of shape memory metal operates as a one-shot fuse by moving to a switch open position at an elevated temperature and remaining open even though the temperature returns to normal.
In accordance with another arrangement, a switch having a current carrying actuator of shape memory metal includes an improved dielectric spacer sandwiched with an actuator mounting portion between a pair of metal plates. The spacer is a very hard electrical insulating material with high tensile strength, low water absorption and a high heat deflection temperature.
In a preferred arrangement for a one-shot switch, the current carrying actuator of shape memory metal is deformed to a strain that is less than 8% and most preferably not greater than about 4%.
In part, low resistance is obtained by coating all current carrying metal parts of the switch, including both outer and peripheral surfaces thereof, with a precious metal.
In one arrangement, the contact member of shape memory alloy is substantially E-shaped in plan view, and the elongated contact arm is defined by the center leg of the E.
In accordance with a preferred arrangement, the peripheral portion of the contact member is circumferentially interrupted so that its circumferential extent is substantially less than 360° but at least 270°.
An electrical insulator is positioned between a peripheral portion of the contact member and one of the outer members. The contact arm engages the one outer member when it is in its deformed shape at normal temperatures, and moves out of engagement with the one outer member when it is in its recovered shape at an elevated temperature.
In one arrangement, the pair of outer members have a cavity therebetween in which the elongated contact arm is located, and the one outer member has a depression therein facing the cavity.
The outer members have outwardly extending circumferential flanges surrounded by a sealing ring of electrical insulating material. A metal clamping ring surrounds the sealing ring and is crimped over the flanges on the outer members for holding same together while firmly compressing the sealing ring against the flanges to seal the cavity.
In one arrangement, the pair of outer members include a cap member and a base member. The base member is of aluminum and the cap member is of cold rolled steel that is plated with a precious metal.
It is a principal object of the present invention to provide a switch having a current carrying actuator of shape memory metal with a very low resistance.
It is a further object of the invention to provide such a switch that does not reclose once it has opened so that it operates as a one-shot fuse.
It is an additional object of the invention to provide such a switch with a dielectric spacer that is very hard, has a high tensile strength, low water absorption and a high heat deflection temperature.
It is another object of the present invention to provide a self-contained sealed thermal switch assembly that has a contact member of shape memory alloy.
It is also an object of the invention to provide a unitary thermal switch assembly that can be attached to batteries and other devices.
It is a further object of the invention to provide a thermal switch assembly of the type described that is reliable in operation and relatively easy to manufacture and assemble.
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Kalapodis James B.
Quinn William F.
Picard Leo P.
Thermo-O-Disc Incorporated
Vortman Anatoly
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