Electricity: electrothermally or thermally actuated switches – Electrothermally actuated switches – With longitudinally expansible solid element
Reexamination Certificate
2000-04-25
2001-10-09
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
06300858
ABSTRACT:
BACKGROUND OF THE INVENTION
This application relates to the art of switches and more particularly, to thermal switches that automatically move from one position to another in response to an elevated temperature. The invention is particularly applicable to a normally closed switch that moves to an open position for preventing thermal runaway 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 normally open switches as well as for other purposes.
Overcharging batteries of certain types may cause thermal runaway that raises the internal battery temperature. The internal pressure within the sealed battery case increases with increasing temperature and an explosion may occur when the internal pressure exceeds the bursting strength of the battery case. A battery explosion releases by-products that are caustic, toxic and flammable, and the force of the explosion may cause injury to persons and damage to property.
Thermal protectors of various types have been proposed and used in batteries for interrupting the battery circuit when a predetermined temperature is exceeded. The space available for small rechargeable batteries in modern electronic devices such as cellular telephones requires miniature temperature protection devices. Miniaturization is limited in thermal protectors that use bimetals or thermistors having a positive temperature coefficient of resistance because such devices do not operate properly or reliably if they are made too small.
Arrangements have been proposed for incorporating a thermal switch actuator of shape memory alloy into the physical structure of a battery internally of the battery case. Such arrangements cannot be used with existing battery designs and it would be desirable to have a self-contained thermal switch assembly with a current carrying actuator of shape memory alloy that could be used for a variety of purposes including external attachment to a battery case.
SUMMARY
A normally closed miniature thermal switch has a current carrying switch blade actuator of shape memory metal that automatically moves to an open position in response to an elevated temperature or an overcurrent condition.
The switch blade is on a switch blade member that is sandwiched with a dielectric spacer between a pair of substantially flat metal plates with the switch blade projecting into a cavity between the plates that is formed by an opening in the spacer. A mounting portion of the switch blade member engages one of the plates and the outer end of the switch blade engages the other plate. The switch blade responds to an elevated temperature to move its outer end away from the other plate and open the circuit.
The switch blade preferably remains in its open position after cooling so that it functions as a one-shot fuse that cannot be reset.
In a preferred arrangement, the entire outer surfaces of the metal parts of the switch assembly are coated with a precious metal. The precious metal coating preferably is on the outer peripheral surfaces of the metal parts as well as the opposite surfaces thereof.
The entire switch assembly may be encapsulated or wrapped in an electrical insulating material with electrical terminals on the switch assembly projecting through the insulating material.
The current carrying switch blade actuator of shape memory metal has an outer end that engages an inner surface of a metal plate with substantially line contact in the switch closed position.
The thermal switch in accordance with the present application preferably has a resistance that is less than 20 milliohms, more preferably less than 10 milliohms and most preferably not more than about five milliohms.
In a preferred arrangement, the one-piece dielectric spacer has integral fastener projections that cooperate with the metal plates to hold the switch assembly together.
In one arrangement, the fastener projections on the dielectric spacer extend through holes in the metal plates and are deformed on the opposite sides of the metal plates from the spacer to complete the assembly.
In accordance with another aspect of the application, the dielectric spacer has at least one reduced thickness end portion defining at least one notch in which a mounting portion of the switch blade member is received. The depth of the notch preferably is not greater than the thickness of the mounting portion of the switch blade member and most preferably is slightly less than the thickness thereof. This insures good compressive engagement between the mounting portion of the switch blade member and the inner surface of a metal plate.
The switch blade member preferably has a thickness that is not greater than the thickness of the spacer, and the spacer has a thickness that is not greater than about 0.050 inches.
In a preferred arrangement, the current carrying switch blade actuator in its deformed martensitic state is under less than about 8% bending strain and most preferably not more than about 4%.
The current carrying switch blade actuator is placed in bending stress between the metal plates to insure good contact and provide low resistivity.
The metal plate of the switch assembly that engages the mounting portion of the switch blade member is attached to a battery terminal or a battery case externally of the battery case. Heat is conducted from the battery case or battery terminal to the switch blade member through the metal plate. Although the switch blade member may be designed to have many different operating temperatures, it may be designed to open at any temperature up to about 125° C. when used with conventional small rechargeable batteries, and most preferably at a temperature in the range of 65-75° C.
It a principal object of the present invention to provide an improved thermal switch assembly.
It is another object of the invention to provide a self-contained thermal switch assembly that uses a current carrying switch blade actuator of shape memory alloy.
It is a further object of the invention to provide a thermal switch assembly that can be attached externally of a battery for protecting the battery against thermal runaway.
It is an additional object of the invention to provide a miniature thermal switch assembly that has a current carrying switch blade actuator of shape memory metal and is relatively simple and economical to manufacture and assemble.
It also is an object of the invention to provide a current limiting switch assembly that has a current carrying actuator of shape memory metal.
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Kalapodis James B.
Polles Jerry L.
Quinn William F.
Harness & Dickey & Pierce P.L.C.
Picard Leo P.
Thermo-O-Disc Incorporated
Vortman Anatoly
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