Electricity: electrothermally or thermally actuated switches – Electrothermally actuated switches – With longitudinally expansible solid element
Patent
1993-10-19
1995-08-01
Donovan, Lincoln
Electricity: electrothermally or thermally actuated switches
Electrothermally actuated switches
With longitudinally expansible solid element
337395, 361103, H01H 6106
Patent
active
054383099
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to circuit protection devices that limit or shut off current flow in conditions of over-current and/or over-temperature.
DESCRIPTION OF RELATED TECHNOLOGY
Raychem Corporation, Menlo Park, Calif. markets a circuit protection device called a "polyswitch." Raychem's "polyswitch" includes a polymeric material loaded with conductive material, such as carbon particles, which is normally conductive. If the current load increases beyond a predetermined value, the polymer heats up and expands with the result that the conductive particles are separated enough to prevent flow of current through the polymer. A problem with this polymeric type device is that it has an undesirable slow response time due to low thermal conductivity of the polymeric materials. Accordingly, there is a need in the art for a device which quickly changes from a low resistance to a high resistance when an over-current or an over-temperature condition exists.
Another problem with the polymeric device is internal "arcing" which occurs when the current flow is interrupted between adjacent particles. This internal "arcing" leads to breakdown of the polymer and hence limits the upper voltage which can be applied to the device. Accordingly, there is a need in the art for a more reliable switch capable of performing under higher voltage and current conditions.
Another inherent problem of polymeric devices is that the conductivity is relatively low, even in its most conductive state. As a result, high current devices are undesirably large in size when low resistance levels are required.
Ceramic PTC (positive temperature coefficient) devices based on barium titanate perform very similarly to polymeric devices and also display catastrophic breakdown when exposed to elevated voltage and/or current conditions.
Various types of mechanical switching arrangements are known in the art. For instance, U.S. Pat. No. 3,544,943 ("Hoagland") discloses an over-current responsive device which includes a pair of terminals electrically connected together by a thermally responsive element. The thermally responsive element includes two elongated cantilevered members supported at one end to a pair of posts. The posts are electrically connected to the terminals. The first elongated member is electrically insulated from the posts. One end of the second elongated member is welded to a free end of the first member. The second member is also bifurcated into two arms, one arm being electrically connected to one post and the other arm being electrically connected to the other post. Current flows from one terminal, along one arm, then along the other arm to the other terminal. The size, shape, and/or materials of the first and second members are chosen such that the second member is heated and the two members swing in one direction to activate a snap-action switch under overload conditions.
Shape memory alloys have been used in electrical connectors. For instance, U.S. Pat. No. 4,621,882 ("Krumme '882") discloses an electrical connector wherein a first strip which terminates in a split tube is removably connected to a second strip. The split tube includes a shape memory alloy layer which opens or closes the tube. For instance, the tube can include a metal layer which acts as a spring to close the tube when the shape memory layer is in its ductile and soft martensitic state, and the shape memory layer changes shape and overpowers the force of the metal layer when the shape memory layer is heated to its austenitic state. The tube can include a flexible heater for heating the shape memory layer.
U.S. Pat. No. 4,643,500 ("Krumme '500") discloses a multi-contact zero insertion force electrical connector. In a first embodiment, the connector includes a pair of flexible spaced-apart sidewalls, slides having camming surfaces extend along inner surfaces of the sidewalls, pairs of spaced-apart contacts are provided between the sidewalls, upper ends of the contacts are attached to the respective sidewalls by extensions on the sidewalls, and
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