Electrical resistors – With switch
Patent
1987-08-27
1989-07-11
Tolin, Gerald P.
Electrical resistors
With switch
200 83P, 337354, 337377, 361400, 338272, H05K 116
Patent
active
048475874
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a bimetal thermoswitch having the features recited in the prior art part of claim 1.
PRIOR ART
Such switch has been described in DE-29 16 516 A1 and consists of a thermoswitch which is mounted on a printed circuit board as a carrier. One or more heating resistors may be provided on the printed circuit board and may permit the thermoswitch to operate as a relay or as a time limit relay.
The known switch has the disadvantage that the printed circuit board tends to be distorted under thermal stress so that a satisfactory function of the thermoswitch is not ensured and at least its switching temperature may be changed.
DISCLOSURE OF THE INVENTION
It is an object of the invention to provide a bimetal thermoswitch which is of the kind described first hereinbefore and can be made as a very flat structure and will remain dimensionally stable even under different temperature stresses and can be used for and adapted to numerous applications.
That object is accomplished by a bimetal thermoswitch having the features recited in claim 1. Desirable further improvements of the invention are covered by the dependent claims.
The novel bimetal thermoswitch uses as a carrier a thin slab of alumina ceramics, which is also described as a wafer in semiconductor technology. The manufacture of such alumina ceramic slabs having an extremely high ultimate strength is known in the art. But in spite of their advantages, which will be described hereinafter, such slabs have not been used thus far as carriers for protective bimetal switches.
Owing to their strength, the alumina ceramic slabs used as carriers for the bimetal thermoswitches may be much thinner than the carriers of injection-molded plastic which have often been used thus far for small, open thermoswitches, and may also be thinner than printed circuit boards. As a result, the carrier used in accordance with the invention has only a relatively small heat capacity, which has a desirable influence on the response time of the switch. Such a carrier of alumina ceramics has a high thermal stability also and particularly when used in the form of a thin slab. It may be used up to much higher temperatures than plastic carriers or printed circuit boards and nevertheless will not be distorted. Another advantage resides in that it has a lower coefficient of expansion than plastics. Besides, a slab of alumina ceramics can inherently be made to have a higher dimensional stability than an injection molding of plastic so that the tolerance problems are alleviated in the manufacture of a bimetal switch having an alumina ceramic slab as a carrier.
Moreover, an alumina ceramic slab can be provided with conductor strips and with electric components, just as a printed circuit board, and resistors and other active or passive circuit components and even complete circuits can be integrated in the carrier slab so that the switch constitutes a compact and economical hybrid component.
An embodiment of the switch in accordance with the invention which is particularly simple but important for practical use is characterized in that the alumina ceramic carrier carries on one side those elements of the bimetal thermoswitch which are known in the art and is provided on the opposite side (on the underside) with a resistor layer so that the switch constitutes a thick-film resistor. If the resistor layer is arranged to shunt the switch, the latter can be used as an overtemperature switch which after a temperature rise above a predetermined value will not close automatically but will remain open because when the switch has opened the current will flow only through the resistor layer so that the latter is heated and generates and transmits to the bimetal element such a quantity of heat that said element is maintained above its switching temperature. When such a switch is designed in accordance with the invention, it will be much more compact and economical than the switch which has been described in German Patent Specification No. 32 31 136, which also remains ope
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Limitor AB
Tolin Gerald P.
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