1. Field of the Invention
The present invention concerns an apparatus for protecting an electrical device from overtemperature, having a temperature-dependent switch with a housing on whose base a first connected contact is provided, a second connecting contact provided on the housing, and a holder which carries the housing and has an external terminal connected to the second connecting contact.
2. Related Prior Art
An apparatus of the kind mentioned above is known from DE 38 17 080 C2.
The known apparatus comprises a holder made of plastic, in which a temperature-dependent switch with a two-part metal housing is arranged. The connecting contacts of the temperature-dependent switch are constituted on the one hand by the metal cover and on the other hand by the metal lower part, a bimetallic switching mechanism (known e.g. from DE-A-21 21 802) being arranged in the interior of the switch.
The known apparatus is slid onto connector lugs of an electric motor and itself has a connector lug which is accessible from outside.
The connector lug of the holder is connected to a thermal contact plate which, in the case of an apparatus placed on the electric motor, rests directly on the energizing winding. In the interior of the holder, the metal housing rests with its base on the contact plate, so that there is thermal contact with the electric motor, and electrical contact with the connector lug of the holder.
Pressing on the metal cover is a connecting spring which is joined to a connector lug of the electric motor but is no longer accessible from outside once the apparatus has been put in place. The second connector lug of the electric motor, and the connector lug of the holder, serve as external terminals of the electric motor equipped with the known apparatus.
With the arrangement described, the temperature-dependent switch is arranged in series with the electric motor between the two external terminals. If the temperature of the energizing winding increases above a defined value, the bimetallic switching mechanism then opens the electrical contact between the metal cover and metal lower part, so that an electrical voltage applied to the external terminals can no longer power the electric motor. This prevents the electric motor from heating up, during operation, above a defined temperature; i.e. it is protected from overtemperature.
For many applications the known apparatus is not suitable, since the holder used therein is on the one hand much too bulky to be mounted on small electrical household appliances, for example a coffee machine, an electric hotpot, or electrical hotplates. A further disadvantage of the known apparatus lies in the fact that it cannot track rapid temperature changes without delay, since heat is dissipated to the connector lugs via the metal cover and the metal lower part.
Rapid reaction to a temperature rise is, however, required specifically in the case of electric hotpots or hotplates, for example in order to prevent the hotpot, or a pot sitting on the hotplate, from boiling dry. Protection from boiling dry cannot be achieved by monitoring power consumption, since the amperage remains relatively constant regardless of whether or not water is still present; the temperature of the electrical device to be protected simply rises, rapidly and steeply, when the water has boiled off.
Apparatuses which protect household appliances from overtemperature are subject in general to particularly stringent demands in terms of electrical, mechanical, and thermal reliability, which are not met by the known apparatus.