The invention relates to an electric heater, expecially for small electric appliances, such as hair roller heaters, egg boilers, or the like, with at least one flat, square heating element with contact-making zones on opposite surfaces, as well as with electrical connection elements in the form of essentially planar contact plates placed on contact-making zones of opposite sides of the heating elements.
In electric warmup or heating appliances, especially small electric devices such as hair roller heaters or the like, it is desirable to provide a satisfactory and uniform heat transfer from the heating elements to the regions to be heated over a relatively large area. Therefore, flat, elongated heaters are utilized. The heating elements employed usually are to an increasingly greater extent so-called PTC elements, i.e. heating elements of PTC material or heat conductors having a positive temperature gradient. Such PTC elements customarily exhibit a prismatic shape with two opposed planar-parallel surfaces and a round or polygonal outline and are contructed, in particular, as square plates. Such PTC elements ordinarily consist of a ceramic material, especially on the basis of barium titanate, and have the property that they are so to speak self-stabilizing with respect to their rate of electric power input, because the electric resistance increases greatly within certain temperature ranges so that the conductivity and the heat transmission of the heating element are constantly self-regulated.
DOS No. 2,845,965 (corresponding to U.S. Pat. No. 4,327,282) discloses such a heater wherein the contact plates, resting on both sides on the resistance heating elements, are held together elastically along the edge zone. This design per se is satisfactory, but cannot be utilized with equally good effects in every case of application, for example in those instances where heat transfer to another body is to be carried out by direct physical contact with the heater. This is so because the clamping members on the edge side necessitate a certain spacing between the heated surface of the heater and a body contacted by the heater, unless the body to be heated up is fashioned in a suitable manner, which is not always possible.
Therefore, an object of the invention is to solve the problem of providing a technically simple and thus inexpensive heater exhibiting flat heat-transfer surfaces which can be placed in a simple and practical way against a body to be heated.
This problem has been solved according to a preferred embodiment of the invention in a heater of the type mentioned in the foregoing by the feature that the heating elements and the contact plates resting on both sides are clamped fast between the legs of a holding member bent into a U-shape and provided on the inside with an insulating layer, this holding member extending over the heating elements and contact plates along their entire widths. By this construction, the heating elements and the contact plates are, first of all, reliably and securely held in place in a simple way. Moreover, the heater of this invention ensures the presence of a flat and completely planar contact surface via which the heat generated by the heating elements can be further transferred uniformly in the entire length of the heater. The heater of the present invention also exhibits the advantage that the heat output can be extensively selected arbitrarily in a suitable way by inserting and retaining a larger or smaller number of heating elements in the holding member between the contact plates.
The provision is made as a preferred feature that the insulating layer is a heat-conductive, electrically insulating coating of the holding member, the coating being resilient in a certain way so that excessively strong forces during compression of the holding member into its U-shaped form, wherein it holds the contact plates and the heating elements together, are not transmitted to the heating elements. The heat-conductive, electrically insulating coating consists preferably of silicone rubber rendered heat-conductive by appropriate additives, such as, for example, magnesium oxide or the like. This is of advantage, in particular, if the provision is to be made in a further preferred embodiment that the space between the U-legs at the ends of the holding member is sealed by an insulating material and/or that the holding member is sealed off in the area of the ends of the U-legs over its entire length by insulating material. If the entire heater is sealed off at the open sides of the U-shaped holding member, the heater of this invention can also be utilized in a moist or wet environment, which is not readily possible in case of the heater according to DOS No. 2,845,965. The latter would first have to be separately sealed off completely.
A further feature of the preferred embodiment is that provision is made for the contact plates to be equipped with contact reeds [tongues] formed integrally therewith, which reeds are laterally extended from the holding member, the contact reeds being fashioned especially in the form of plug-in elements. In this case, one plug-in element can be constructed as a flat plug and the other as a flat plug socket or receptacle. Preferably, the contact reeds are furthermore provided with insulating sleeves placed thereon in the zone where they are extended out of the U-shaped holding member. These insulating sleeves consist, for example, of shrunk-on film [sheeting] insulating the contact reeds reliably against each other as well as with respect to the U-shaped holding member; this is important if the holding member consists of a conductive material, e.g. aluminum sheet.
These and further objects, features and advantages of the present invention will become obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, a single embodiment in accordance with the present invention.