Abstract:
A mirror heater which is suitable for bathrooms and humid spaces includes electrically insulating layers which are partially provided by double-sided adhesive tapes. The double-sided adhesive tapes fix the various layers to one another and provide an insulative layer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a resistance heating device for flat objects such as mirrors. 
     2. Discussion of the Background 
     Mirror surfaces often steam up following showering or bathing which prevents the mirrors from fulfilling their function. In order to keep a mirror from steaming up it is known to heat the mirror surface from the rear with heating layers so that the surface does not steam up, even in high air humidity. However, these heating devices have been restricted in countries that impose high safety requirements. For mirror heaters, this may mean that the power-carrying elements, i.e., the heating elements, must be safeguarded by insulating layers which are separated from one another. For example, embedding a serpentine resistance heater between three insulating layers separated from one another on both sides leads to a comparatively thick construction, which in turn results in less efficient heating. The large overall height and the loss of efficiency have kept flat heating devices of this type from becoming widespread, particularly for mirrors. 
     SUMMARY OF THE INVENTION 
     Accordingly, one object of this invention is to provide a resistance heating device which has a low overall height and satisfies the safety requirements of the German Technical Inspection Authority (TUV). 
     A plurality of double-sided adhesive tapes and an intermediate layer made of insulating material arranged between two adhesive layers provides an insulation layer with a high breakdown voltage. The overall height increase is small since the double-sided adhesive tapes perform both a retaining or adhesive function and an insulative function. The physical integration of adhesion and insulation results in a significantly lower overall height while achieving an adequate transfer of heat to the flat object such as a bathroom mirror. At the same time, the safety requirements of the TUV are fulfilled. 
     According to an advantageous feature of the invention, the heating layer can be connected directly to the mains or to a power source by feed lines. This means that the resistance heater is designed in accordance with the available operating voltage, e.g., 230 volts, 12 volts, or 4 volts. Thus, the resistance heater can be connected directly to the power source and furnish the desired heating power. Since regulating devices of any kind are not necessary, a cost-effective design is achieved. 
     According to an advantageous feature of the invention, an ON/OFF switch is provided in the feed line because the air humidity and the temperature conditions in the bathroom are not always such that they cause the mirror to steam up. The mirror heater can therefore be switched on as required. 
     According to another advantageous feature of the invention, temperature and/or moisture sensors are provided. The sensors can cause the mirror heaters to be switched on or off when the temperature or moisture exceeds or falls below appropriate threshold values. The temperature and moisture sensors can be arranged directly in the mirror surface, on the mirror surface, or close to the mirror. 
     According to one particularly advantageous embodiment of the invention, the second adhesive layer of the second double-sided adhesive tape is covered with a protective film that can be pulled off, permitting the heating device to be packed, despatched, and handled in any manner before being fitted to the mirror. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
     FIG. 1 shows a sectional illustration through the layers of an exemplary embodiment of the invention; 
     FIG. 2 shows a top view of the resistance heater in the case of a round mirror; and 
     FIG. 3 shows a top view of the resistance heater in the case of a rectangular mirror. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, and more particularly to FIG. 1, thereof, an electrically insulating substrate 1 has a first main surface 2 and a second main surface 3. A flat resistance heating layer 4 made of a material with a known electrical resistance is applied to the first main surface 2 of the substrate 1. The resistance heating layer 4 is covered by a first double-sided adhesive tape 6 with first and second adhesive layers 7 and 8, respectively, and an insulating intermediate layer 9. In this case, the first adhesive layer 7 contacts the resistance heating layer 4. The second adhesive layer 8 of the first double-sided adhesive tape 6 is connected to one side of a first insulation layer 12. The other side of the insulation layer 12 is connected to a double-sided adhesive tape 14 with first and second adhesive layers 15 and 16, respectively, and an insulating intermediate layer 17. In this case, the first adhesive layer 15 contacts the first insulation layer 12. A protective film 18 that can be pulled off is arranged on the second adhesive layer 16 of the second double-sided adhesive tape 14. The second main surface 3 of the electrically insulating substrate 1 is connected to a third double-sided adhesive tape 20, which has first and second adhesive layers 21 and 22, respectively, and an insulating intermediate layer 23. In this case, the first adhesive layer 21 is connected to the second main surface 3 and the second adhesive layer 22 is connected to a second insulation layer 25. Electric connection points 26 and 27 penetrate the second insulation layer 25, the third double-sided adhesive tape 20 and the substrate 1 from the second insulation layer 25, and make contact with corresponding connections on the resistance heating layer 4. 
     The resistance heater according to the invention and according to FIG. 1 can be connected to the object or mirror to be heated immediately following its production, with the result that a mirror with resistance heating is delivered as a single package to the market. When the protective film 18 is used, existing mirrors can be equipped with resistance heaters. Since the appropriate TUV regulations are satisfied by the triple insulation layer on both sides, the resistance heater according to the invention can also be sold in Do-It-Yourself stores to laymen, i.e., directly to the final user. The resistance heater according to the invention is preferably constructed so that it can be connected directly to the mains in the household via silicone-insulated feed lines. In this case the connection can be made through a distribution box or by a standard plug. 
     FIG. 2 shows a round resistance heater according to the invention wherein the heating elements of the resistance heating layer 4 are spiral. FIG. 3 shows a rectangular resistance heater for rectangular mirrors. In this case, the heating elements of the resistance heater 4 are arranged in serpentine fashion. The serpentine configuration of the resistance heater 4 may also be applied to round mirrors and mirrors of other shapes, and the spiral configuration may also be applied to rectangular mirrors and mirrors of other shapes. 
     The adhesive layers of the double-sided adhesive tapes 6, 14, and 20 are preferably acrylate adhesives, synthetic rubber, or natural rubber. The electrically insulating substrate 1, the first and second insulation layers 12 and 25, and the intermediate layers 9, 17, and 23 of the double-sided adhesive tapes 6, 14, and 20, are flexible plastic films having a thickness in the range from 0.008 mm. to 0.09 mm. The electric breakdown strength of the individual insulating layers is preferably at least 2000 volts. The resistance heating layer 4 consists of an electrically conductive material having a specific electrical resistance. For a given supply voltage, the necessary heating power is achieved by adjusting the dimensions of the heating elements. The material of the heating elements may consist of, for example, copper, silver, aluminum or other conductive alloys and substances. The heating elements may also be PTC elements, i.e., elements made of a material having a positive resistance/temperature coefficient. This provides additional safety since PTC elements are self-regulating. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.