Abstract:
A glass or glass ceramic cooking top  1  has on its underside  6  at least one heating unit  7 . The heating unit  7  is in flat, heat-conducting contact with the underside  6 , and contains an electrical resistance heating element exhibiting PTC behavior, the temperature in zone  3  being limited to a desired value without the use of a temperature limiter.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to a glass or glass ceramic cooking top having at least one zone to be heated, in particular a cooking, grilling, or warming zone, and an electrical heating unit provided on the underside of the zone.  
         BACKGROUND OF THE INVENTION  
         [0002]    Such apparatus as described above are known commercially. The flat, smooth, pore-free surface of the glass or glass ceramic cooking top allows for easy cleaning. The cooking, grilling, or warming zones according to the prior art are heated by radiant heating elements having spirally wound resistance wires, or tubular heating elements, heating films, or halogen lamps. A temperature limiter is provided to protect the glass or glass ceramic cooking top from overheating. A cutoff temperature of 560° C. to 600° C. is selected for cooking operations. For a warming zone, a cutoff temperature of approximately 100° C. to 150° C. is selected.  
           [0003]    Temperature limiters are expensive, with costs in the same range as the heating unit. To avoid temperature limiters, the specific heating capacity of the heating unit could be designed low enough so that overheating of the glass or glass ceramic cooking top would be prevented under all operating conditions. However, this would have the disadvantage of a very lengthy heat-up time, as well as considerable thermal inertia in supplying heat to a cold cooking utensil.  
           [0004]    U.S. Pat. No. 5,220,155 describes a heating unit for a glass ceramic cooking top in which heating elements exhibiting PTC behavior are provided on a heat sink disk. The heat sink disk is made of an electrically insulating material with good thermal conductivity, such as aluminum nitrite. A disk made of such a material is costly.  
         OBJECTS AND SUMMARY OF THE INVENTION  
         [0005]    The object of the present invention is to provide a glass or glass ceramic cooking top of the aforementioned type, having a heating unit which operates in an intrinsically safe manner by virtue of a simple design and which in spite of a short heat-up time does not result in overheating of the glass or glass ceramic cooking top, so that a temperature limiter is unnecessary.  
           [0006]    The above-referenced object is achieved by the present invention The PTC (positive temperature coefficient) behavior of the resistance heating element causes the affected zone of the glass or glass ceramic cooking top to first heat up quickly, after which the electrical resistance of the heating element becomes so high that no significant further heating of the affected zone occurs. The glass or glass ceramic cooking top is thus protected from overheating without a temperature limiter being necessary.  
           [0007]    The described design is economical and robust. The geometry of such a heating unit may be easily adapted to different shapes of the affected zone.  
           [0008]    The heating unit is preferably pressed onto the underside by spring elements or adhesively bonded to the underside. A layer of heat sink paste may be provided to improve the heat transfer from the heating unit to the zone.  
           [0009]    The heating unit may be formed from a flat, metallic, heat-conducting support element which is adapted to the shape of the zone and lies flat against the underside of the zone. The heating element(s) are positioned on or in the support element.  
           [0010]    It is also possible to apply the heating element to the underside of the zone using thick-film technology.  
           [0011]    Further advantageous embodiments arise from the following description and the drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0012]    [0012]FIG. 1 shows a top view of a glass or glass ceramic cooking top having four cooking zones and one rectangular warming zone;  
         [0013]    [0013]FIG. 2 shows a top view of a glass or glass ceramic cooking top having four cooking zones and one circular warming zone;  
         [0014]    [0014]FIG. 3 shows a top view of a glass or glass ceramic cooking top having four gas cooking zones and one electrical warming zone;  
         [0015]    [0015]FIG. 4 shows a perspective view of a tabletop unit having a warming zone;  
         [0016]    [0016]FIGS. 5 through 8 show sections of a glass or glass ceramic cooking top with a heating unit;  
         [0017]    [0017]FIG. 9 shows a perspective view of the heating unit, showing a) a support element, b) an insulation element, and c) the heating unit; and  
         [0018]    [0018]FIG. 10 shows two side-by-side heating units according to FIGS. 1 through 4 for heating a warming zone. 
     
    
     DETAILED DESCRIPTION  
       [0019]    A glass or glass ceramic cooking top  1  for a cooking range has four cooking zones  2  and one warming zone  3  (see FIGS. 1, 2, and  3 ). Warming zone  3  can be provided at various locations on glass or glass ceramic cooking top  1 . In FIG. 1, the warming zone is provided in the edge region of glass or glass ceramic cooking top  1 , and in FIGS. 2 and 3 it is located between cooking zones  2 . Warming zone  3  may have various shapes; in FIGS. 1 and 3 it is rectangular, and in FIG. 2 it is circular.  
         [0020]    Cooking zones  2  in FIGS. 1 and 2 are heated by electrical radiation heating elements known per se. However, these cooking zones may also be heated by heating units, which are as described below for warming zone  3 .  
         [0021]    Cooking zones  2  may also be heated by gas in a manner known per se (see FIG. 3).  
         [0022]    [0022]FIG. 4 shows a table-mountable portable unit  4  having a housing  5  which supports a glass or glass ceramic cooking top  1  on which a rectangular warming zone  3  is provided. This zone may also be designed as a grilling zone upon which foods to be grilled are directly placed.  
         [0023]    [0023]FIGS. 1 through 4 show embodiments by way of example only. The heating unit described below for a warming zone  3  may also be used in cooking zones or grilling zones of various shapes, such as circular, square, or oval, with various unit designs such as built-in cooking tops, tabletop cooking units, free-standing stoves, outdoor cooking units, or camping units.  
         [0024]    A heating unit  7  is provided on underside  6  of glass or glass ceramic cooking top  1  in the region of warming zone  3  or in zones used for other purposes. This heating unit has a surface region  8  which extends on underside  6 , essentially over the entire area of warming zone  3 . This extension may also be accomplished by positioning two or more heating units  7  side by side on underside  6  (see FIG. 10). Surface region  8  is in flat, heat-conducting contact with underside  6  in the region of cooking zone  2  in order to achieve good heat transfer from heating unit  7  to warming zone  3 .  
         [0025]    In the embodiment according to FIG. 5, surface region  8  is pressed tightly against underside  6  by spring elements, in particular a plurality of pressure springs  9  situated in the edge region. Pressure springs  9  rest on a base part or a fastening crossmember (not shown) in the housing which supports glass or glass ceramic cooking top  1 .  
         [0026]    In the embodiment according to FIG. 6, a layer  10  of heat sink paste is also provided between the bottom  6  and the heater unit  7  to improve the heat transfer compared to FIG. 5.  
         [0027]    In the embodiment according to FIG. 7 spring elements are omitted, and surface region  8  is instead adhesively bonded at certain points  11  to underside  6 . A layer  10  of heat sink paste is provided here as well. Layer  10  of heat sink paste is particularly advantageous when underside  6  has a textured, for example napped, surface.  
         [0028]    In the exemplary embodiment according to FIG. 8, the entire area of surface region  8  of heating unit  7  is glued to underside  6  with an adhesive layer  12 , compared to adhesive bond  11  which is achieved by glued points or glued seams as shown in  
         [0029]    [0029]FIG. 7. FIG. 9 shows heating unit  7 . A shaft  13  is provided on surface region  8  (see FIG. 9 a ). Surface region  8  and shaft  13  are produced from a continuous-cast aluminum profile, for example. Shaft  13  runs approximately along the lengthwise centerline of surface region  8 . An electrically insulating insulator  14  (see FIG. 9 b ), which may be designed to have two or more layers, is to be inserted into shaft  13 . Insulator  14  forms a receiving space  15  into which resistance heating element  18 , which is electrically contacted using electrodes flat on both sides, is to be inserted. Heating element  18  is rod-shaped.  
         [0030]    Heating element  18  has a positive temperature coefficient of electrical resistance. Its electrical resistance therefore increases with increasing temperature. Such PTC heating elements are known, and are usually made of doped polycrystalline ceramic using barium titanate, for example, as base material. Electrodes  16 ,  17  are used to conduct current as well as to transfer heat from heating element  18  to surface region  8  via insulator  14  and shaft  13 .  
         [0031]    Heating element  18  has a characteristic curve which represents the electrical resistance of the heating element as a function of temperature (PTC characteristic curve or RT characteristic curve). A heating element  18  is used in which the operating range lies predominantly in the low-impedance portion of the characteristic curve. The selection is made according to the desired function of the heating unit for the warming zone or for a cooking zone. At an initial low temperature, heating occurs with high heat output due to the correspondingly low resistance. As the temperature increases, the heat output diminishes in accordance with the characteristic curve, as a result of which no further heating of zone  3  occurs above a temperature specified by the choice of the characteristic curve. Heating unit  7  has self-regulating characteristics with regard to the temperature of zone  3 . When a cold cooking utensil or cooking material is placed on zone  3 , which has been heated to its final temperature, the temperature of heating element  18  is reduced on account of the heat conduction, so that the heat output of the heating element again increases. Altogether, a sensitive regulation and limitation of the temperature in zone  3  is thus achieved, although glass or glass ceramic cooking top  1  itself has poor heat conduction properties.  
         [0032]    The heat-up speed of zone  3  may be increased by the invention, using two or more PTC heating elements. FIG. 10 shows, similarly to FIG. 9, two heating units  7   a  and  7   b  situated side by side which cover zone  3  with their combined surface region  8 . A heating element  18   a ,  18   b  is situated in shaft  13   a ,  13   b , respectively. The number of PTC heating elements  7   a, b  can be further increased to achieve a higher output.  
         [0033]    In another embodiment it is possible to apply heating unit  7  to underside  6  of zone  3  using thick-film technology. Heating unit  7  has an electrical resistance layer, exhibiting the described PTC behavior, situated between two electrically conductive electrode layers. The one electrode layer may be applied directly to underside  6 .  
         [0034]    This is particularly the case when the glass or glass ceramic cooking top produces sufficient electrical insulation in the temperature range of interest, for example in a warming zone. However, if zone  3  is provided as a cooking zone with higher temperatures than a warming zone, an electrically insulating intermediate layer can be provided between the electrode layer and underside  6  in order to ensure electrical insulation, even at temperatures at which the electrical conductivity of the glass or glass ceramic cooking top increases.