Abstract:
A status indicator ( 4 ) for a transparent ceramic glass cook top is disclosed which is able to display a range of colors. Typically such indications are used with red or smoked glass cooktops and are limited to red. The present invention is positioned directly underneath the transparent glass ( 22 ) which has an opaque layer ( 23 ) on the underside and wherein a portion ( 24 ) of the opaque layer has been removed thereby allowing the light source ( 2 ) to be visible directly above said cook top. The indicator may be used for a range of functions, including indicating the element is on and also that the heating region is too hot to be safely touched.

Description:
FIELD OF THE INVENTION 
     This invention relates to the use of an element status indicator for a ceramic glass cook top. 
     DESCRIPTION OF RELATED ART 
     Conventional ceramic glass cook tops generally employ either a dark red or brown ceramic glass top. With red coloured ceramic glass only red light will be permitted to pass. This has meant that in conventional cook tops generally red neon lamps have been used underneath the sheet of glass as indicators for a number of conditions. 
     In particular it has proven useful to provide indication of whether it is safe to touch the surface of the cook top. The generally accepted “safe” temperature is approximately 50-60° C., above which any such indicator would be lit. In some cases however because all the indicators (for various different conditions eg: dual element) will be red it can be somewhat confusing as to what each indication relates to at a quick glance. In the worst case this may lead to inadvertently placing ones hand or other inappropriate objects onto the cook top when it is hot. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an element status indicator for a ceramic glass cook top which goes some way to overcoming the abovementioned disadvantages or at least provides the public with a useful choice. 
     In a first aspect the present invention may broadly be said to consist in a cooktop including at least one heating means beneath a substantially colourless and transparent ceramic glass surface with an opaque layer on sections of the underside thereof and a status indicator, said status indicator comprising: 
     indication means positioned directly underneath and proximate to said surface wherein said opaque layer is not present directly above said indication means, allowing said indication means to be visible directly above said cook top, and 
     a control means configured to determine the temperature of said surface above said heating means and energise said indication means when said surface above said heating means is above a predetermined temperature and de-energises said indication means when said surface above said heating means is below said predetermined temperature. 
     Preferably said control means comprises an electric circuit fed from a transformer less supply. 
     Preferably the colour emitted by said indication means is dependent on whether said heating means is energised. 
     Preferably said indication means is at least one light emitting diode. 
     Preferably said control means includes heat sensing means positioned in close proximity to said heating means, the electrical characteristics of which are temperature dependent. 
     Preferable said heat sensing means is a bimetallic switch. 
     Alternatively said heating sensing means is a thermistor. 
     In a further alternative said heating sensing means is a positive temperature coefficient paste coated on the underside of said surface or said opaque layer. 
     Preferably said predetermined temperature is the maximum temperature for which human skin can safely be exposed to. 
     Preferably said maximum temperature is 50° C. 
     To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     One preferred form of the present invention will now be described with reference to the accompanying drawings in which; 
     FIG. 1 is a plan view of the cook top, 
     FIG. 2 is a cross section of the cook top using a surface mount LED, 
     FIG. 3 is a cross section of the cook top using a PCB mounted LED, 
     FIG. 4 is a cross section of the cook top using a bezel mounted LED, 
     FIG. 5 is a circuit diagram of the control circuit for the element status indicator, 
     FIG. 6 is an equivalent circuit with the element on, 
     FIG. 7 is an equivalent circuit with the element cooling, 
     FIG. 8 is an equivalent circuit with the element off, and 
     FIG. 9 is a circuit diagram of the complete circuit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the preferred form of the present invention an element status indicator is provided on a ceramic glass cook top in order to indicate when the surface of the cook top is safe to touch. The indicators are preferably of the LED variety which provides the ability to use different colours to indicate different conditions. In order that this can be achieved the cook top is of a clear glass variety coated on the underside with a layer of enamel. 
     A glass surface cook top  1 , shown in FIG. 1, includes four circular elements  2  as would be typical for a conventional cook top. Generally the elements will be of the radiant ribbon variety, chosen for their quick response and high efficiency. The surface of the cook top  3  above the element  2  will initially be cool but will quickly heat up due to radiation from the element  2  and conduction from the heating vessel (not shown). Once the element  2  is turned off and the heating vessel removed, the thermal inertia of the glass  3  means that it will stay significantly hot for quite some time—up to 15 minutes. Obviously this is not that safe because there will be no visible indication of the surface temperature—if the element  2  is not glowing it would normally be assumed the element  2  is off and the surface  3  therefore cold. 
     The preferred embodiment of the present invention includes a multicoloured indicator  4  for each element  3 . The indicator  4  is positioned beside the control knob  5  for easy reference as to that elements status. Different colours can be used to indicate different conditions eg: orange for element on, red for element too hot to touch. Alternatively the indicators could be positioned separately to mimic the spatial orientation of the respective elements. 
     The cook top surface in the preferred embodiment is constructed from glass ceramic, e.g. borosilicate, of approximately 4 mm thickness, and clear or “smoky” in appearance (substantively transparent). The top surface is smooth and the rear either smooth or slightly textured, which reduces stress concentrations from scratches preferably resulting in a bending strength greater than 100N/mm 2 . The overall colour of the panel arises from a ceramic ink which is screened on the rear of the panel. The areas outside the elements  2  are screened with a black or dark colour to make the panel suitably opaque (specifically not to show a colour change from moisture or glue in contact with the lower surface). The area of the ceramic glass above the heated area of the element  2  needs to be able to cope with more arduous conditions eg thermal load of 700° C., class  1  restraint to acid, alkali and water without discolouration. A different (and more limited) palette of colours would be used for this area, to help delineate it from the main area. It is also important that this area is more translucent to enable the element to be seen when glowing (the ability to t radiant energy in the 500-4000 nm range is clearly of prime importance too). 
     The indicator  4 , seen in FIG. 1, is preferably a surfaced mounted LED  20 , seen from the side in FIG.  2 . The LED  20  is mounted on a printed circuit board  21  which might also include provision for touch controls, or a rotary knob to control the elements  2 . Directly above the LED  20  the enamel  23  is absent from the underside of the glass  24 , to allow the LED  20  to be visible from above. Since the glass is clear this means that any colour of indication can be visible, rather than only red as was conventionally available. In a similar fashion the indicator could be a convention PCB mount LED  30 , shown in FIG. 3, or an LED mounted in a bezel  40 , shown in FIG.  4 . 
     The indicator maybe controlled by a number of means many of which are known in the art. The preferred embodiment of the present involves a bimetallic switch on the body of the element which closes when the glass surface temperature is above 50° C. 
     It will be appreciated many other variations would be equally applicable. A thermistor or other types of thermally responsive sensor could also be used in conjunction with an electronically controlled cooktop. Especially where such a cooktop included a microprocessor, such devices would also be used to control the energisation of the elements themselves. 
     A circuit module is used to control the LEDs for each element, seen in FIG. 5, which emit an orange light when the element is on and a red light when the element is cooling. The orange light is an optical combination or a green LED  50  and a red LED  51 . The circuit is more simply analysed in its three modes of operation: 
     i) element on 
     ii) element cooling and 
     iii) element off 
     Firstly in the element on configuration, shown in equivalent form in FIG. 6, the element switch  52  is in an closed state. This effectively ties the base of (pnp) T 1   53  and (npn) T 2   54  to the phase rail  55 . The maximum emitter to collector voltage of T 1   53  is clamped to a maximum of the on state voltage of the red LED  51  during the positive half of the phase voltage, which is approximately 1.5V. Thus for T 1   53  to be in an on state the base voltage  58  must be lower than 0.9V. The actual voltage  59  driving the base from the voltage divider must be slightly lower than 0.9V to account for the voltage drop across the base resistor  60 . The voltage divider voltage  59  is designed such that relative to the collector voltage  61  T 1   53  is prevented from attaining the necessary base to emitter voltage to turn on. The mirror of this applies for T 2   54  and the green LED  50  during the negative half cycle. 
     This means during the positive half cycle the red LED  51  will be on and in the negative half cycle the green LED  50  will be on. This cycling of green and red at 50 Hz will appear to the human eye as orange light. 
     When the element is cooling (temperature above 50° C.) the bimetallic switch  56  is in a closed state and the element switch  52  is in an open state, shown in equivalent form in FIG.  7 . This effectively ties the base of T 1   53  and T 2   54  to the phase rail  55  in the positive half cycle and to the reference rail  57  in the negative half cycle. This means that during the positive half cycle T 1   53  will biased off allowing the red LED  51  to be on. During the negative half cycle T 2   54  will be biased on which bypasses the green LED  50 . 
     Therefore with only the red LED allowed to operate the human eye will see red light. 
     Finally in the element off configuration, shown in equivalent form in FIG. 8, the element switch  52  and the bimetallic switch  56  are in an open state. This effectively ties the base of T 1   53  and T 2   54  to the reference rail  57 . This means that T 1   53  will be biased on in the positive half cycle and T 2   54  will be biased on in the negative half cycle. Thus for both half cycles the LEDs will be bypassed and therefore the operator will see no light. 
     Equally applicable would be the use of a thermistor with an electronic controller to calculate the surface temperature and activate the LED at the appropriate times. Similarly a timer based system whereby the LED is kept on for a “typical time to cool” might also be used to control the LED. 
     In the preferred embodiment a number of such modules will be connected together in series and connected to a power supply. In one embodiment, shown in FIG. 9, three such modules  80  are connected as such in respect of a three element cooktop. Each module is connected with its reference terminal  72  connected to the corresponding supply terminal  71  of the preceding module—these correspond to the reference rail and collector voltage referred to in the description of each module. Also a connection  70  to the phase terminal  81  of the supply is used in each module to supply the switches  52 ,  56 . 
     The supply itself is of the transformer less variety, instead utilising a capacitor to ensure a constant current supply. In the preferred embodiment, again referring to FIG. 9, a single capacitor  82  is connected in series in the neutral rail  83  with safety resistor  84 , and surge resistor  85  for this purpose. Two further zener diodes  86  are connected in parallel across the supply for transient over voltage protection.