GB2153555A - Temperature control of heating apparatus - Google Patents

Abstract

A temperature sensor (13) senses the temperature of a heated area (15) eg of a cooking vessel and a switching device 22 switches between different combinations of heating elements 26-29 to provide power outputs, in dependence on the difference between the sensed temperature and a selected power output set by a user-operable control 17. The heating apparatus includes a generally circular tray (1) containing a layer (2) of thermally-insulative material and supporting four infra-red-emitting, tungsten-halogen lamps (5), each having a tungsten filament (7). The apparatus is mounted beneath a layer (11) of glass ceramic to form a cooking hob. On/off switching of the lamps (5) provides temperature control and energisation and de-energisation of each of the lamps (5) may be implemented in a sequential manner. <IMAGE>

Description

SPECIFICATION Heating apparatus This invention relates to heating apparatus and in particular, though not exclusively, to such apparatus of the kind described in our copending Application No. 8320717, incorporating a number of sources of infra-red radiation.

Heating apparatus of this kind consists of, in one example, a shallow tray member containing thermally-insulative material and supporting four tubular, infra-red-emitting, tungsten-halogen lamps.

The tray member, together with three similar members, is preferably mounted below a continuous layer of glass ceramic, so as to form a cooking hob.

Temperature control of the heating apparatus is preferably implemented by a switching arrangement, which provides a number of series and/or parallel combinations of the respective filaments of the lamps, so as to produce a number of discrete control settings, which correspond to different power outputs to provide an optimised heat output curve.

However, switching between the filament combinations is achieved by a manual control, so that it is necessary to rely substantially on a user of the cooking hob to implement the temperature control thereof, so as to maintain a desired temperature of a cooking utensil disposed on the hob.

Furthermore, it may be desirable to utilise on/off switching of the lamps to provide temperature control of the heating apparatus. This type of temperature control is not, however, suitable for the switching of relatively high powers of 2KW, for example, because legislation only permits relatively low switching rates for such powers, which tend to cause visible, and thus disturbing, flickering effects of the lamps.

It is an object of the present invention to provide heating apparatus of the above-mentioned kind incorporating an improved temperature control arrangement.

It is another object of the invention to provide heating apparatus of this kind incorporating a temperature control arrangement which alleviates the afore-mentioned problem of undesirable flickering effects of the lamps.

According to one aspect of the invention there is provided heating apparatus including a number of sources of infra-red radiation, said sources being capable of interconnection into various combinations so as to provide a range of power outputs indicative of respective operating temperatures of the apparatus, a user-operable control for setting a selected one of said power outputs, temperature sensing means for sensing the temperature assumed by an article exposed to said radiation but said sensing means itself being substantially shielded from directly incident radiation emitted from said source, and electronic switching means for switching between said various combinations in dependence on the differences between said sensed temperature and the selected power output set by said user-operable control.

The temperature sensing means preferably consists of a platinum temperature sensor positioned in thermal contact with the undersurface of a shielded area of a glass ceramic material disposed above the said source, so that the sensor monitors the temperature of a cooking utensil disposed on the glass ceramic through the shielded area rather than the temperature of the glass ceramic heated by the infra-red sources.

The electronic switching means preferably includes a solid-state switching device for switching between the filament combinations, so as to maintain the utensil temperature, as sensed through the shielded area of the glass ceramic, in accordance with the user-operable control setting.

According to a second aspect of the invention there is provided heating apparatus incorporating a number of sources of infra-red radiation and switching means for energising and de-energising respectively said sources so as to maintain substantially a set operating temperature of said apparatus, said switching means being arranged to energise and de-energise each of said sources in a sequential manner so as to reduce substantially the amount of power being switched on or off at any one time.

The invention will now be further described by way of example only with references to the accompanying drawings, wherein: Figure 1 shows a sectional view of one embodiment of the present invention, Figure 2 shows various filament combinations for power output control of the embodiment shown, and Figure 3 shows schematically a circuit arrangement suitable for implementation of temperature control of the embodiment shown.

Figure 1 shows a generally circular tray 1, which is preferably formed from metal and has a layer 2 of a suitable thermally insulative material deposited therewithin, on the base thereof. The tray 1 has two extending flanges, 3 and 4, arranged on opposite sides of the rim thereof, which support respectively each end of a number of sources of infra-red radiation, one being shown at 5. In a preferred example, four sources, shown at 26 to 29, respectively, in Figures 2 and 3, are arranged across the circular area of the tray 1.

Each source of infra-red radiation comprises a tungsten-halogen lamp including a 500W tungsten filament 7 supported within a quartz envelope 6.

Each lamp has moulded ceramic end caps, 8 and 9, which each enclose a pinch seal (not shown) with an electrical lead connected to the respective end of the filament sealed therein, the lead being welded to an appropriate electrical connector.

A moulding 10 of ceramic fibre material is disposed above the tray 1 and press-fitted around the ends of each source 5 to provide a suitable packing therefor.

The tray 1 is urged upwardly, by a resilientlymounted support plate (not shown), towards the undersurface of a layer 11 of glass ceramic, which forms the top surface of a cooking hob.

Temperature control of the heating apparatus shown in Figure 1 is implemented by a switching arrangement which provides six series and/or parallel combinations 23 of the four 500W filaments of the lamps, as shown at 26 to 29, respectively, in Figure 2. This provides six discrete control settings on a user-rotatable control knob, shown schematically at 17 in Figure 3, which correspond to six power outputs, as shown in Figure 2, to produce an optimised characteristic heat output curve. Figure 2 also indicates the percentage of each power output relative to the maximum output of 2KW.

A diode 12 is used in one or more of the six combinations to ensure that each control setting, particularly the lower settings, provide an aesthetically-pleasing balanced effect of the visible radiation emitted from the filaments as seen through the layer 11 of glass ceramic, as well as enabling the lower powers, which are suitable for simmering purposes, to be achieved by the combinations.

The heating apparatus so far described is disclosed in our copending Application No. 8320717.

As described in our copending Application No.

8327872, the filament combinations generating control setting no. 3 preferably includes two oppositely-directed diodes 24 and 25, which suppress substantially one or more harmonics of the main power supply which would otherwise cause harmonic disturbance to be carried on the main supply.

However, one example of one aspect of the present invention provides a temperature sensor, shown at 13 in Figure 1, which is preferably formed from platinum and disposed in thermal contact with the undersurface of the layer 11 of glass ceramic. The sensor 13 is accommodated within a thermally insulative surround 14, so as to shield the sensor 13 and area 15 of the layer 11, enclosed within dotted lines 16, from directly incident radiation emitted from the infra-red lamps 5.

It can thus be envisaged that the shielded area 15 would only become heated by a cooking utensil (not shown) disposed over this area 15 on the layer 11 and heated by the infra-red lamps 5, so that the temperature sensed by the sensor 13 is substantially only indicative of the temperature of the cooking utensil disposed thereabove.

Figure 3 shows a circuit arrangement suitable for implementation of temperature control of the heating apparatus in accordance with the temperature of the utensil, as sensed by the sensor 13.

By setting the user-rotatable control 17 to a selected control setting, as shown in Figure 2, a temperature reference signal 18 is passed to a comparator 19, wherein the signal 18 is compared with a temperature sensor output signal 20, which is indicative of the sensed temperature of the area 15 (and thus of the cooking utensil).

If the amplitudes of signals 18 and 20 differ by more than a permissable extent, a control signal 21 is generated to control the operation of a solidstate switching device 22.

Switching device 22 incorporates suitable components for providing switching of the four filaments 26 to 29 from one of the six filament combinations 23 to another thereof, in accordance with the control signal 21. The switching of the four lamps 26 to 29 between each of the six combinations 23 thus causes an increase or a decrease in the power output of the infra-red lamps 26 to 29, whichever is appropriate to render the sensed temperature of the area 15 substantially equal to the desired temperature set by control 17.

It can therefore be seen that once the desired temperature has been set, temperature control of the apparatus is electrically implemented by switching between the filament combinations so as either to increase or decrease the power output, should the temperature of the cooking utensil being heated by the apparatus fall below or rise above, respectively, the desired set temperature.

However, it may be preferable to use on/off switching of the lamps to provide temperature control of the heating apparatus so as to maintain the operating temperature of the apparatus substantially at a temperature set in accordance with a user-operable temperature control. To this end, a second aspect of the invention provides respective energisation and de-energisation of each of the lamps in a sequential manner, instead of energisation or de-energisation of all of the lamps at one time. This enables switching of the lamps to be carried out on one lamp at a time and thus at lower power levels, of 500W instead of 2KW for example, for which legislation permits higher switching rates of the lamps, thereby alleviating the disturbing flickering effect of the lamps caused at relatively low switching rates.

The sequential switching of the lamps is preferably implemented at successive intervals of one cycle of the main power supply and at zero-crossing points thereof.

Claims (8)

1. Heating apparatus including a number of sources of infra-red radiation, said sources being capable of interconnection into various combinations so as to provide a range of power outputs indicative of respective operating temperatures of the apparatus, a user-operable control for setting a selected one of said power outputs, temperature sensing means, substantially shielded from directly incident radiation emitted from said sources, for sensing the temperature assumed by an article exposed to said incident radiation, and electronic switching means for switching between said various combinations in dependence on the differences between said sensed temperature and the selected power output set by said user-operable control.

2. Heating apparatus as claimed in Claim 1 wherein said apparatus is mounted beneath a layer of infra-red- transmissive material, said temperature sensing means being in thermal contact with the under-surface of an area of said layer shielded from said directly incident radiation.

3. Heating apparatus as claimed in Claim 1 or 2 wherein said electronic switching means includes a solid-state switching device for switching between said combinations.

4. Heating apparatus including a number of sources of infra-red radiation, temperature sensing means for monitoring the operating temperature of said apparatus, and switching means for energis ing and de-energising respectively said sources, in dependence on the monitored operating tempera ture, so as to maintain substantially a set operating temperature of said apparatus, said switching means being arranged to energise and de-energise each of said sources in a sequential manner so as to reduce substantially the amount of power being switched on or off at any one time.

5. Heating apparatus as claimed in Claim 4 wherein said sequential switching of said sources is implemented at successive intervals of one cycle of the mains power supply and at zero-crossing points thereof.

6. Heating apparatus as claimed in any preceding claim wherein said temperature sensing means consists of a platinum temperature sensor.

7. Heating apparatus as claimed in any preced ing claim wherein each of said sources of infra-red radiation consists of a quartz, halogenated lamp having a tungsten filament.

8. Heating apparatus substantially as herein de scribed with reference to the accompanying draw ings.