GB2170665A

GB2170665A - Heating apparatus

Info

Publication number: GB2170665A
Application number: GB08502718A
Authority: GB
Inventors: Bernard Frederick Fellerman
Original assignee: Thorn EMI Domestic Appliances Ltd
Current assignee: Thorn EMI Domestic Appliances Ltd
Priority date: 1985-02-02
Filing date: 1985-02-02
Publication date: 1986-08-06
Also published as: GB8502718D0; GB2170665B

Abstract

A heating unit includes four tungsten halogen lamps emissive of infra-red radiation, mounted substantially parallel to each other and each including a tungsten filament 9 to 12 respectively, and a control circuit including switches 21 to 31 for connecting the filaments 9 to 12 into series and/or parallel combinations to achieve a number of discrete power outputs of the unit. In a number of the power outputs, only two of the filaments 9 to 12 are required to be energised and, in these filament combinations, the circuit is arranged to energise the outer filaments 9 and 12, rather than the two inner filaments 10 and 11, to achieve an aesthetically-pleasing balanced effect of visible radiation emitted from the unit. The switches 21 to 31 give the different power outputs, and eleventh and lowest output being achieved by phase control of a triac 32 with all filaments 9 to 12 then being in series with one another (Fig. 3). <IMAGE>

Description

SPECIFICATION Heating apparatus This invention relates to heating apparatus and in particular, though not exclusively, to such apparatus including a number of sources of infra-red radiation in the form of tungsten-halogen lamps.

Heating apparatus of this kind is disclosed in our copending U.K. Patent Application No.

83 20717 (Publication No. 2132060A), wherein a heating unit, including in one example four tungsten-halogen lamps, is mounted beneath a glass ceramic cook-top to form a hotplate of a cooking hob.

A switching arrangement is provided for switching the filaments of the lamps into various series and/or parallel combinations producing six discrete power outputs, and therefore six temperature control settings, which achieve an optimised characteristic heat output curve.

It may be desirable to provide a more accurate temperature control, which will require an increased number of discrete power outputs, which thus necessitates the provision of an increased number of filament combinations.

However, an important consideration when selecting the filament combinations is that they should produce an aesthetically pleasing, balanced effect of the visible radiation emitted from the filaments, as seen through the glass ceramic cook-top.

It is therefore an object of the present invention to provide heating apparatus of the above-mentioned type, including a switching arrangement which is capable of providing an increased number of discrete power outputs from the lamp filaments than has been provided hitherto, whilst still providing a substantially balanced effect of the visible radiation emitted therefrom.

In order that the invention may be clearly understood and readily carried into effect, one embodiment thereof will now be described by way of example only, with reference to the accompanying drawings, wherein: Figure 1 shows a plan view of one embodiment of heating apparatus, in accordance with the invention, Figure 2 shows a switching arrangement therefor, and Figure 3 shows a number of filament combinations, and corresponding power outputs, produced by the arrangement.

In Fig. 1, a heating unit includes a generally circular shallow tray 1 containing a layer 2 of thermally-insulative material, such as a microporous material, on the base thereof. Mounted above the insulative material are four infra-red, tungsten-halogen lamps 3 to 6, which are retained in the correct position by two pieces of suitably-shaped insulative material 7 and 8.

The lamps 3 to 6 include tungsten filaments 9 to 12, respectively, supported within quartz tubes 13 to 16 respectively, with pinch seals (not shown) at either end thereof, each being enclosed by a ceramic end cap, such as 17.

The heating unit is preferably mounted beneath a glass ceramic cook-top (not shown) to form a hotplate area of a cooking hob. The unit also preferably includes a thermal limiting device 18 for limiting the operating temperature of the glass ceramic cook-top to prevent damage thereto by overheating.

Fig. 2 shows a control circuit for providing a range of discrete power outputs of the lamp filaments 9 to 12, each power output corresponding to a power control setting number, as shown in Fig. 3, to be set by a useroperable control of the cooking hob. The control circuit includes eleven switches 21 to 31, inclusive, which provide eleven discrete power outputs of the filaments 9 to 12, which preferably each have a power output of 450W, by selectively closing a number of the switches 21 to 31 as shown in Fig. 3.

The series and/or parallel combinations of the filaments 9 to 12, which provide the eleven power outputs, are also shown in Fig.

3 and it can be seen that, to achieve four of the outputs, either one or both of diodes 19, 20 are required to be switched into the filament combination. Both diodes 19, 20, oppositely directed as shown, are required in the combinations for control setting nos. 5 and 6 to suppress one or more harmonics of the mains power supply which would otherwise cause harmonic disturbance to be carried on the supply. The diodes 19, 20 each preferably have a 3 amp, 600 volt rating.

To achieve the lowest control setting No.

11, a triac 32 provides phase control of the filament combination for this setting to generate a low power output of approximately 80W as described in our copending U.K. Application No. 84 22902.

In control setting nos. 3, 5, 7 and 9 only two of the lamp filaments are used and, to maintain an aesthetically-pleasing balanced effect of the visible radiation emitted by the filaments and transmitted through the glass ceramic cook-top, the switching is arranged so that the two outer filaments 9 and 12, rather than the inner filaments 10 and 11, are used.

It can thus be seen that the present embodiment of the invention, including an elevenswitch control circuit, is advantageous in that it provides eleven discrete power outputs of the four lamp filaments so that, together with zero power output in the "OFF" position, twelve control settings are provided.

The present invention therefore provides heating apparatus wherein an increased number of power outputs, which conform to a characteristic heat output curve, can be generated, whilst a balanced effect of the visible radiation emitted from the lamp filaments is still maintained for each output.

Claims

1. A control circuit for a heating unit including a plurality of tungsten-halogen lamps mounted in a substantially parallel formation relative to each other, said control circuit including switching means for connecting the filaments of said lamps into series and/or parallel combinations to achieve a number of discrete power outputs of said unit, at least one output being generated by a combination of less than all of said lamps, said control circuit being arranged to generate said at least one output by energisation of only the outer lamps of said formation to provide an aestheticallybalanced effect of visual radiation emitted from said unit.

2. A circuit as claimed in claim 1 wherein first and second oppositely-directed diodes are connected into one or more of said combinations to suppress one or more harmonics of the mains power supply, which would otherwise cause harmonic disturbance to be carried on said supply.

3. A circuit as claimed in claim 1 or 2 wherein a triac is connected into one or more of said combinations to provide phase control thereof.

4. A control circuit substantially as herein described with reference to the accompanying drawings.

5. A heating unit including a plurality of tungsten-halogen lamps mounted in a substantially parallel formation relative to each other and a control circuit for controlling the output of said lamps, as claimed in any preceding claim.

6. A heating unit as claimed in claim 5 wherein four lamps are provided and only the two outer lamps of said formation are energised to generate said at least one output.