Radiant heater having multiple heating zones

In a radiant heater having multiple heating zones there is provided a first heating zone incorporating at least one heating element and a second heating zone incorporating at least first and second heating elements. A thermal cut-out device includes a temperature sensor which passes through at least the first heating zone and which is responsive solely to heat emitted in the first heating zone. A manually operable switch permits switching between first and second heating states. In the first heating state, the at least one heating element in the first heating zone is energised alone, while in the second heating state the at least one heating element in the first heating zone is electrically connected in series with the second heating element of the second heating zone, the at least one heating element and the second heating element being energised in parallel with the first heating element of the second heating zone.

The present invention relates to a radiant heater having multiple heating 
zones which may be used, for example, in a cooking appliance having a 
glass ceramic cooking plate. 
BACKGROUND TO THE INVENTION 
Radiant heaters having multiple heating zones are known for example from 
United Kingdom Patent Specification No. 2 069 300 and European Patent 
Specification No. 0 103 741. European Patent Specification No. 0 103 741 
describes a heater having inner and outer concentric heating zones, the 
inner heating zone containing one heating element and the outer heating 
zone containing two heating elements. A temperature sensor of a thermal 
cut-out device extends over both the inner and outer heating zones and is 
sensitive to heat emitted in both zones. The thermal cut-out device has 
two switches operating at upper and lower cut-out temperatures in order to 
protect the glass ceramic cooking surface against overheating. 
When the inner heating element is used alone, for example to heat a small 
cooking utensil, the inner heating element is operated at full power. In 
this condition, the inner heating element is connected to the thermal 
cut-out device by way of its switch operable at the lower cut-out 
temperature. When both the inner and outer heating zones are to be used 
together, for example to heat a large cooking utensil, one of the heating 
elements in the outer zone is electrically connected in series with the 
heating element in the inner zone, and the two heating elements in series 
are connected in parallel with the other heating element in the outer 
zone. In this condition, the heating elements are connected to the thermal 
cut-out device by way of its switch operable at the upper cut-out 
temperature. The effect of this is to reduce the specific heating surface 
loading in the inner zone as compared with the outer zone. 
This arrangement has the disadvantage that two switches on the thermal 
cut-out device are required to control the operation of the heating 
elements, one of the switches being a changeover switch rather than a 
simple make-and-break switch. This precludes the possibility of using the 
second switch on the thermal cut-out device as a signal switch, for 
example to warn the user of the cooking appliance that the glass ceramic 
cooking surface is at an elevated temperature and may be too hot to touch. 
OBJECT OF THE INVENTION 
It is an object of the present invention to provide a radiant heater having 
multiple heating zones in which it is possible to modify the specific 
heating surface loading of one of the heating zones in a manner which only 
uses a single switch of the thermal cut-out device. 
SUMMARY OF THE INVENTION 
According to the present invention there is provided a radiant heater 
having multiple heating zones comprising: 
a first heating zone provided with at least one heating element; 
a second heating zone provided with at least first and second heating 
elements; 
a thermal cut-out device including a temperature sensor passing through at 
least the first heating zone and responsive solely to heat emitted in the 
first heating zone; and 
switch means for switching between first and second heating states, the 
arrangement being such that in the first heating state the at least one 
heating element in the first heating zone is energised alone and that in 
the second heating state the at least one heating element in the first 
heating zone is electrically connected in series with the second heating 
element of the second heating zone, the at least one heating element and 
the second heating element being energised in parallel with the first 
heating element of the second heating zone. 
The heating element in the first heating zone may be a coil of bare 
resistance wire, an infra-red lamp, or a coil of bare resistance wire 
electrically connected in series with an infra-red lamp. 
The first heating element of the second heating zone may be a coil of bare 
resistance wire, an infra-red lamp, or a coil of bare resistance wire 
electrically connected in series with an infra-red lamp. 
The second heating element of the second heating zone may be a coil of bare 
resistance wire. 
The temperature sensor may pass through the second heating zone in a manner 
which renders the sensor substantially unresponsive to heat emitted in the 
second heating zone. For example, the temperature sensor may comprise a 
differential expansion member, the differential expansion of the sensor 
being substantially eliminated in that region of the sensor passing 
through the second heating zone. Alternatively, that region of the 
temperature sensor passing through the second heating zone may be isolated 
from heat emitted in the second heating zone by means of a block of 
thermal insulating material at least partly surrounding the sensor. As a 
further alternative, that region of the temperature sensor passing through 
the second heating zone may be at least partly surrounded by a thermally 
conducting element arranged to conduct heat externally of the heater. 
According to another alternative, that region of the temperature sensor 
passing through the second heating zone may be isolated from heat emitted 
in the second heating zone and exposed to heat emitted in the first 
heating zone. 
The first and second heating zones may be separated by a wall of thermal 
insulating material. 
The first heating zone may be circular and the second heating zone may be 
annular, the second heating zone surrounding the first heating zone. 
For a better understanding of the present invention and to show more 
clearly how it may be carried into effect reference will now be made, by 
way of example, to the accompanying drawings in which:

DESCRIPTION OF PREFERRED EMBODIMENTS 
The radiant heater shown in FIGS. I and 2 is arranged beneath a cooking 
surface 1, for example of glass ceramic material, and comprises a metal 
dish 2 containing a base layer 4 of electrical and thermal insulating 
material. Against the side of the dish 2 is located a peripheral wall 6 of 
thermal insulating material. The area within the peripheral wall 6 is 
divided into a first or inner, generally circular heating zone 8 and a 
second or outer, annular heating zone 10 by means of a circular wall 12 of 
thermal insulating material. Extending over the inner heating zone 8 and 
over at least a part of the outer heating zone 10 is a thermal cut-out 
device 14 for protecting the cooking surface against excessive 
temperatures. The thermal cut-out device will be explained in more detail 
hereinafter. 
Within the inner heating zone 8 are arranged two heating elements 16 and 
18. Element 16 is in the form of a coil of bare resistance wire located in 
a groove formed in the base layer 4 and arranged within an infra-red lamp 
18 of generally circular configuration. The lamp 18 is positioned within, 
but generally not in contact With, a recess formed in the base layer 4. 
Where the lamp 18 passes across the outer heating zone 10, the envelope of 
the lamp 18 is coated with a substantially opaque material in order to 
confine any visible light emitted by the lamp 18 to the inner heating zone 
8. 
In the outer heating zone 10 are arranged two heating elements 20 and 22. 
Element 20 is in the form of a coil of bare resistance wire located in a 
groove formed in the base layer 4 and is generally in the form of two 
concentric arcs, the inner arc extending substantially around the 
circumference of the outer heating zone and the outer arc extending 
substantially around 300 degrees of the outer heating zone. Element 22 is 
also in the form of a coil of bare resistance wire located in a groove 
formed in the base layer 4 and is generally in the form of an arc 
extending substantially around 60 degrees of the outer heating zone in 
that portion not occupied by the heating element 20. 
The thermal cut-out device 14 comprises a differential expansion probe-type 
temperature sensor 24 comprising a rod 25 of material having a high 
coefficient of thermal expansion, such as an iron-chrome alloy, arranged 
within a tube 27 of material having a low coefficient of thermal 
expansion, such as quartz, and a switch assembly 26 operable by the sensor 
24. The sensor is configured in such a way that it is sensitive 
substantially only to heat emitted by the heating elements 16 and 18 in 
the inner heating zone 8 and is isolated from any heat emitted by the 
heating elements 20 and 22 in the outer heating zone 10. 
Isolation of the temperature sensor 24 can be achieved in a number of ways. 
As shown in FIG. 1, the effective length of the temperature sensor 24 can 
be designed to terminate substantially at the boundary between the inner 
and outer heating zones, for example by substituting for the low expansion 
tube 27 in the outer heating zone a high expansion tube 36, for example 
made of the same material as that of the high expansion rod 25. As shown 
in FIGS. 3a and 3b, the temperature sensor can be isolated by enclosing 
that part of the sensor passing through the outer heating zone 10 in a 
block 28 of thermal insulating material. As shown in FIG. 4, the 
temperature sensor can be isolated by enclosing that part of the sensor 
passing through the outer heating zone 10 in a heat conducting material, 
such as a copper tube 30, such that the copper tube acts as a heat sink 
and heat absorbed is conducted outside the radiant heater. As shown in 
FIG. 5, the temperature sensor can be isolated by extending the thermal 
influence of heat emitted in the inner heating zone to that part of the 
sensor passing through the outer heating zone 10, for example by providing 
a block 32 of thermal insulating material having a tapering tunnel 34 
formed therein and communicating with the inner heating zone. It will be 
noted, however, that some minor alteration to the configuration of the 
heating element 20 may be required. 
Because the temperature sensor 24 is isolated from heat emitted by the 
heating elements 20 and 22 in the outer heating zone 10, it is necessary 
only to provide a single set of switch contacts in the switch assembly 26. 
The use of a thermal cut-out device 14 having only a single set of switch 
contacts in the switch assembly 26 results in a device which is more 
economical to manufacture compared with a thermal cut-out device such as 
that described in European Patent Specification No. 0 103 741 which 
requires a switch assembly with an additional changeover switch for 
switching power to the heating elements. Where a second set of 
make-and-break contacts is available, as in FIG. 1, these can have a lower 
power capacity and can be employed to switch at a considerably lower 
temperature, for example 60.degree. C., to give an indication to the user 
that the cooking surface I may be too hot to touch. 
In use, the radiant heater is incorporated in a circuit such as that shown 
in FIG. 6. FIG. 6 shows that electrical energy is supplied to the radiant 
heater by way of an energy regulator 38 having a manually adjustable 
control knob 39 which determines the mark-to-space ratio of the switched 
output from the regulator. The energy regulator also incorporates a 
manually operable changeover switch 40 for switching between a first 
heating state in which only the heating elements 16 and 18 in the inner 
heating zone 8 are energised, for example for heating a relatively small 
cooking utensil, and a second heating state in which all the heating 
elements 16, 18, 20 and 22 are energised, for example for heating a 
relatively large cooking utensil. 
In the first heating state as illustrated, in which only the heating 
elements 16 and 18 in the inner heating zone 8 are energised, electrical 
power passes through the switch 40 to the heating elements 16 and 18 which 
are electrically connected in series. The heating elements 16 and 18 are 
electrically connected in series because the lamp 18 has a very low 
electrical resistance at low temperatures and thus draws a very high 
starting current. It is often desirable to limit the starting current by 
incorporating a conventional heating coil in series with the lamp. For an 
inner heating zone 8 having a diameter of some 145 mm the combined heating 
power of the heating elements 16 and 18 is typically 1200 watts giving a 
specific surface loading of some 0.073 watts/mm.sup.2. The temperature in 
the inner heating zone 8 is monitored by the temperature sensor 24 of the 
thermal cut-out device 14. When the temperature detected exceeds a first 
predetermined temperature the first set of contacts in the snap switch 
assembly 26 is actuated to energize a warning light 42, and when the 
temperature detected exceeds a second predetermined temperature the second 
set of contacts in the snap switch assembly 26 is actuated to cut off 
power to both the heating elements 16 and 18. In the second heating state, 
in which the heating elements 20 and 22 in the outer heating zone 10 are 
energised in addition to the heating elements 16 and 18 in the inner 
heating zone, electrical power passes through the switch 40 to the heating 
element 20 and electrical power passes directly to heating elements 22, 16 
and 18 which are electrically connected in series. The heating element 20 
is connected in parallel with the series connected elements 22, 16 and 18. 
Heating element 22 is designed to generate typically 117 watts of power in 
the outer heating zone 10 and to reduce the power generated in the inner 
heating zone 8 by the heating elements 16 and 18 to typically 1000 watts, 
giving a specific surface loading of some 0.061 watts/mm.sup.2. Heating 
element 20 is designed to generate typically 1083 watts in the outer 
heating zone 10, making the total heat generated in the outer heating zone 
10 some 1200 watts. For a radiant heater having a diameter of some 210 mm, 
and an internal wall 5 mm thick where it is in contact with the underside 
of the glass ceramic cooking surface, the specific surface loading in the 
outer heating zone 10 is some 0.076 watts/mm.sup.2, that is about 25 per 
cent above the specific surface loading for the inner heating zone 8. As 
with the first heating state, the temperature in the inner heating zone 8 
is monitored by the temperature sensor 24 of the thermal cut-out device 
14. When the temperature detected exceeds a first predetermined 
temperature the first set of contacts in the snap switch assembly 26 is 
actuated to energize a warning light 42, and when the temperature detected 
exceeds a second predetermined temperature the second set of contacts in 
the snap switch assembly 26 is actuated to cut off power to all the 
heating elements 16, 18, 20 and 22. However, it will be noted that in the 
second heating state the heat generated in the inner heating zone is 
reduced from 1200 watts to 1000 watts. This has the effect of modifying 
the specific surface loading of the inner heating zone and permits the 
heat distribution in the inner and outer heating zones to be optimized in 
each of the first and second heating states. Use of the radiant heater in 
the circuit according to FIG. 7 is similar to that of FIG. 6, except that 
the switch 44 in the energy regulator is a simple make-and-break switch 
rather than a more complex changeover switch. In order to use the radiant 
heater with the switch 44 in the second heating state as illustrated, 
electrical power from the switch 44 is connected across a relay coil 46 
and relay contacts 48 are employed as a substitute for the switch 40. 
Numerous modifications are possible to the radiant heater described above. 
For example, the heater need not have a concentric circular configuration. 
Other configurations include an arrangement where the inner heating zone 
and the outer heating zone are not concentric or an arrangement where a 
circular zone is provided for the first heating zone and a second heating 
zone is provided in the form of an additional zone on one or opposite 
sides of the circular zone so as to form a generally oval or rectangular 
heater. 
Although the invention has been described with two heating elements 16 and 
18 in the first heating zone this is not necessary and the first heating 
zone may alternatively be provided with a single coil of bare resistance 
wire or a single infra-red lamp. Moreover, the invention has been 
described with a single heating element 20 generating the major part of 
the power in the second heating zone, but this may alternatively comprise 
an infra-red lamp or a coil of bare resistance wire in series with an 
infra-red lamp. 
The major benefit of the radiant heater according to the present invention 
is that the specific surface loading of the first heating zone is capable 
of being modified with a thermal cut-out device having a snap switch 
assembly with only a single set of contacts. This permits the heater to 
give improved performance over existing heaters that employ thermal 
cut-out devices having a snap switch assembly with only a single set of 
contacts. The invention also permits the heater either to be manufactured 
more economically than known radiant heaters that are able to modify the 
specific surface loading of one of the heating zones or to be more 
versatile in providing the well known facility for indicating to the user 
that the cooking surface may be too hot to touch.