GB2218605A

GB2218605A - Control means for an electric heater unit for an electric ceramic hob

Info

Publication number: GB2218605A
Application number: GB8910368A
Authority: GB
Inventors: Alan Wilfred Welsh
Original assignee: Redring Electric Ltd
Current assignee: Redring Electric Ltd
Priority date: 1988-05-13
Filing date: 1989-05-05
Publication date: 1989-11-15
Also published as: GB8811440D0; DE8905879U1; GB8910368D0

Abstract

Control means for an electric heater unit comprising at least one electric heater element (3,4) positioned below the ceramic hob plate (1), comprising a thermally conductive rod (14) within an electrically-insulating tube (15) positioned below the hob plate in proximity to the heater element for exposure to its heat, and a thermally responsive sensor, preferably a switch (11), for controlling the heat output of the element or elements (3,4). The sensor is connected thermally to one end of the rod and is located further than the rod from the heater element. The heating element controlled may be a booster element that is switched off when a predetermined temperature is reached. <IMAGE>

Description

Thermal Control Device This invention relates to thermal control devices for use in controlling the temperature of heater units incorporating one or more electric heater elements, more especially, though not exclusively, for use with electric hobs. In such hobs, the heater units are usually of the type incorporating a thermally and electrically insulating support carrying one or more of said electric heater elements, generally of coiled wire form, and designed to be mounted beneath an associated hob plate, for example of glass ceramic material, with the heater element or elements spaced from the lower surface of the plate.

In order to prevent the glass ceramic plate from being damaged due to overheating, the heater unit associated therewith has comnonly been provided with a thermal control device incorporating a heat sensitive rod which extends across the unit close to the heater element surface within an electrically insulting tube. As the temperature rises, the rod expands against an end wall of the tube, thus activating contacts disposed at the opposite end, so as to de-energise some or all of the heater element or elements.

Such thermal control devices are, however, expensive and an object of the present invention is to provide an alternative less expensive means of preventing overheating of the glass ceramic plate associated with a heater unit of the above kind. The invention is, however, of wider application as will be apparent from the following description.

The invention provides control means for an electric heater unit comprising at least one electric heater element positioned below a ceramic hob plate, the control means comprising a thermally conductive elongate probe comprising a rod held within an electrically insulating tube which in use is positioned below the hob plate in proximity to the heater element for exposure to its heat, and a thermally responsive sensor for controlling the heat output of the element, the sensor being connected thermally to one end of the probe ànd preferably being locateable further than the probe from the heater element in use.

The rod, preferably straight, is preferably held within an electrically insulating tube formed from quartz. The sensor is preferably a thermally-responsive switch, conveniently in the form of a disc type thermal sensor. Relatively inexpensive mass produced ceramic or thermoplastic bodied thermal sensors may be used for this purpose, such sensors being pre-set to trigger at a temperature in the region of 100" to 200"C.

Conveniently, where the heater unit is designed for use with an electric hob, the heater element or elements define a plane lying parallel to, but spaced from, the lower surface of the hob plate, and the rod extends substantially parallel to the surface and extends completely across the heater element or elements between the latter and the plate.

One end of the rod is conveniently brazed to one face of a metal mounting plate, with the thermal sensor held in contact with the opposite surface of the mounting plate, for example by a spring or a rubber strap.

In a heater unit incorporating an insulating support sealed within a metal casing having a base and a peripheral wall, the mounting plate is conveniently secured to the wall of the casing.

The rod may be formed from a nickel chrome alloy.

It will be understood that the invention includes within its scope not only thermal control devices per se, but also heater units for electric hobs incorporating such devices.

A thermal control device embodying the invention will now be described by way of example with reference to Figures 1 and 2 of the accompanying schematic drawing, which illustrate a plan view and a sectional view of one form of heater unit incorporating a thermal control device.

The heater unit Is similar to that described in co-pending Patent Application No. 2186167A, and comprises two heater elements 3, 4, supported in spiral grooves on the upper surface of a ceramic support 5 accommodated within a sheet metal casing 6. The support 5 has an upstanding side wall the top edge of which is arranged'to be held against the lower surface of a glass ceramic hob plate 1 (Fig. 2) in any convenient manner. The elements 3, 4 are connected to respective terminals of a terminal box 7 attached to the side of the casing 6.

As in the case of the unit described in said co-pending patent application, both heating elements are energisable together when the unit is first switched on to a maximum heat condition, to produce a rapid heating of the glass ceramic plate 1, and a thermally responsive switch device is arranged to switch one of the elements (the "booster" element) off when a predetermined temperature is reached, below that at which is likely to damage the glass ceramic plate (for example 800 C), leaving only one element operative. The latter is insufficient, by itself, to heat the glass ceramic plate 1 to such a temperature, but is capable of maintaining the upper surface of the plate at a temperature (say between 500"C and 600"C) which is adequate for normal cooking purposes.

The thermally responsive switch device, in this case, comprises a disc-type thermal sensor 11 mounted on a sheet metal thermally conductive plate 12, for example of stainless steel, and held in place against one surface of the plate 12 by a spring, as at 13 (Fig. 1). To the opposite surface of the plate there is brazed one end of a thermally conductive rod 14, for example of 80/20 nickel-chrome, which is arranged to extend across the heater unit between the heater elements 3, 4 and the glass ceramic plate 1, the rod being accommodated within quartz tube 15 which is supported within close fitting openings at diametrically opposite regions of the side wall 16 of the heater element support 5. A bracket 21 secures the assembly to the casing 6. The tube 15 is spaced vertically from the plate 1.

Terminals 17 of the thermal sensor 11 are connected via leads 18 to terminals 19 on the terminal box 7 so that the sensor 11 is in series with the booster element. It has been found that the use of the thermally conductive rods 14 rapidly conveys changes of temperature to the thermal sensor, which provides a relatively inexpensive form of temperature control.

The rod 14 can extend completely across the heater unit as shown, although it has been found that this is not necessary, and a rod which extends across only a part of the unit has been found to result in a satisfactorily rapid response of the thermal sensor to temperature changes of the unit.

Although in the embodiment described the heater unit is of the construction forming the subject of said co-pending patent application, the thermally responsive switch arrangement of the invention may be used to advantage with other forms of ceramic hob heating units for providing a protective device for switching off one or more heater elements at a predetermined high temperature.

Thus the coiled wire heating elements may in some cases be partially or wholly replaced by one or more halogen lamps.

Claims

1. Control means for an electric heater unit comprising at least one electric heater element positioned below a ceramic hob plate, the control means comprising a thermally conductive elongate probe comprising a rod held within an electrically insulating tube which in use is positioned below the hob plate in proximity to the heater element for exposure to its heat, and a thermally responsive sensor for controlling the heat output of the element, the sensor being connected thermally to one end of the probe.

2. Control means according to Claim 1, in which the sensor is a thermally-rsponsive switch.

3. Control means according to Claim 2, in which the switch is a disc type thermal sensor.

4. Control means according to Claim 2, in which the switch is set to trigger at a temperature of the probe which corresponds to a predetermined temperature of the hob plate in use.

5. An electric heater unit for an electric hob, the unit comprising a generally planar electric heater element, and control means according to Claim 1, of which the probe is parallel and adjacent to the plane of the heater element, and the sensor is arranged to control the heat output of the element.

6. An electric heater unit according to Claim 5, in which one end of the probe is brazed to one face of a metal mounting plate, and the thermally responsive sensor is held in contact with the opposite face of the mounting plate.

7. An electric heater unit according to Claim 6, incorporating an insulating support for the heater element sealed within a metal casing having a base and a peripheral wall, the mounting plate being secured to the casing wall.

8. An electric heater unit according to Claim 5, incorporating an insulating support for the heater element sealed within a metal casing having a base and a peripheral wall, wherein the sensor is located outside the metal casing.

9. An electric heater unit according to Claim 5, in which the said element is a booster element arranged to be switched off when the sensor reaches a predetermined temperature corresponding to a cooking temperature of the hob, and further comprising a cooking element which is energised independently of the sensor for maintaining a desired cooking temperature.

10. Control means for an electric heater unit, substantially as described herein with reference to the accompanying drawings.

11. An electric heater unit, substantially as described herein with reference to the accompanying drawings.

12. An electric hob comprising an electric heater unit substantially as described herein with reference to the accompanying drawings.