GB2308023A - Hotplate control circuit providing reduced warm-up time - Google Patents

Abstract

If a heat-setting selector 6 is set to a desired setting at a time when a temperature detector 1 senses that the hotplate is cool, a control circuit 8 causes the hotplate heating element 2 or elements to be energised for a predetermined time in a manner to achieve a fast warm-up. The predetermined time may increase as the selected heat setting increases. If the element 2 is normally duty cycle controlled to achieve the required heat setting, then the element 2 is energised continuously for the predetermined period. If the desired heat setting is achieved by programmed control of one or more of a plurality of heating elements (10,11,12, Fig.2A), then one or more of the heating elements are turned on so that the total energy output during the predetermined period is greater than the programmed output for the selected heat setting. The detector 1 may have a first set of contacts which are used to detect when the hotplate is cool, and are also used to operate a neon indicator 9 to advise a user that the hotplate is still hot, and a second set of contacts which operate at the maximum safe working temperature.

Description

A CONTROL CIRCUIT This invention relates to control circuits for heating elements of hotplates.

In a conventional system, when a user selects a desired heat setting, for example by turning a control knob, a heating element of the hotplate switches on. The current is supplied via a bimetallic strip in the energy regulator, which heats up, bends and eventually opens the circuit contacts, thereby switching off the heating element. With the heating element switched off, the bimetallic strip cools and straightens, causing the contacts to close and switch the heating element on once more. The point at which the contacts open and close is determined by the setting, which therefore controls the duty cycle.

According to the present invention there is provided a control circuit for a heating element of a hotplate, the circuit comprising a heat-setting selector and a hotplate temperature sensor, the circuit being arranged such that, when the selector is set to a desired heating setting whilst the sensor indicates the temperature of the hotplate is below a predetermined value, the heating element is switched on for a predetermined period of time.

By employing the present invention the slow warm-up time associated with conventional energy regulators is avoided for the normal duty is overridden for a predetermined period, which period is preferably dependent on the setting. Therefore a cold pan will be brought to, or near to, the desired temperature as quickly as possible before the control circuit returns the heating element to its normal mode of operation.

Preferably, the invention includes means for visibly warning a user when the hotplate temperature exceeds a certain value. This feature is advantageously incorporated in conventional circuits for glass/ceramic hotplates.

It is also preferable that the control circuit establishes a duty cycle of the heating element which is dependent upon the particular heat-setting chosen, which duty cycle is exceeded by the predetermined time period. On culmination of the predetermined period, the control circuit returns to the duty cycle.

In an alternative embodiment the control circuit may determine a normal energy output of the heating element or heating elements, if more than one is present, which output is dependent on the heat-setting chosen. In this embodiment, the circuit causes the heating element, or elements, to exceed the normal energy output during the predetermined time period.

Both the above described embodiments have the advantage that the control circuit overrides the conventional energy regulator in order to cause the hotplate to quickly warm up, and then returns to the normal mode of operation in order to maintain the hotplate at the desired setting.

The control circuit preferably comprises a limiter, which may incorporate the temperature sensor, which causes the heating element or elements to be switched off should the hotplate temperature exceed a maximum safe working temperature. This is a safety feature, which prevents the hotplate from overheating if no pan is placed on it.

One embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1A is a schematic diagram of a control circuit for a heating element in accordance with the present invention; Figure 1B is a sectional view along the line I-I' of Figure 1A; Figure 2A is a schematic diagram of an alternative circuit for a plurality of heating elements; Figure 2B is a sectional view along the line II-II' of Figure 2A; and Figure 3 is a circuit diagram for the arrangement of Figures 2A and 2B.

Referring to Figures lA and 1B, a temperature limiter, indicated by the reference numeral 1, is shown interposed between heating element 2 and glass/ceramic hotplate 3. Temperature limiter 1 comprises a metal rod 4 within a sleeve 5, the rod being free to expand along the length of the sleeve. The expansion and contraction of the rod opens and closes two sets of contacts at respective different temperatures, the first set at a temperature above a predetermined "cool" temperature where injury is likely to occur by bodily contact with the hotplate 3, and the second set at the maximum safe working temperature of the hotplate. When the hotplate temperature exceeds the cool value, the contacs within limiter 1 open and this is detected by a processing unit 8.In response processing unit 8 activates a visible warning by switching on neon light 9, which informs the user that the hotplate is hot. The light 9 remains on until the temperature sensor indicates the temperature of the hotplate is below the aforementioned predetermined value.

If hotplate 3 is cool whilst control knob 7 of heat-setting selector 6 is rotated to indicate a desired heat-setting, processing unit 8 switches on heating element 2 for a predetermined period of time, the length of which depends upon the heat-setting chosen.

This period of time is designated as a warm-up time, and a typical warm-up time for each heat-setting is shown in the table below. During this period the heating element 2 is on continuously unless the limiter indicates the maximum safe working temperature has been reached, which could occur if a pan was removed, or an empty pan had been placed on the hotplate.

Heat Setting Warm-up Time (s) cold 0 0 1 1 7 10 2 20 3 30 4 40 5 50 6 60 7 70

8 80 9 90 10 100 11 110 12 130 13 140 hottest setting 14 150 Because the element 2 is permanently on during the predetermined warm-up period, hotplate 3 quickly reaches the desired heat level.

After completion of the warm-up period, processing unit 8 operates the heating element 2 at a duty cycle determined by the desired heat-setting.

Again, if at any time limiter 1 indicates that the temperature of the hotplate 3 exceeds a maximum safe working temperature, typically 575 C, processing unit 8 switches off heating element 2.

Referring to Figures 2A and 2B, hotplate 9 is heated by three heating elements 10, 11, 12, the operation of which is described in our co-pending patent application number allocated the reference P/60699/RED in the records of GEC Patent Department. Heatsetting selector 13 comprises control knob 14 which may be turned by the user to the desired heat-setting as before. Processing unit 15 is programmed to control one or more of the heating elements in order to produce a specific energy output for each heat setting. Limiter 16, identical to limiter 1 of Figures 1A and 1B, interposed between heating elements 10-12 and hotplate 9, indicates when the temperature of the hotplate is above the aforementioned predetermined "cool" value, and processing unit 15 switches on neon light 17 as before.If the limiter 16 indicates that the hotplate temperature is below this value, that is to say the hotplate is cool, and the knob 14 is subsequently turned to indicate a desired heat-setting, the processing unit 15 switches on one or more of heating elements 10-12 such that the total energy output during the warm-up time exceeds the normal pre-programmed energy output for that particular setting. In this manner, the hotplate heats up quickly and reaches the desired level of heat during warm-up time as before.

Temperature sensor 16 also again indicates when the hotplate temperature exceeds the maximum safe working temperature, and should this occur the processing unit 15 switches heating elements 10-12 off.

A circuit diagram for the control circuit of Figures 2A and 2B is shown in Figure 3.

Processing unit 13, heat-setting selector 13 and temperature sensor 16 are each connected to a 5V rail. In this diagram, the heat-setting selector 13 is a potentiometer whose output, along with the output of the temperature sensor, is input to processing unit 15. This unit has four outputs, one of which is connected to neon light 17. The remaining three outputs are connected to resistors R " R and R3 respectively and each of these resistors represents the resistance of one of the heating elements 10-12.

Thus, when sensor 16 indicates that the temperature of the hotplate is above the predetermined cool value, processing unit 15 switches on neon light 17. Below this value, the hotplate is cool and the light is switched off. If the potentiometer is adjusted whilst the hotplate is cool so that a component of current is input from the potentiometer to unit 15, the normal pre-programmed settings are overridden and the unit increases the current fed to any one of or a combination of R, R2 and R This increases the overall energy output, which causes the hotplate to attain quickly the required heat level.

The above two examples of the present invention given by way of example only, and it will be appreciated that variations can be made within the scope of the appended claims.

Claims (8)

1. A control circuit for a heating element of a hotplate, the circuit comprising a heatsetting selector and a hotplate temperature sensor, the circuit being arranged such that, when the selector is set to a desired heating setting whilst the sensor indicates the temperature of the hotplate is below a first predetermined value, the heating element is switched on for a predetermined period of time.

2. A control circuit as claimed in claim 1, comprising a visible warning means arranged to operate when the sensor indicates the temperature of the hotplate is above the predetermined value.

3. A control circuit as claimed in claim 1 or 2, wherein the control circuit determines a duty cycle of the heating element in dependence on the heat-setting and wherein the predetermined period associated with a particular heat-setting exceeds the duty cycle for that setting, the control circuit returning to the duty cycle after the predetermined period expires.

4. A control circuit as claimed in claim 1 or 2, wherein the control circuit determines a normal energy output of one or more heating elements in dependence on the heat setting and is arranged such that the energy output during the predetermined period exceeds the normal energy output for that setting.

5. A control circuit as claimed in any one of claims 1 to 4, further comprising a limiter arranged to sense the temperature of the hotplate and switch the heating element off when the temperature exceeds a maximum safe working temperature.

6. A control circuit as claimed in any preceding claim wherein the predetermined period is dependent on the heat-setting.

7. A glass/ceramic hob unit incorporating a control circuit as claimed in any preceding claim.

8. A control circuit substantially as hereinbefore described with reference to, or as illustrated in, the accompanying drawings.