GB2246033A - Control circuit for an electric heating appliance - Google Patents

Abstract

The appliance, particularly an electric grill has heating elements R1, R2 normally energised with half wave rectified A.C. via respective rectifying devices D1, D2 and in order to achieve a rapid warm up the rectifying devices are rendered inoperative for a predetermined period or until a predetermined temperature is reached, whereby each element R1, R2 initially receives both positive and negative half cycles from AC supply LN. in an alternative embodiment, the elements R1, R2 are energised via respective triacs (TR1), (TR2), (Fig 2), which initially each pass both half cycles of the AC supply until a timer (CL) times out whereupon the triacs are controlled so that one passes only positive, and the other only negative half cycles. The timer (CL) is retriggered each time an energy regulator (ER) operates but an enable/disable circuit (E/D) prevents the triacs returning to their full cycle mode unless the output of the regulator (ER) has been zero for at least five minutes. <IMAGE>

Description

A HEATING CIRCUIT This invention relates to a heating circuit and in particular but not exclusively to a circuit for use in an electric grill.

With circuits employing electrical heating elements it is often desirable that when the heating element is initially switched on it reaches its operating temperature in the shortest possible time; this is especially true when the circuit is incorporated in such appliances as electric cooking hobs, electric fires, and electric grills.

The appliances described above have two modes of operation either on or off. In the case of variable setting appliances such as electric hobs then this is achieved by switching between the two states. At equilibrium in the on position the heat radiated from the heating element is equal to the electrical energy input, the resistance of the heating element being chosen such that the applied voltage across the appliance causes the appliance to operate at the desired maximum temperature. It will be realised that when the heating element is initially switched on all the electrical energy input will in the first instance go into heating the element, for the element will at this stage be at equilibrium with its surroundings and will not be radiating any heat.However as the element gets hotter the amount of heat radiated will increase and the rate of increase of its temperature will decrease until equilibrium is reached when the element is at its designed working temperature. It is this that necessitates a fairly long warming up period for the appliance.

In addition to the problem described above the rate of heating is also dependent upon the mass of the heating element, because a substantial amount of energy is required to heat the element to its normal operating temperature especially in the case of mineral insulated elements, traditionally used in electric hobs and grills.

This delay in appliances reaching their operating temperature has long been recognised as a problem and various "super fast" grills and hobs are today in the market place. Such appliances reduce the heating time of the heating elements by replacing the mineral insulated elements with elements having a lower mass, for example Halogen hobs and elements having ceramic cores with low mass wire resistances wound around them. However none of these methods deal with the former problem outlined above, and furthermore they limit the selection of heating elements that the designer may use in such appliances.

According to the present invention there is provided a heating circuit comprising a heating element and, in series with the heating element, rectifying means which can be selectively rendered inoperative. Such a circuit enables rapid warm up of the heating elements since full, unrectified power can be supplied initially, and rectified power to maintain a required temperature thereafter. For example, the above circuit enables a heating element to be used in an appliance with a resistance low enough that without the activation of the rectifying means the element would exceed the designed maximum operating temperature.

However because of the rectifying means reducing the power supplied after an initial "warm up" period the element is designed not to exceed the desired operating temperature.

This enables the element to very rapidly reach its operating temperature before the current to the heating elements is rectified. If the current is half wave rectified then the switching may be performed when half of the energy supplied is being transferred into increasing the temperature of the heating element, this occuring whilst the rate of temperature increase of the element is still relatively high.

Preferably the circuit comprises two heating elements each associated with a respective rectifying means, the two rectifying means being arranged to be rendered inoperative together, and to function in opposite sense. In this way when the rectifying means are operative they respectively rectify different halves of the alternating cycle avoiding unbalancing the power supply and the losses that would be associated with such an unbalanced supply.

The circuit preferably comprises a temperature sensor which determine the temperature of the heating element and controls the period of time for which the rectifying means is inoperative, or alternatively the rectifying means may be rendered inoperative for a predetermined warm up period.

A heating circuit for an electric grill will now be described by way of example only with reference to Figures 1 and 2 of the accompanying drawings in which: Figure 1 shows schematically a simple circuit of the heating circuit; and Figure 2 shows schematically an alternative circuit for the heating circuit.

Referring now to Figure 1 there is shown a circuit comprising live and neutral terminals L and N to be connected across an AC power supply, a switch S1 and a diode D1 connected in parallel and which are in turn connected in series with a heating element R1. These three components are connected between terminals L and N in parallel with an equivalent set of components S2, D2 and R2 similarly arranged.

When it is required for the heating elements R1, R2 to be energised, an AC supply is connected across the terminals L and N and switches S1 and S2 are closed such that R1 and R2 are connected in parallel between the terminals L and N.

The resistive values of R1 and R2 are selected such that a half wave rectified AC signal at the applied voltage heats the elements to the desired operating temperature. However with the switches S1 and S2 closed the unrectified AC signal causes the elements to rapidly reach this operating temperature whereupon switches S1 and S2 are opened such that diodes D1 and D2 half wave rectify the AC supply to the respective heating elements R1 and R2 such that they are maintained at the desired working temperature. D1 and D2 are arranged in opposite sense and do not therefore effect the balance of the alternating supply.

Referring now to Figure 2 there is illustrated a circuit in accordance with the invention for use in an electric grill.

The circuit comprises a live and neutral terminal to be connected across an AC power supply, an energy regulator ER, an enable disable circuit E/D, a forty second timer CL, two identical heating elements of the grill R1 and R2 each having associated with it a respective power triac TR1 and TR2, and a power supply PS for supplying operating power to the enable disable circuit, the timer and a triac firing circuit TFC.

In operation the power is switched on via the energy regulator ER which energisers heating element R1 and R2, the enable/disable circuit E/D and triggers the timer CL. The timer sets the triac firing circuit TFC to its boost position such that unrectified current flows through R1 and R2 allowing them to very rapidly reach their desired operating temperature. After forty seconds the timer CL deactivates the triac firing circuit and the triacs rectify the AC current through the heating element R1 and R2 such that they are maintained at the desired working temperature.

Each time the energy regulator operates, the timer is retriggered. However the triac are not deactivated until the enable circuit returns to enable mode. The output of the energy regulator must be zero for five minutes or more before the enable circuit returns to the enable mode and will allow the timer to operate again, this provision applying even if the main power is switched off. It is therefore impossible for a non-rectified AC supply to be passed through the heating elements R1 and R2 without there first being a period when the circuit is not energised. The period being sufficient to allow the heating elements R1 and R2 to substantially cool such that further boosting will not cause them to exceed the normal desired operating temperature.

Claims (9)

1. A heating circuit comprising a heating element and, in series with the heating element, rectifying means which can be selectively rendered inoperative.

2. Circuit as claimed in claim 1 comprising two heating elements each associated with a respective rectifying means, the two rectifying means being arranged to be rendered inoperative together and to function in opposite sense.

3. A circuit as claimed in claims 1 or 2 comprising a temperature sensor which determines the temperature of the heating elements and controls the period of time for which the rectifying means is rendered inoperative.

4. A circuit as claimed in claim 1 or 2 in which the rectifying means is rendered inoperative for a predetermined warming up period and operative only thereafter.

5. A circuit as claimed in claim 4 wherein the warming up period is between 20 and 60 seconds in duration.

6. A circuit as claimed in any proceeding claim wherein the rectifying means comprises a TRIAC.

7. A heating circuit as claimed in any preceding claim1 in which the heating element is a grill heating element.

8. A circuit substantially as herein before described with reference to figures 1 or 2 of the accompanying drawings.

9. A grill unit comprising a circuit as claimed in any preceding claim.