GB2135143A - Electric heating appliance - Google Patents

Abstract

The heater Lo of an appliance, particularly a kettle, is turned off by a relay RL controlled by a transistor TR1 when steam is sensed by a temperature sensing diode D7. A boil-dry sensing diode DX may also be provided. The heater Lo is initially energised in response to closure of a push button switch SW1, a capacitor C3 charging more slowly than a capacitor C1 so that there is an irritive delay before transistor TR1 turns on, thereby allowing the power supply for the control circuit to reach a stable state. A permanent connection may be provided instead of switch SW1 so that the circuit is changed from one-shot operation to cycling operation to maintain a selected temperature when the appliance is an electric iron, a water heater, or a cooling appliance such as a fryer. A switch may and cycling operation. A thermistor may be used in an electric iron in place of the temperature responsive has a diode (D11), (Figure 5) to sense water temperature and a temperature selector (R21), the heater Lo being turned off in response to sensor D7 when selector (R21) is at a maximum position and in response to sensor (D11) below the maximum. In other embodiments the relay is replaced by a triac (TH1) or (TH2), (Figure 6,7) triggered at zero crossings. Other types of appliance referred to are spaced heater, toasters and hot plates on cookers. <IMAGE>

Description

SPECIFICATION Electric appliance This invention relates to electronic control circuits for electric appliances and to electric appliances incorporating such control circuits. The invention has particular reference to electronic control circuits for domestic electric appliances, for example electric space heaters, electric water heaters including kettles, electric smoothing irons, toasters and fryers.

It is an object of the present invention to provide an electronic control circuit that is reliable in operation, and able to respond to one or more different control conditions.

According to the present invention, an electronic control circuit for an electric applicance includes a first switching device for controlling the application of electric power to the appliance and a switching circuit for actuating the first switching device to a conducting condition, the switching circuit being such as to provide a potential to produce actuation of the switching device to the conducting condition via a second switching device under the control of a temperature responsive device.

In one embodiment of the invention, the second switching device is closed after the expiry of a predetermined time interval following the establishment of the said potential, control over the second switching device being subsequently exercised by the temperature responsive device.

The switching circuit may include a manually operable switch biassed to an "OFF" position and momentary closure of which establishes the said potential. Subsequently, control of the second switch by the temperature responsive device is such that, when the latter device reaches a preset temperature, power is removed from the appliance which remains unpowered until the manually-operable switch is again momentarily closed.

Alternatively, the switching circuit may include an ON/OFF switch which may be on the appliance and which is operable by a user of the appliance and movement of which to an "ON" position establishes the said potential. Subsequently, control of the second switch by the temperature responsive device is such that, when the latter reaches a preset temperature, power is removed from the appliance and is re-applied once the temperature of the device falls beiow the preset temperature.

In a further embodiment, the switching circuit may include a multi-position switch with an "OFF" position and capable, in one position, of momentary closure only to establish the said potential, and, in another position of providing an "ON" condition to establish the said potential. Subsequently, control of the second switch by the temperature responsive device depends on the position to which the multiposition has been set. In the one position of the multi-position switch, the temperature responsive device controls the second switch in the manner described above for momentary closure. In the other position of the multi-position switch, the temperature responsive device controls the second switch in the manner described in the immediately preceding paragraph.

In one embodiment of the invention, the switching device comprises a switching relay whose operating coil is energisable by the said potential.

The switching relay may be of the change-over type having a first position in which connection is established via the second switch to the switching circuit, and a second position in which electrical power is applied to the appliance.

When electrical power is applied to the appliance, the relay is held in the second position by a latching circuit.

The switching relay may, alternatively, be a solid state switch for example a triac or a thyristor whose control electrode is connected to the switching circuit.

By way of example only, embodiments of the invention will now be described in greater detail reference to the accompanying drawings of which: Figure 1 is a circuit diagram of a first embodiment, Figure 2 is an end view, with a cover member removed, of a domestic jug kettle incorporating the control circuit of Figure 1, Figure 3 is a section on the iine Ill-Ill of Figure 1, Figure 4 is a plan view of the kettle with a lid and cover member removed, Figure 5, 6 and 7 are circuit diagrams of a second, third and fourth embodiments respectively, of the control circuit.

The control circuit shown in Figure 1 is suitable for controlling resistive loads represented in Figure 1 by block LO. In the example now to be described, the load is the resistive heating element of an electric kettle.

The load LO is powered from the A.C. mains supply which is connected across input terminals L, N. The power supply to the load LO is controlled by a relay RL with a contact member RL1 movable between fixed contacts RL2 and RL3. In the deenergised condition of relay RL, member RL1 is in contact with fixed contact RL2 as shown.

Contact RL2 is connected via a voltage dropping resistor R1, normally open switch SW1 and rectifier diode D1 to the junction of voltage dropping resistors R2, R3 in series connection as shown with rectifying diode D2 and electrolytic capacitor C1 across the load LO. In parallel with the load LO is a visual indicator V in the form of a neon. Alternatively, a light emitting diode may be used instead of the neon and connected at an appropriate position in the control circuit.

Switch SW1 is of the "push-button" type that is to say it releases to the open position shown immediately after actuation.

The junction between resistor R3 and capacitor C1 is joined to one terminal of the operating coil of relay R1 and also to paralleled resistors R4, R5. The other terminal of the relay coil is connected to the collector of NPN transistor TR1 whose emitter is joined to the mains input N. Connected in parallel with the relay coil is a diode D3 which short circuits the coil when the latter is de-energised so preventing transients and possible damage to transistor TR1 arising therefrom.

Where, as referred to above, the visual indicator is an LED, it may be in series connection with the collector of TR1.

In series connection with resistors R4, R5 across capacitor C1 is Zexer diode D4 which acts to provide a smoothed voltage at the junction with resistors R4, R5. Connected to that junction is a voltage dropping resistor R6 which is joined via reverse biassed Zener diode D5 and diode D6 to mains input N. Zener diode D5 has a positive temperature coefficient whilst diode D6 has a negative temperature coefficient which precisely compensates that of diode D5 thereby providing temperature stability for the reference voltage appearing at the junction of resistor R6 with diode D5.

That reference voltage powers circuits the first of which comprises a temperature responsive component shown in Figure 1 as diode D7 and fixed resistor R7 while the other of which includes fixed resistor R8, preset potentiometer R9 and resistor R10. The potential at the junction of D7 with R7 is connected to theinverting input of integrated circuit IC1 - Type LM 311 - which is a voltage comparator.

The voltage comparator is a circuit arrangement which includes an open collector output transistor, an inverting input port, and a non-inverting input port. In operation, the output transistor assumes its high-impedance state when the potential difference between the input ports is such as to make the non-inverting input positive with respect to the inverting input, and the state of the output transistor is reversed on reversal of the potential difference between the output ports. Reversal of the state of the output transistor occurs at about zero potential difference between the input ports but the circuit arrangement is usually such thatit exhibits hysteresis in order to ensure reversal of the output state without oscillation.

The other, non-inverting input of integrated circuit IC1 is connected to the tapping point of potentiometer R9. A capacitor C2 is connected across the input terminals of integrated circuit IC1 for the purposes of A.C. rejection. Resistor R11 provides positive feedback and introduces a degree of hysteresis into the response of integrated circuit IC1.

The output of integrated circuit IC1 is applied to the junction of resistors R12 and R13 as shown.

Resistor R13 is connected to the base of transistor TR1 whose emitter is joined to the mains input N.

Connected in parallel across the base-emitter of transistor TR1 are electrolytic capacitor C3 and a second "push-button" type switch SW2 similar to switch SW1.

To energise the load LO, a user presses pushbutton switch SW1 and the resulting positive potential appearing at the junction of resistors R2 and R3 reverse biasses diode D2 and commences to charge capacitor Cl. At the same time, capacitor C3 starts to charge via resistors R4, R5, R12 and R13. The time constant of the resistor-capacitor combination comprising resistor R12, R13 and capacitor C3 is greater than the time required to establish voltage stability at the junction of resistor R3 and capacitor C1 and across the circuit just mentioned. Consequently, the potential at the junction of resistor R13 and capacitor C3, and thus on the base of transistor TR1, is insufficient to switch the latter into a conducting state.

Capacitor C1 is of a value sufficient to provide a considerable degree of smoothing of the rectified supply but is not so great as to interfere with the operation of the circuit as hereinafter described.

The smoothed potential at junction R3, C1 is reduced in value by the resistor combination R4, R5 and is stabilised by diode D4. The effect of diodes D5 and D6 is, as is explained above, to ensure a stabilised voltage at the junction R4, D5 and thus across the two circuits powered by the stabilised voltage. In that way, a reliable reference voltage for integrated circuit IC1 is obtained.

As capacitor C3 charges, the potential at the junction rises and when the capacitor C3 is charged sufficiently, transistorTR1 conducts over its collector-emitter path thereby allowing current to flow through the coil of relay R1 from capacitor C1.

Contact member RL1 moves over into contact with RL3 thereby applying full mains voltage across load LO and maintaining the coil of relay RL1 energised via diode D2, resistors R2, R3 and conducting transistor TR1.

In the case of a kettle, temperature sensitive diode D7 is so located that when the load LO has heated water in the kettle to boiling, steam flowing over the diode D7 raises the latter to a temperature sufficient to bring the potential at junction D7, R7 to a value greater than that of the tapping point of potentiometer R9 and the output of integrated circuit IC1 switches from high to low impedance thereby effectively grounding the junction of resistor R12, R13 and discharging capacitor C3 via resistor R13.

The grounding of the junction switches transistor TR1 off and the coil of relay RL is de-energised causing contact member RL1 to return to fixed contact RL2 thereby de-energising the load LO. The kettle is now switched off and cannot be switched on again until the user again depresses switch SW1.

Capacitor C1 is fully discharged and the change over contact RL2 also de-energises the integrated circuit IC1 and its associated components which are thereby rendered "safe".

Preset potentiometer R9 is normally set in the factory to allow the potential at junction D7, R7 to cross over that at the tapping point of potentiometer R9whenthewatertemperature reaches 100 C.

It will be appreciated that the initial delay before the coil of relay RL is energised allows the supply potentials for the bridge circuit to stabilise thereby eliminating the risk of spurious response from the integrating circuit.

Switch SW2 is a switch, depression of which by a user grounds the base of transistor TR1 rendering the latter non-conductive and thereby de-energising the load LO. Switch SW2 enables a user to switch off at any time before the normal heating cycle as described above is terminated. The switch SW2 may also be operated by a user to de-energisethe load LO if the control circuit has been inadvertently set into operation.

Conveniently, the control circuit may be mounted on a printed circuit board or thick film hybrid module and will preferably be assembled and listed as a unit before being mounted in the kettle.

The kettle shown in Figures 2,3 and 4 is of the so-called jug variety. The jug 1 is of generally oval shape when seen in plan as in Figure 4 having a pouring lip 2, a handle 3 and a detachable lid 4. The lid has a downwardly extending tongue 5 whose lower extremity 6 marks the maximum ievel to which the jug may be filled.

The handle 3 is hollow, the upper open end being closed by a detchable cover 7 held in place by a screw (not shown) which passes through the cover 7 and into a boss 8.

The jug is of a moulded heat-resisting plastics material, for exam pie, that known under the Trade Mark "KEMATAL", the body of the jug and the handle being of one-piece construction but with a double wall formed adjacent the handle 1 thereby providing a thermally insulating space 9 adjacent the handle 3.

A heating element 10 located adjacent the floor of the jug is carried by a mounting secured in an aperture in the curved wall of the jug below the handle 3. The mounting incorporates a "boil dry" sensor. In the Figure 1 circuit described above, such a sensor may be of the "cycling" type because, on operation, it switches off power to the appliance and to the control circuit thereby de-energising the latter.

The appliance cannot be re-energised until switch SW1 is re-operated.

It is also possible to provide a second temperature responsive device in place of the bi-metal "boil dry sensor". Such a device, referenced DX, is shown in dotted lines in Figure 1 together with its calibrating resistor RX.

The element 10 is separated from the floor of the jug buy a metallic shield 11.

A clamp ring 12 that secures the heating element mounting in position includes an integral extension strip 13 which supports a printed circuit board 14 whose upper edge is located in a support groove 15 moulded internally within the handle 3. The printed circuit board carries the components of the control circuit described above with reference to Figure 1 except the temperature responsive diode D7 which is located beneath the cover 7 being positioned between moulded supports 16,17 and above an inclined trough 18formed in a transverse wall 19.

The diode D7 is joined to the junction between resistors R6, R8 and to resistor R7 by flexible insulated conductors 20 that pass downwardly through the handle 3.

Printed circuit board 14 and the clamp ring 12 are enclosed by a detachable cover 21 shaped to blend smoothly into the contour of the jug. Moulded into the cover 21 is a recessed socket 22 into which extend connector pins by which electrical connection is made to the control circuit and the element 10.

Further moulded recesses 23 each having a hollow cylindrical extension 24 accommodate actuating buttons 25 by means of which a user can actuate switches SW1 and SW2 referred to above. The buttons are suitably identified to enable a user to distinguish the one from the other. The cover 21 is also apertured to accommodate lenses of a clear plastics material through which a user can view the visual indicating device V mounted upon the board 14 and which is energised only when element 10 is energised. One of the apertures 27 is shown in Figure 3.

After the kettle has been filled with a required quantity of water to be heated, the user presses the upper one of the buttons 25 that actuates switch SW1 and the control circuit operates in the manner described above to produce energisation of the heating element 10. When the water in the kettle boils, steam given off is channelled along the trough 18 by a downwardly extending lip 26 on the cover 7 and over the diode D7 causing de-energisation of the heating element as described above.

It is, of course, possible to locate the diode D7 at positions other than that described above and in which the diode responds to the presence of vapour.

By locating the diode at some other position on the body of the kettle or a position in which the diode responds to the temperature of a liquid in the kettle, control at temperatures other than that of the vapour may be provided. The kettle may then be used to heat liquids to temperatures other than the boiling temperature. Manual adjustment of the potentiometer R9 may also be allowed thereby enabling a user to select such other temperature.

The control circuit as described above with referpence to Figure 1 is of the so-called "one-shot" type that is to say once the diode D7 has responded to a predetermined temperature, the load is deenergised and cannot be re-energised until the control circuit is reset. An electric kettle and an electric toaster are examples of appliances requiring "one-shot" type control.

In an alternative form of control circuit suitable for electrical appliances where a continuously acting form of control is required, switch SW1 is replaced by a "hard wire" or permanent connection shown in dotted lines in Figure 1. In this case, the temperature responsive diode D7 will act to maintain the temperature at a preset value. Additionally, adjustment of the potentiometer R9 may now be under user control. A control circuit modified as just described is suitable for electric irons, electric water heaters, electric cooking appliances and fryers.

A control circuit for a multi-purpose electric appliance requiring both one-shot and continuous operation may incorporate a switch providing, in one position, the one-shot operation of switch SW1 and, in another position, the "permanent" connection for the maintenance of a predetermined temperature.

In the second embodiment shown in Figure 5, components having afunction similar to that of components of the embodiment shown in Figure 1 have the same reference numbers. The embodiment shown in Figure 5 is suitable for incorporation in an electric cooking vessel for example or other liquid container.

The control circuit shown in Figure 5 controls the energisation and de-energisation of an electric heating element of the vessel, the element being shown as block LO. The element LO is powered from the A.C. mains supply which is connected across input terminals L, N. The supply of power to the element is controlled by a relay RL having a contact member RL1 movable between fixed contacts RL2 and RL3. In the de-energised condition of relay RL, member RL1 is in contact with fixed contact RL2 as shown.

Contact RL2 is connected via rectifying diode D1, voltage dropping resistor R1 and normally open switch SW1 to the junction between voltage dropping resistors R3 and rectifying diode D2. In series connection with resistor R3 and diode D2 across the element LO are a further voltage dropping resistor R2 and an electrolytic capacitor C1.

Connected to the junction between resistor R3 and capacitor C1 is voltage dropping resistor R15 which provides a smoothed power supply to the junction of voltage dropping resistor R6 and Zener diode D4 whose anode is connected to terminal N. Diode D4 is thus reverse-biassed and serves to stabilise the voltage at the junction with resistor R6.

In series connection between resistor R6 and input terminal N is a further voltage stabilising circuit comprising Zener diode D5 and rectifying diode D6.

Diodes D5 and D6 have similar but opposite voltage temperature coefficients thereby providing temperature stability.

The fully-stabilised voltage supply available at the junction between resistor R6 and diode D5 powers circuits some containing temperature responsive sensors.

The first of such circuits comprises sensor D7 and resistor R7. A second circuit comprises resistor R8, pre-set potentiometer R9 and resistor R10. The juncton of sensor D7 with resistor R7 is connected to the inverting input of a voltage comparator V forming part of a quad comparator circuit comprising voltage components S, V and D and inverter I.

Comparators S, V and D are similar in operation with comparator ICI. Capacitor C2 connected across the input terminals of comparator V provides A.C.

rejection by decoupling any A.C. in the output of the first bridge circuit.

The quad comparator is powered from the smoothed supply voltage at the junction of resistors R6 and R15.

Athird circuit comprises sensor D11 and resistor Rl 8 and the remaining circuit comprises resistors R19-R24. R21 and R22 are potentiometers whilst series connected resistors R23 and R24 parallel connected across potentiometer R21 provide, with potentiometer R21 a subsidiary circuit whose function will be dealt with below.

The junction of diode D11 with resistor R18 is applied to the non-inverting input of voltage comparator S to whose inverting input is connected the tapping point of potentiometer R21.

The subsidiary circuit also provides a potential at the tapping point of potentiometer R21 and at the junction of resistor R23 with resistor R24. The tapping point is connected to inverting input of comparator D while the junction R23, R24 is joined to the non-inverting input of comparator D.

Decoupling of A.C. across the inputs of comparators S and D is provided by individual capacitors C2 as shown.

The output of comparator S is connected via resistor R26 to the non-inverting inputs of invertor I.

To that same input of invertor I, the output of comparator D is directly connected. The junction between resistors R6 and R15 is also joined to the output of comparator V via resistor R27 and the output of invertor I is also joined to the output of comparator V.

Invertor I is provided with A.C. coupling at its inputs by another capacitor C2.

The second input of inverter I is connected to the voltage stabilised supply at the junction between resistor R6 and diode D5.

One terminal of the operating coil of relay RLl is connected to the junction between resistors R3, R15, while the other terminal of the coil is connected to the collector of NPN transistor TR1. In parallel connection with the terminals of the operating coil is diode D3.

The outputs of comparator S and invertor I are connected to the base of transistor TR1 while the emitter thereof is joined to terminal N of the power supply via Zener diode D10.

Connected between the emitter of transistor TR1 and the output of comparator D is a light emitting diode LD1 in series connection with rectifying diode D9. The outputs of comparators Sand Dare also interconnected via resistor R26.

Joined across the base-emitter of transistor TR1 are series connected resistor R17 and electrolytic capacitor C3, these latter components being parallel connected with resistor R16.

A further resistor R28 in series connection with switch SW2 is joined across capacitor C1.

The sensor D7 is so located in the vessel as to be exposed to steam and water vapour emitted from water in the vessel as the water boils. Potentiometer R9 is preset to a position requiring sensor D7 to attain the temperature of the water vapour as the potential at junction D7, R7 reaches that at the tapping point of potentiometer R9.

Sensor Dll similarto sensor D7 is located in a position in which it is responsive to the temperature of water in the vessel and can be used to control the temperature to which that water is heated. Sensor Dull, also similar two sensor D7, may be accommodated in the wall of the vessel or it may be housed in a probe positioned inside the vessel at a suitable point close to the surface of the water when the vessel is filled. The position of the sensor Dl 1 is chosen so that it provides a reasonably accurate indication of the average temperature of the water in the vessel. Potentiometer R2l is settable by the user to a desired water temperature and may be calibrated in degrees Centigrade and/or Fahrenheit.

In the use of the vessel, switch SW1 is pressed after the vessel has been connected to the A.C.

mains supply and filled with liquid, for example, water, to a desired level.

Momentary closure of switch SW1 allows capacitors C1 and C3 to charge as is described above.

Capacitor C3 charges at a lower rate than capacitor C4thus ailowing the latter to reach full charge first and establish voltage stability at the junction with resistor R3 and thereby a voltage stabilised power supply to the circuits. At the same time, the potential on the base of transistor is insufficient to switch the transistor into a conducting condition until capacitor C3 is fully charged at which time the transistor conduct over its emitter-collector circuit and the operating coil of relay RL is energised.

Contact member RL2 moves into contact with fixed contact RL3 thereby applying full mains voltage to the element LO. The operating coil of relay RL1 is maintained energised via resistor R2, diode D2, resistor R3 and the conducting emitter-collector path oftransistorTRl.

Potentiometer R21 is a user control, providing a range of desired temperature settings below boiling point, and with a "boil" setting providing the maximum desired temperature.

At the "boil" setting, the slider of the potentiometer R21 will be at or near the highest attained potential, that is, the potential as the junction of resistor R20 and the potentiometer R21, creating a condition in which the potential at the inverting input of comparator D is above that of its non-inverting input, and resulting in the output of the comparator D being in its low impedance state. The output condition of comparator S becomes immaterial while comparator D is in its low state because the end of resistor R26 remote from comparator Swill be held at a low potential by comparator D. The inverting input of inverter I is therefore held low by comparator S and invertor I presents a high impedance output to the base of the transistor.

As explained above, at the "boil" setting, the invertor I is held in its high impedance output state, and control of the transistor TR1 is exercised by the comparator V which is also connected to the base of the transistor TR1. The comparator V and its associated circuits operate in the same manner as the comparator IC1,the comparator V presenting its high output state so long as the temperature sensing diode D7 is below the "boil" temperature, the transistorTR1 being turned on by base drive from the supply line by way of resistor R27. The emitter current of the transistor passes to the comparator D by way of the light emitting diode LD1 and the rectifying diode D9. The diode LD1 is energised and signals to the user that the vessel is energised in the boil mode.The output of the comparator V will, as explained above, go to its low impedance state after steam reaches the temperature sensing diode D7 and terminate the operation.

As explained above, the comparator S is ineffective so long as the position of the slider of the potentiometer R21 is such as to hold the comparator D in its low impedance state, but should the slider of the potentiometer R21 be set at or near the low potential end of the potentiometer, i.e. the user has selected a temperature well below the boiling temperature, the input conditions of the comparator D will then be such as to cause the output of the comparator D to achieve its high impedance state, permitting the comparator S to exercise control of the invertor I. The connections of the comparator S to its associated input circuits are such that it provides a low impedance output when the temperature of the liquid in the vessel, as sensed by the diode Dl 1, is below the desired temperature as set by the use of potentiometer R21.The comparator S exercises control of the invertor I by way of the resistor R26 causing the invertor I to go to its high impedance state, permitting the switching-on of the transistor TR1, as explained previously. The compa rator V will also be in its high impedance state and will not interfere with the conditions set by the comparator S.At the same time, conduction of current through the light emitting diode LD1 is limited by the resistor R26 and the light emitting diode LD1 is held extinguished, emitter current for the transistor TR1 flowing mainly through the Zener diode Dl 0. As the temperature rises, the potential at the inverting input of the comparator Swill rise (as the voltage drop across the diode D11 falls with temperature rise in the known manner) and the output of the inverter Swill change to its high impedance state as the desired temperature is attained. The input condition of the invertor I will change with the change in the output condition of the comparator S and the transistor TR1 will be switched off at the desired temperature.The turning off of the transistor TR1 is effective to de-energise the relay RL, which causes contact RL1 to revert to fixed contact RL2 so de-energising the element LO.

As in the control circuit of Figure 1, switch SW2 enables a user to terminate a water heating operation at will. Actuation of switch SW2 effectively short-circuits C1 thereby de-enegising the control circuit and turning-off transistor TR1.

The quad comparator circuit is suitable of type LM339.

It will be appreciated that instead of employing a continuously variable control for the preset water temperature, a number of pre-set temperature positions may be provided. That may be effected by replacing potentiometer R21 with a multi-position switch connected to bring into operation each of a number of fixed resistances of different values.

It will be noted from the above description that the function of the comparator D is to act as a switch to select either the comparator S or the comparator V as the effective controller to the transistor TR1 according to the user control setting R21. Selection between the comparator Sand the comparator V may, of course, be effected by means of a mechanical switch instead of the comparator D. A mechanical selector switch to replace D would, of course, be coupled to any user control such as, for example, the multi-position switch referred to above, in order to provide "automatic" changeover of the comparators S and V as the user operates the temperature-select control. As an alternative modification, a single comparator may be used to perform the functions of the comparators S and V by arranging for cross-over switching to take place at the input of the comparator as the position of the user's temperature setting control moves through the position at which the system would switch automatically between its "boil" mode and its "heat" mode. The cross-over switching referred to may, of course, be effected by either mechanical or electrical switches.

The control circuit described with reference to Figure 5 is of a so-called "one-shot" or single action type. That is to say, at the end of a boiling mode operation or a hot water mode, the element LO is de-energised and remains so until the cycle is reactivated by a user. This is because the control circuit is de-energised when the heating element is de-energised.

In another embodiment of the invention shown in Figure 6, only the heating element or load is de-energised.

As before, the power is suppled via A.C. mains input terminals Land N. Joined across the input terminals are series resistor R25, R26, Zener diode D12 and rectifying diode D12A. In parallel across resistor R25 and diode D12 is an electrolytic capacitor C4 which corresponds with capacitor C4 of the Figure 5 embodiment.

Connected across diode D12 is resistor R27 in series connection with Zener diode D13 and rectifying diode D14. Diodes D13 and D14 correspond with diodes D5 and D6 of the embodiment described above with reference to Figure 5 and operate in like manner.

The junction between resistor R27 and diode D13 provides a source of a stabilised voltage for two potential divider circuits comprising, in one circuit, a sensor D15 and resistor R31, and in the other circuit resistors R28, R29 and R30 of which resistor R29 is a pre-set potentiometer. The potential at the junction R31, D15 relative to the potential at the tap of potentiometer R29 is applied to the input terminals of a voltage comparator D. D5 is part of a quad comparator circuit that also comprises comparator L and zero crossing detectors Z1 and Z2.

The quad comparator is powered from the junction R25, D12 via resistor R32 as shown and its output is connected via diode D16 to the base of transistor TR2 whose emitter-collector circuit is in series connection with resistors R33 and R34 between input terminal Land the junction D12, R26.

The junction between resistors R33, R34 is connected to the gate electrode of a triac TH1 in series connection across the terminals L, N with the heating element LO.

The base of transistor TR2 is joined via voltage dropping resistor R35 to the junction with resistor R25 of diode D12.

Also connected to the stabilised voltage supply point between resistor R27 and diode D12 is one output of voltage comparator L of the quad comparator. The other input to comparator L is connected via capacitor C5 to the junction D12, R26, and via switch SW4 to the junction R25, D12.

The output of comparator L is connected to diode D16 and also via resistor R36 to junction R25, D12.

The output of comparator L is also returned to one of the inputs thereby via resistor R47 as shown. The outputs of comparators D and L are also interconnected.

Also connected to the base of transistor TR2 are the outputs of zero crossing detectors Z1 and Z2. In both cases, one of the inputs of the detectors is connected to the junction R25, D12 via rectifying diode D18 while the other input is joined to tapping points on a voltage dropping chain comprising resistors R37, R38, R39, R40 connected in series between the junction R25, D12 and the junction D12, R26. As can be seen from Figure 6, the junction R37, R38 is joined to the non-inverting input of detector Zl, whilst.junction R39, R40 is joined to the inverting input of detector Z2. Junction R38, R39 is connected to diode D18 to the inverting input of detector Z1, and the non-inverting input of detector Z2.Junction R37, R38 is also connected to the inverting input of detector Z1 via resistor R41.

As in the case in the embodiments described above with reference to Figures 1 and 5, the power supply to the bridge is very stable.

Sensor D5 is a silicon diode as in the case of sensor Dl 1 of the Figure 5 embodiment and has similar characteristics of sensor D11.

The control circuit of Figure 6 can be used on a cooking vessel or other container whose liquid content is to be maintained at boiling temperature for as long as a user requires.

After filling the vessel with the liquid and connecting it to the mains supply, the user depresses switch SW3 momentarily. That action connects the noninverting input of comparator L to the junction of R25 and D12 and capacitor C5 commences to charge positively, but the inverting input of the comparator L is held at the immediately available potential of the junction of R27 and D13 and the comparator L goes to its low impedance state until C5 charges enough to reverse the condition.The output of comparator L then changes from low to high impedance and this via resistors R36 and R50 applies a positive pulse to the positive input of the comparator to hold the latter in its high impedance output condition after the user releases switch SW3. During this short period the voltage sources will have achieved stabilised values.

Capacitor C4 also commences to charge but at a higher rate than capacitor C5 so ensuring a stabilised voltage at junction R27, D13 before capacitor C5 becomes fully charged.

Transistor TR2 is also energised but does not conduct over its emitter-collector circuit until comparator L latches at about the time that capacitor C5 is fully charged. When latching occurs, the virtual earth on the base of the transistor is removed.

The output of comparator D is at high impedance and this allows resistor R36 to raise the potential of the commoned outputs of comparators D and Lto a high impedance condition.

As has been explained above, the outputs of detectors Z1 and Z2 are connected together and the circuits are so arranged that when the mains voltage on terminal L is approximately zero (from either + or - side of actual zero), the outputs of the detectors change to high impedance. At other times, the output of at least one of the detectors is at low impedance.

The simultaneous existence of a high impedance from the outputs of comparators D and L removes the virtual earth from the base of transistor TR2 which conducts. The resultant current flow through resistors R33 and R34 and the gate of triac TH1 switches the latter to conduction and element LO is energised. Detectors Z1 and Z2 are so arranged that triggering current flows to fire the triac just before, through and just after the zero crossing. Thus the triac is maintained in a conducting condition.

However, when sensor SD15 reaches the preset temperature, the potential difference across the inputs of the comparator D is reversed, and the output impedance of the comparator D changes to low impedance and a virtual short circuit is applied, via diode Dl 6, to the base of transistor TR2 which ceases to conduct and triac TH 1 is allowed to turn off at the next zero crossing point.

The control circuit remains powered so that as long as comparator L remains in its high impedance state, the element LO will be re-energised when the temperature of sensor D15 drops below the preset temperature and the potential difference across the inputs of the comparator D is such as to place the comparator D in its high impedance state.

To terminate the heating cycle, it is necessary to depress switch SW4. That action short circuits capacitor C5 and places comparator L in its low impedance state.

Figure 7 shows an embodiment of the invention providing zero crossing detectors Z1, Z2 with biassing resistors similar two those of the embodiment of Figure 6 for controlling a triac TH2.

With reference to Figure 7, it will be observed that triac TH2 and the heating element are series connected across the mains input terminals Land N.

Terminal L is also joined via rectifying diode D1, dropping resistor R1 and switch SW1 to the junction with rectifying diode D18 of electrolytic capacitor C6.

Capacitor C6 is also connected to mains input terminal N. Diode D18 is also joined via resistor R41 to the junction with triac TH 1 of the element LO.

Resistor R41 is a voltage dropping resistor.

Switch SW1 is connected via the primary winding of pulse transformer T1 and voltage dropping resistor R42 that are in series connection with the emitter collector circuit of transistor TR3. The emitter of the transistor is joined to terminal N.

Aflywheel diode D19 is provided in parallel connection with the primary winding of the pulse transformer T1 whose secondary is bridged across resistor R43 one end of which is connected to input terminal I, the other end being connected to the gate of triac TH2.

Switch SW1 is also connected to resistor R44 itself joined via Zener diode D20 to input terminals N.

To the junction R44, D20 are connected resistors R45 and R46. Resistor R45 is series connected with Zener diode D21 and diode D22 across diode D20.

Diodes D21 and D22 have equal but opposite voltage temperature coefficients giving full stabilisation to the potential at junction R44, R46.

That stabilised potential powers two potential divider circuits comprising sensor D23 and resistor R47 in one circuit and resistors R48, R49 and R50 in the other circuit. Resistor R49 is a preset potentiometer whose tap is joined to the non-inverting input of a voltage comparator D. The inverting input of comparator D is joined to the junction D23, R47. The output of comparator D is connected to the noninverting input of a comparator T.

Comparator D forms part of a quad comparator circuit that also comprises voltage comparator T and zero crossing detectors Z1, Z2.

The inverting in put of comparator T is connected to the junction R45, D21 and the non-inverting input is connected to the junction of R46 and a capacitor C7 itself joined to the mains input terminal N.

The output of comparator T is joined to the junction R44, R46 via resistor R51.

The output of comparator T is joined to the base of transistor TR3 and to the mains input terminal N via switch SW5.

Bridged across the input terminals L, N is the series connected combination of resistor R52 and diode D24, the junction between those components being connected as one input to zero crossing detector Z1 and Z2.

A voltage dropping chain comprising resistors R53, R54, R55 connected between the junction R44, R46 provide inputs to detectors Z1 and Z2 as follows.

Junction R53, R54 is connected to the non-inverting input of detector Z1 while junction R54,R55is connected to the inverting input of detector Z2.

These inputs are interconnected by resistors R56 and R57. The inverting input of Z1 and the non-inverting input of Z2 are connected together, and via diode D25, to the junction R44, R46.

Sensor D23 is a silicon diode similar to the silicon diode described above and is positioned so as to be exposed to the temperature to be controlled which might be a boiling temperature or some lower temperature, for example that of a liquid to be heated.

The control circuit of Figure 7 may be used with a kettle or some other vessel which, when it has been filled with water and connected to the mains supply, can be brought into use by depressing SW1.

That action charges capacitor C6 to a voltage determined by resistor R1 to provide power for the control circuit via resistor R44. A fully voltage stabilised supply at the junction R45, D21 is connected to the bridge circuit. Capacitor C7 also commences to charge via resistor R46 but at a slower rate than capacitor C6. When capacitor C7 is fully charged, the output of comparator T changes from a low to a high impedance state. The existence of a high impedance state at the output of comparator T transfers control of transistor TR3 to the zero crossing detectors Z1, Z2, which, as in Figure 6 above, supply pulses to the transistorTR3, which via the pulse transformer T1, provides trigger pulses for the triac TH keeping it in conduction as long as the output of comparator T remains high.

The output of comparator D is, at this time, of high impedance.

The action of the zero crossing detectors Z1 and Z2 is similar to the action of detectors Z1 and Z2 described above in connection with the embodiment shown in Figure 6. Thus at the first zero crossing detected after the switching of comparator T, the output of the zero crossing detectors Z1, Z2 turns on transistor TR3. Currnt flow through the primary of transformer T1 results in current flow through the secondary and consequently through the gate of triac TH1 which turns on so permitting energisation of element LO.

As with the circuit of Figure 6, the zero crossing detectors Z1, Z2 are arranged to fire the triac before, through and after each zero crossing. When the "holding current" of the triac is overcome, the output from the zero crossing detectors terminates.

When sensor D23 reaches the preset temperature, the output of comparator D switches to low impedance and this effectively grounds the non-inverting input of the comparator T. The base of the transistor TR3 is therefore pulled down the transistor ceases to conduct as does triacTHi. Load LO is, therefore, de-energised.

The heating cycle may be terminated by operation of switch SW5 closure of which earths the base of transistor TR3.

A control circuit embodying the invention may also be fitted to an electric cooker or hob to give the so-called pan sensing control for one or more of the heating rings or hot plates. A temperature sensing element may be located so as to contact automatically a pan placed on the ring or plate or the element may be connected to the control circuit via a flexible lead so ailowing the user to locate the sensor as required, for example in thermal contact with the contents of the pan.

In the case of a smoothing iron embodying the continuous control, potentiometer R9 is the control by which the user sets the iron temperature according to the nature of the material to be ironed. Power supply to the iron is controlled by the ON/OFF switch on the socket by which the iron is connected to the mains. In this case, the sensing element will be one able to operate at the higher temperatures required during ironing, for example the element may be a thermistor.

A control circuit embodying the invention may also be fitted to a wall mounted water heater, for example, which would be suitable for several uses requiring heater water, such as boiling eggs and the like.

Claims (30)

1. An electronic control circuit for an electrical appliance, the circuit comprising a first switching device for controlling the application of electrical power to the appliance, and a switching circuit for actuating the first switching device to apply power to the appliance orto disconnect it, the switching circuit being such as to provide a signal to cause actuation of the first switching device via a second switching device under the control of a temperature responsive device.

2. A control circuit as claimed in claim 1 in which the second switching device is actuated after the expiry of a predetermined time interval, to allow the signal to cause actuation of the first switching device, following the appearance of the said signal, and in which control over the second switching device is subsequently exercised by the temperature responsive device.

3. A control circuit as claimed in claim 2 in which the switching circuit includes a manually operable switch, actuation of which establishes the signal.

4. A control circuit as claimed in claim 3 in which control by the temperature responsive device is such that when the latter device reaches a preset tempera ture the first switching device is actuated to prevent the supply of power to the appliance and to disable the second switch.

5. A control circuit as claimed in claim 3 in which control by the temperature responsive device is such that when the latter device reaches a pre-set temperature the first switching device is actuated to prevent the supply of power to the appliance, and such that the first switching device is re-actuated to permit the supply of power to the appliance when the temperature responsive device falls below the pre-set temperature.

6. A control circuit as claimed in any one of claims 2-5 in which the temperature responsive device is part of a potential dividing circuit such that the latter circuit responds when the temperature sensitive element reaches the pre-set temperature.

7. A control circuit as claimed in claim 6 in which the potential dividing circuit is fully enabled shortly before the expiry of the predetermined time delay.

8. A control circuit as claimed in claim 7 in which the switching circuit includes means for supplying power to the potential dividing circuit to enable the latter prior to actuation of the first switching device.

9. A control circuit as claimed in claim 8 in which actuation of the first switching device also results in the supply of power to the potential dividing circuit to maintain the latter in an enabled condition.

10. A control circuit as claimed in claim 8 or 9 in which said power supply means is the source of said signal, the latter comprising an energising potential to actuate the first switching device.

11. A control circuit as claimed in claim 10 in which the first switching device comprises an electro-magnetic relay whose operating coil is energisable by said energising potential.

12. A control circuit as claimed in claim 11 in which the relay is of the change-over type having a first contact position in which connection is establishable via the second switch to the switching circuit, and a second position for establishing the supply of power to the appliance.

13. A control circuit as claimed in claim 10 in which the first switching device is a semi-conductor switching device, the application to which of the signal switches the device into a conducting condition.

14. A control circuit as claimed in any one of the peceding claims in which the second switching device includes a second thermally responsive device connected to control the second switching device and means for setting the second device to respond to a required temperature.

15. An electric control circuit for an electrical appliance, the circuit comprising a first switching device for controlling the application of electrical power to the appliance, a switching circuit for actuating the first switching device to apply power to the appliance or to disconnect it, the switching circuit being such as to provide a signal to cause actuation of the first switching device via a second switching device under the control of first and second temperature responsive devices.

16. A control circuit as claimed in claim 15 in which a user operated control is provided for setting the temperature to which the second temperature responsive device responds.

17. A control circuit as claimed in claim 15 or 16 in which indicating means are provided for indicating when the second switching device is under the control of the first temperature responsive device.

18. A control circuit as claimed in any one of claims 15-17 in which the second switching device is actuated, after the expiry of a predetermined time interval after the appearance of the signal, to allow the latter to cause actuation of the first switching device, and in which control over the second switching device is subsequently exercised by the first or the second temperature responsive device.

19. A control circuit as claimed in claim 18 in which control by the first temperature responsive device is such that, when the latter device reaches a preset temperature, the first switching device is actuated to prevent the supply of power to the appliance and to disable the second switch.

20. A control circuit as claimed in claim 18 in which control by the second temperature responsive device is such that, when the latter device reaches a preset temperature, the first switching device is actuated to prevent the supply of power to the appliance, and such that the first switching device is re-actuated to permit the supply of power to the appliance when the temperature of the second temperature responsive device falls below the preset temperature.

21. A control circuit as claimed in any one of claims 15-20 in which the first and second temperature responsive devices are parts respectively of first and second potential dividing circuits such that the circuits respond when the first and second temperature responsive devices reach preset temperatures.

22. A control circuit as claimed in claim 21 in which the switching circuit includes means for supplying power to the potential dividing circuits to enable the latter prior to actuation of the first switching device.

23. A control circuit as claimed in claim 22 in which actuation of the first switching device also results in the supply of power to the potential dividing circuits to maintain the latter in an enabled condition.

24. A control circuit as claimed in claim 22 or 23 in which the said power supply means is the source of said signal, the latter comprising an energising potential to actuate the first switching device.

25. A control circuit as claimed in claim 24 in which the first switching device comprises an electro-magnetic relay whose operating coil is energisable by said energising potential.

26. A control circuit as claimed in claim 25 in which the relay is of the change-over type having a first contact position in which connection is establishable via the second switch to the switching circuit, and a second position for establishing the supply of power to the appliance.

27. A control circuit as claimed in claim 26 in which the first switching device is a semi-conductor switching device, the application to which of the signal switches the device into a conducting condition.

28. A control circuit substantially as herein described with reference to and as illustrated by Figure 1, or Figure 5, or Figure 6, or Figure 7 of the accompanying drawings.

29. Avessel incorporating a control circuit as claimed in any one of claims 1-28.

30. A vessel substantially as herein described with reference to and as illustrated by Figures 1-4 of the accompanying drawings.