GB2073455A - Electrical power control systems - Google Patents

Abstract

A control system for controlling the supply of electrical power to a load (19) includes a temperature sensor (11) and a temperature dial or setting element (13) operatively connectable to a processor means (10) including a microprocessor. The processor means (10) provides an output signal and operates a load control device (18). The load control device (18) regulates the supply of electrical power from a supply (17) to the load (18) e.g. by means of a triac and a burst-fire system. The sensor (11) is a thermistor and the system is designed to avoid overshoot by adjusting the supplied power in accordance with the rate of change of the sensed temperature and by reducing the applied power as the desired temperature is approached. The system may be applied e.g. to a smoothing iron or to a cooking pan, or to space heating. <IMAGE>

Description

SPECIFICATION Electrical power control systems The present invention relates to a control system and in particular to a control system for electrical energy which is supplied to a load.

The present invention will hereafter be particularly described with reference to domestic appliances such as frypans, irons and the like, although it will be appreciated that it is not thereby limited to such applications. The control system may be readily adapted to other heating applications including room heaters and other appliances, for example saucepans, kettles, jugs, toasters, ovens and the like.

The control system is particularly useful in applications where the electrical energy is supplied to the load in pulsed form or when the energy fluctuates periodically.

Domestic irons typically include a sole plate, a heating element positioned in heat conducting relationship with the sole plate and a control switch to turn the heating element on and off. The control switch commonly has associated with it a bi-metallic thermostat positioned in the sole plate to switch off power to the heating element when a temperature set at the control switch has been reached by the sole plate.

Similarly, frypans and other electrically operated cooking appliances will generally include a base and thermostatically controlled heating element in heat conducting relationship therewith.

Prior art control switches have a disadvantage in that they do not provide adequate control at relatively low temperature settings. Bi-metallic thermostats cause too great an excursion of temperature which causes boiling and then completely off as the element is switched on or off. Prior art control switches also do not adequately control thermal overshoot and undershoot of the sole plate or base temperature associated with the thermal delay of the sole plate or base of the appliance.

It is an object of the present invention to alleviate the disadvantages of the prior art.

The present invention accordingly provides a control system which allows control of temperature over a broad range of settings and a more rapid response. It provides a control means suitable for an iron, frypan or similar appliance adapted to regulate the supply of energy to the appliance.

According to the present invention there is provided a control system for controlling supply of electrical power to a load comprising: temperature sensing means; operator setting means; and processor means to which said temperature sensing means and operator setting means are operatively connectable, said processor means providing an output signal, and load control means operated in response to said output signal to control the supply of electrical power to said load.

The temperature sensing means is adapted to monitor inter alia, the sole plate or base temperature of the appliance. The temperature sensing means may include a sensor in the form of a temperature responsive element preferably comprising a high temperature thermister embedded in heat conducting relationship with the sole plate or base of the appliance. The thermister may have a positive or negative temperature coefficient. Preferably the thermister exhibits a negative temperature coefficient. If the control system of the present invention is adapted to control a room heater or the like, the temperature responsive element may be positioned in any convenient location at which it is desired to monitor changes in temperature.

The temperature sensing may be adapted to measure the temperature at the surface of the sole plate or base. It will be noted in this regard that the temperature variation at the surface of the sole plate or base is smaller than at the temperature sensor due to the largerthermal path. The physical position of the temperature sensor may be selected by experiment in order to minimize thermal delay.

The operator setting means may comprise a control panel including a keyboard and/or a temperature dial element such as a variable potentiometer.

The temperature dial element may enable the operator to select the required operating temperature.

The keyboard may include a plurality of press of touch activatable switch elements which are generally of a low current type. The switch elements may comprise conductive rubber touch pads or touch control electrodes suitable for use with Integrated Circuit elements. An on/off switch for the appliance may be provided on the control panel.

The control system of the present invention may incorporate a timer facility adapted to switch off power to the load automatically after a preset time has elapsed. Where a timer facility is provided the control apparatus preferably includes a manual override facility. The manual override facility may include a manual switch element. Alternate actuation of the manual switch element may be adapted to turn the control system on and off.

The operator setting means thus may include a plurality of timing switch elements. The timing switch elements may be adapted to select timed runs having variable duration.

The control system of the present invention may include display means. The display means may comprise an arrangement of display elements such as a plurality of light emitting diodes (LEDs) to indicate the state of the control system. Preferably at least one of the display element is used to prdvide an intermittent output whenever the temperature is below or above the required setting. The or each display element may provide a constant output whenever the temperature of the appliance reaches the required temperature setting.

In one form, the display element may provide a fast intermittent output whenever the temperature is above the required setting and a slow intermittent output whenever the temperature is below the required temperature. In another form the display elements may provide the same or similar intermittent output when the temperature is above or below the required temperature e.g. a 2Hz flash rate. The display means may be arranged so that it is activated only when the temperature setting is altered. Preferably the display means is activated when the dial element setting is altered at least 5% in value during a given period, say two seconds.

The display element may be illuminated steadily when the sole plate or base temperature reaches the desired setting. Preferably the steady display is not affected by temperature variations in the sole plate or base due to changes in thermal load (e.g. in steaming).

The display means thus may be adapted to provide indication of a temperature which is excessive for a given cooking application (frypan) or a particular fabric (iron). The display means preferably is arranged with the operator control panel for convenience.

The display elements may be associated with the switch elements of the control panel. A display element may be associated with each switch element such that actuation of a switch element causes illumination of an associated display element to indicate that the switch element has been actuated.

The control system of the present invention may include audible warning means. The audible warning means preferably is associated with the visual display means to provide audible warning of the temperature changes of the appliance. The audible warning means may comprise a piezo hooter turned to any convenient audio frequency.

In one form the piezo hooter may be adapted to provide intermittent output at the same or similar rate as the display elements of the visual display means. When the temperature reaches a new setting the piezo hooter may be adapted to emit a tone for a set period e.g. one second.

According to one embodiment of the present invention the processor means includes trigger means adapted to switch the load control means.

According to a further embodiment of the present invention the processor means includes a microprocessor. The microprocessor may include discrete LSI integrated circuits or may comprise a single chip microprocessor.

The processor means preferably incorporates a control unit and a memory store. The processor means may include a read only memory ROM. The microprocessor and ROM may be provided on a single chip microcomputer. In one form of the present invention the microcomputer comprises a four bit single chip device such as a Hitachi circuit type HMCS 42. In another form the microcomputer comprises a four bit National Semiconductor device of the COPS family such as a circuit type COP 311 L.

The processor means may be adapted to receive and process information from the temperature sensing means and the operator setting means. The processor means preferably incorporates an arrangement whereby the sensed temperature is compared on a continuous or recurring basis with the dial element of the setting means.

The control system of the present invention may include a clock means to supply timing pulses to the processor means. The clock means preferably includes a timing circuit such as an RC oscillator circuit connectable to the processor means. In one form the RC circuit may be adapted to provide an oscillator rate of 420KHz. The clock means may include means whereby zero crossings of the power supply may be detected. In one form a 50Hz (or 60Hz) square wave signal may be derived from the power supply for timing and zero crossing reference.

The load control means of the present invention preferably is operably connectable to the processor means. The load control means may be adapted to regulate electrical power to the load, which may comprise a heating resistance element. The proves; sor means may operate the load control means to regulate supply of power to the load. Where electrical power is supplied to the load in the form of pulses, it is preferable that the processor means operates the load control means when available power is at a minimum. This arrangement minimizes switching noise and the use of Ref filters and the like.

The load control means preferably include a solid state switch such as a silicon controlled rectifier or a triac to switch the supply of power to the load. The processor means may be operably connectable to the load control means via an interface means. In one form the interface means may include a switching element such as a transistor.

The processor means of the present invention preferably includes a trigger generator. The trigger generator may be adapted to generate trigger pulses to the load control means. Each trigger pulse preferably is generated to coincide with a zero crossing of the power supply to minimize the aforementioned switching noise. The trigger pulses may be of relatively short duration to minimize power dissipation in the trigger circuit. Preferably each trigger pulse is of 100 IlS duration.

The processor means may be adapted to adjust the load control means by determining the duration that power should be applied to the load in a given control period, say one second. In one form the processor means may be adapted to determine a duty cycle between zero and fifty full cycles of power to be applied to the load each fifty cycles.

The processor means preferably determines the duty cycle by reference to the temperature difference between the temperature sensing means and the temperature dial element. In addition the processor means may determine the duty cycle by reference to the rate at which temperature is changing. By taking into account the rate at which temperature is changing at the temperature sensing means, the control system is able to correct the supply of power to the load in a manner which minimizes the thermal overshoot normally associated with the thermal delay of the sole plate or base. For example when there is a sudden change in load or dial element setting. Response to a sudden temperature drop due to addition of cold food is thus improved.

In one form the processor means may be adapted to calculate the number of cycles of power (duty cycle) to be applied to the load during the next control period as follows: N equals number of cycles (0-50) of power applied to load.

PT equals temperature setting of temperature dial element ( C).

FT equals appliance soleplate or base temperature sensed by temperature sensing means ( C).

(T3) suffix value for next control period.

(T2) suffix value for current control period.

(T1) suffix value for previous control period.

n(T3) equals N(T2) plus A ( PT(T2) - FT(T2) ) - B ( FT(T2) - FT(T1)).

Value of coefficients A and B may be 3 and 2 respectively. However it is preferably to determine these on an emulator to achieve optimum response time without excessive overshoot.

Where application of the control system is to an ironing appliance, thermal overshoot due to the thermal inertia of the sole plate, may be further minimized by reducing power to the load when the temperature of the sensing means approaches the temperature of the dial element setting. In one form the processor means may be adapted to shorten the duty cycle such that energy the load is reduced to 75% of full power when the temperature sensing means reaches 400C below the dial element setting.

When the temperature sensing means reading is approximately 1 0 C below the dial element setting, energy may be applied proportionally.

The processor means may be arranged, particularly where the application of the control system is to an ironing appliance, to shut down power to the load whenever the appliance is in the rest position or is not in use. In this respect it has been found that the change in number of cycles of power to the load in a control period is smaller when the iron is not used irrespective of the type of load, than when it is being used.

The processor means accordingly may include means for detecting that the rate of change in number of cycles of power to the load is below a threshold level. The threshold level may be adjustable by the control panel or it may be fixed by the program.

The processor means preferably is arranged to measure the duration of time that the rate of change of cycles of power to the load is below the threshold level. This may be achieved by means of shut down counter. Each time a change than the threshold level occurs the shut down counter may be reset to zero.

When the change is below the threshold level the shut down counter may be incremented by one each control period until it reaches the shut down count.

The shut down count may be chosen as required.

Preferably the shut down count is 200-300 control periods or approximately four to six minutes.

The processor may be arranged to shut down supply of power to the load whenever the shut down counter reaches the shut down count.

Such an arrangement has the adavantage that if the iron appliance if left unattended or in the rest position the control means can switch off power to the load automatically.

The processor means may additionally be adapted to provide faster shut down whenever the iron appliance is not being used and is in the horizontal position. To this end the control system may include means to detect that the iron appliance is in the horizontal position. The horizontal detector means may comprise a mercury (gravity) switch connected to the processor means. The processor means may be adapted to reduce the shut down count whenever the horizontal detector means signals that the iron is in the horizontal position. In one form the processor means may reduce the shut down count to about 30 control periods or about 30 seconds.

The display means preferably is arranged to provide an indication of shut down. For example, the display elements may provide a short burst of intermittent output followed by a relatively larger pause which may be repeated until the appliance is reset. Preferably the display means is adapted to provide an intermittent output of 12 cycles on, 12 cycles off, 12 cycles on, 50 cycles off. This sequence may be continuous. Where an audible warning means is provided, this may be adapted to provide an audible output at the same rate. The control means may be reset by switching power off and then on again which restores normal operation.

The processor means may be arranged to provide a timer facility as previously described. The time facility preferably is adapted to shut down the load control means after a predetermined time has elapsed. In one form the timer facility may be adapted to disable the trigger generator means after a predetermined time. The timer facility may be engageable by actuating timer switch elements on the operator setting means. Preferably the timer facility is not affected by returning the temperature dial element to its off position. This will turn off power to the load but preferably should not affect the status of the timer or manual override facility.

Each timer switch element preferably is adapted to select a different time duration. The effect of actuating multiple timer switch elements may be cumulative. One a timer switch element has been actuated further actuation of the same switch element preferably has no effect until the set time expires or the manual override facility is actuated.

In one preferred form time switch elements may be adapted to select runs of 1/4 HR. 1/2 HR, 1 HR, and 2 HRS duration respectively. Actuation of all four switch elements thus will result in a total run of 33/4 HRS. The processor means may be provided with a select option to alter these times to 1,2,4 or 8 hours respectively, for room heater application etc.

The display elements of the display means preferably are adapted to display the time remaining before the timer facility automatically shuts down the load control means. In one form the LED elements may be arranged to change state to indicate the time remaining before auto shut down.

The manual override facility may be actuated at any time to deactivate the timer facility. The display means may be adapted to provide an intermittent output when the set time expires to indicate for example, that cooking has finished. The audible warning means may be adapted to provide a burst of audible output at the end of the set time. In one form the piezo hooter may be adapted to provide a tone burst for a duration of one second to indicate that the set time has expired. At the expiration of the time set by the timer facility it is preferably to provide a keep warm facility. A keep warm facility may be provided to maintain the load at a steady elevated tempera ture after the set time has expired. The elevated temperature preferably is kept above 600C to prevent growth of bacteria.

In one form the keep warm facility may be provided by means of a fixed element whose resistance corresponds to the desired keep warm temperature. The processor means may be adapted to "read" the fixed element in place of the temperature dial element.

The temperature sensing means preferably is connectable to the processor means via suitable interface means. The interface means may include an Analodue to Digital (A-D) converter. In one form the A-D converter may comprise an integration capacitor. The capacitor may be charged alternatively by the temperature dial element or the temperature sensing element. Preferably the capacitor is reset by a transistor under the control of the processor means.

The A-D converter may include schmitt trigger circuit. The schmitt trigger may be turned on at the commencement of each charging period and terminated at the schmitt trigger turn off level. This may occur for either the dial element or the temperature sensing element.

An advantage of an A-D converter of the above form resides in that the converter only needs to give repeatable results for two successive control periods in order to determined whether the value of the dial element has been altered. An A-D converter of the above form additionally allows large changes in absolute readings due to long term (greater than two seconds) changes in clock rate and input threshold of the processor means.

The temperature sensing means preferably is arranged such that the temperature dial element and the temperature sensing element are linear and are of equal value at the same temperature.

The control system of the present invention prefer ably includes a shut down feature for safety. The processor means may be adapted to disable the load control means under conditions of overtemperature.

Preferably the load control means is disabled whenever the temperature of the sensing element exceeds 220 C. The load control means may additionally be disabled under conditions of open-circuit temperature sensing element, or where the temperature reading corresponds to a very low temperature, say -1 00C. The display means may be adapted to provide an indication of shut down. In one form the display elements may be adapted to provide an intermittent-outputoftwo cycles of 2Hz then a half second delay repeated continuously. The audible warning means may provide an audible output at the same rate if connected.

The power supply for the control system of the present invention may be provided in any suitable form. Preferably the power supply includes a half wave rectifier and a dropping resistance element direct from the mains. The half wave rectifier preferably includes a series diode. The power supply may include a zener diode and one or more filter capacitance elements.

The control system of the present invention prefer ably incorporates a delay circuit and associated elements to reset the processor means once power to the control system has stabilized.

According to another aspect of the present invention there is provided an energy controller for controlling energy or power supplied to the load by utilizing phase control of the A.C. power supply.

The energy controller may include temperature sensing means and load control means as described above.

The energy controller preferably includes trigger means to actuate the load control means. The energy controller may include operator setting means as described above. The operator setting means preferably comprises a temperature dial element to enable the operator to select the required temperature. Preferably the temperature dial element comprises a variable potentiometer.

The load control means preferably comprises a solid state switch such as a triac connected in series with the load. The gate of the triac may be adapted to receive trigger pulses from the trigger means.

As will be appreciated, timing of trigger pulses to the gate of the triac with respect to the zero crossing of the power supply determines the conduction angle of the triac. Generally speaking the longer the delay of the trigger pulses with respect to the zero crossing the less power is supplied to the load.

The trigger means may be adapted to provide trigger pulses having a delay which is adjustable with respect to the zero crossing. The delay of the trigger pulses may be adjustable by means of the temperature dial element The trigger means preferably includes at least one pulse delay circuit. The delay circuit may comprise a low pass filter or an integrator circuit. Preferably the trigger means comprises a pair of delay circuits. The time constant of each pulse delay circuit preferably is significantly smaller than the duration of each trigger pulse. In one form the time constant of said delay circuits may be 1.5% and 3.3% respectively of the duration of each trigger pulse. Each low pass filter may include a resistance and capacitance element. The resistance element of one of the low pass filter circuits preferably includes the temperature dial element.

The trigger means may include a threshold device.

Preferably the threshold device comprises a diac.

The threshold device may be included to minimize the dependance of the trigger pulses on variation in triac gate characteristics.

A pair of delay circuits may be used in the trigger means to allow the triac to turn on for low conduction angles and minimize the hysterysis effect. The temperature sensing means may be adapted to prevent the load over-heating. The temperature sensing means preferably comprises a thermister having negative temperature characteristics. Preferably the thermister is shunted across the output of one of the pulse delay circuits. The thermister may be adapted to prevent trigger pulses from reaching the load control means when an overtemperature condition exists. The temperature at which trigger pulses are shunted by the thermister may be adjust able by providing an additional resistance element in series with the thermister.

The control system or energy controller of the present invention may conveniently be housed in the handle of the iron, frypan or similar appliance.

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings wherein: Figure 1 shows a functional diagram of a load control system incorporating the present invention.

Figure 2 shows a heating control arrangement for an iron, frypan or similar appliance incorporating a preferred embodiment of the present invention.

Figure 3 shows a functional diagram of an energy controller in accordance with a further aspect of the present invention.

Figure 4 shows in graphical form the temperature/ resistance characteristics of a high temperature thermister.

Figure 5 shows in schematic form a heating control circuit for an ironing or similar appliance, in accordance with preferred embodiment of the present invention.

Figure 6 shows in schematic form a heating control circuit for a frypan or similar appliance in accordance with a preferred embodiment of the present invention, and Figure 7 shows in schematic form a heating control circuit for a frypan or similar appliance in accordance with a further embodiment of the present invention.

Referring to Figure 1 a load control system is shown incorporating a processor means 10, a temperature sensing means 11, operator control means 13, clock means 14, memory store 15, display means 16 and load control means 18. The control system is arranged to regulate the supply of power from supply 17to load 19.

Referring to Figure 2, the preferred heating control arrangement according to the present invention comprises microcomputer 20 and a high temperature thermister 21 connected to the microcomputer 20 via an interface 2lea.

An operator control panel 23, LED display 24 and Hooter 28 are connected to the microcomputer 20 via interface elements 23a, 24a and 28a respectively.

The microcomputer is connected to a triac switch element 25 via interface 25a. The triac switch 25 is connected between the AC mains supply 27 and the heating element 26 of the appliance or heating arrangement.

Referring to Figure 3 an energy controller in accordance with further embodiment of the present invention comprises trigger means 30, temperature sensing means 31, operator setting means 32 and load control means 33. The energy controller is arranged to regulate the supply of power from power supply 34 to load 35. Trigger means 30 receives a synchronizing signal from supply 34.

The heating control circuit of Figure 5 includes a signal chip four bit microcomputer 50. The microcomputer is connected to a suitable power supply via pins 11 and 14. The power supply comprises a half wave rectifier including diode D1, resistor R1 and capacitor C1 zener clamped to - 1 0V via zener diode Z1.

The operator control panel includes temperature setting potentiometer VR1 and a main power switch (not shown).

The display means includes indicator LED diode LD1. LED diode current is limited by resistor R5.

Pin 27 of microcomputer 50 applies trigger pulses to the load control means comprising triac TR1 and driver transistor T1. Trigger pulses approximately 100 uS (1% duty cycle) in duration are applied to the base of transistor T1 via resistor R6 to minimize supply current drain. Trigger pulses are supplied by the microcomputer each zero crossing to minimize switching current and associated RF noise.

Supply of trigger pulses is referenced to a zerocrossover input DO (pin 8) to the microcomputer.

The input reference is derived from a clamping circuit comprising diodes D2, D3 and resistor R4.

Triacswitch element TR1 connects power to Heater Element 51 when it is triggered into conduction by the trigger pulses. Power control is achieved by selecting between 0-05 full cycles each control period. For 50Hz supply the control period is one second and for 60Hz the period is .83 second. The microcomputer is wired to reset at power up.

Internal clock frequency is set by external timing elements comprising resistor R3 and capacitor C3.

No changes to the control programme are required for 50 or 60Hz supply.

The temperature sensing means is shown in the upper right hand portion of Figure 5. The temperature sensing means includes thermister 52 connectable to an Analogue to Digital converter (A-D). The A-D converter comprises a schmitt trigger shown generally at 53, reset transistor T2 and capacitor C2.

The microcomputer compares the resistance of thermister 52 and temperature setting potentiometer VR1. Calibration for resistance VR1 and series resistor R9 is chosen so that the resistance of VR1 + R9 is apprioximately equal to the resistance of thermister 52 at any given temperature.

The temperature control programme is adapted to keep the potentiometer and thermister resistance values equal. The microcomputer adjusts the heat input once every control period. The microcomputer calculates the number of cycles adjustment to the heat input using temperature difference and rate of change of temperature. This procedure compensate for thermal delays between the heater element and thermister and minimizes overshoot where there is a large temperature difference due to a change in potentiometer setting of a sudden change in load.

The A-D converter uses integrating capacitor C2 connected to a negative supply. Capacitor C2 is charged alternatively through potentiometer VR1 + R9 or thermister 52 by selecting pin 19 or 20 repsectiveiy of microcomputer 50. The charged capacitor is reset by transistor T2. Schmitt trigger comprising transistors T3 and T4 and resistors R10-R14 is turned on at the start of each charging period and terminated at the schmitt trigger turn off level. This occurs for either the potentiometer setting or thermister value.

The output of the schmitt trigger is pulse width modulated in accordance with the resistance value of the potentiometer or the thermister. This output appears at the collector of transistor T4 and is connected to pin 7 of microcomputer 50. Successive pulses at this pin input are indicative of the thermister and potentiometer resistance value respectively.

At power on and whenever the potentiometer value changes more than 5% in two control periods LD1 flashes to indicate a temperature difference from the setting.

When the thermistertemperature is too high, LD1 flashed twice each control period i.e. on 13 cycles off 12 cycles. When the thermistertemperature is too low LD1 flashes once per control cycle i.e. on 25 cycles, off 25 cycles. When the temperature reaches the required setting LD1 stays on steadily to indicate correct temperature.

The heating control circuit of Figure 5 incorporates a non-use -shut down facility. Microcomputer 50 is programed to shut down power to the load after a delay of 300 control periods (five minutes) of non-use. Microcomputer 50 incorporates a shut down counter as described above. The shut down counter is reset each time the change in number of cycles of power to the heater element 51 is above a preset threshold. When the change falls below the threshold the counter is incremented by one each control period until it reaches the shut down time (300 periods).

Mercury switch 54 is closed whenever the appliance is placed in the horizontal position. The effect of closing switch 54 is to reduce the shut down time from 300 periods to 30 periods (30 seconds). This latter feature minimizes temperature build up by shutting down power should the ironing appliance be placed face down and left unattended.

Microcomputer 50 additionally incorporates a failsafe shut down feature. At a power on, the microcomputer scans the thermister/capacitor charging time and if well below a value which corresponds to a very low temperature (-1 00C) disables power from the heat element.

A hooter or buzzer 55 is connected to the microcomputer to give audible warning of power shutdown. The buzzer is adapted to provide continuous output if power to the heater element is disconnected by the microcomputer for any reason.

Microcomputer 50 is adapted to reduce power to heater element 51 whenever the temperature sensing means determines a temperature reading appro ximately 40"C below the potentiometer setting.

When the reading is approximately 10 C below the potentiometer setting power is applied proportionally.

The heating control circuit of Figure 6 includes a single chip four bit microcomputer 60. Microcomputer 60 is connected to a suitable power supply via pins marked Vcc and GND. The power supply comprises a half wave rectifier including diode D10 resistor R20 and capacitors C10 and C11 zener clamped to 5.6V by zener diode Z2. Diodes D11 and D12 isolate filter capacitor C10 from (50/60Hz) square wave pulses used for zero crossing input L6 and switching inputs L1-L5 of microcomputer 50 respectively.

The operator control panel includes a plurality of timing switch elements S2-S5 and a manual switch element S6. The control panel further includes temperature setting potentiometer VR2 and a main power switch (not shown). To keep warm (pin G2), 1/4/1 her. timer (pin G1) and 50/60Hz (pin S1) select links may be included on the operator control panel or (preferably) they may be located in a position which is not operator accessible.

Timing switch elements S2-S5 actuate the timer facility of the mirocomputer. Actuation of switch elements S2-S5 selects timed runs of 1/4,1/2,1 and 2 HRS respectively. Actuation of multiple timer switch elements produces cumulative runs i.e. actuation of all four switch elements provides a total runs of 33/4 ~ HRS. By linking pin G1 to ground a quardrupling of possible runs is effected. Selection of this link option alters switch elements S2-S5 to provide timed runs to 1,2,4 and 8 HRS respectively. At the completion of a time run microcomputer 60 automatically disconnects power from Heater Element 61.

The display means includes timing LED diodes LD2-LD5 and status LED diode LD6. Timing diodes LD2-LD5 are illuminated upon actuation of timing switch elements S2-S5 respectively. It should be noted that inputs Ll -L5 of microcomputer 60 are bidirectional inputs. Resistors R22-R26 limit current in LED diodes LD2-LD6 respectively.

Pin G3 of microcomputer 60 applies trigger pulses to the load control means comprising triac TR2 and driver transistor T7. Trigger pulses approximately 100 #S (1% duty cycle) in duration are applied to the base of transistor T7 to minimize supply current drain. Trigger pulses are supplied by the microcom puter each zero crossing to minimize switching current and associated RF noise.

Supply of trigger pulses is referenced to the zero crossings of the power supply by means of zero crossing input L6 to the microcomputer. The input reference is supplied via clipping diode D11 from the half-wave rectified output of power diode D10.

Triac switch element TR2 connects power to Heater Element 61 when it is triggered into conduction by the triger pulses. Power control is achieved by selecting between 0-50 full cycles each control period. As described above the control period is one second for 50Hz supply and .83 seconds for 60Hz supply.

The microcomputer is wired to reset after power up. A reset circuit is provided to reset the microcosms puter after power has stabilized. The reset circuit comprises a delay circuit including resistor R30, capacitor C30 and diode D13. The output of the delay circuit is connected to the RESET pin of microcom puter 60. Internal clock frequency is set by external timing element comprising resistor R31 and capacitor C31. This frequency is set at 420KHz.

The temperature sensing means is shown in the left hand portion of Figure 6. The temperature sensing means includes thermister 62 connected to an Analogue to Digital converter (A-D). The A-D converter includes capacitor C20 and reset transistor T6.

The microcomputer compares the resistance of thermister 62 and temperature setting potentiometer VR2. Calibration for resistance VR2 and series resistor R21 is chosen so that VR2 + R21 is approximately equal to the resistance of thermister 62 and parallel resistor R27 at any given temperature. Diodes D14 D16 have identical characteristics.

The temperature control program is adapted to keep the potentiometer (including series R21) and thermister (including parallel R27) values equal. The microcomputer adjusts the heat input once every control period. The microcomputer calculates the number of cycles adjustment to the heat input using temperature difference and rate of change of temperature as described above. Thermister 62 exhibits a negative temperature coefficient. The preferred resistance/temperature characteristics of thermister 62 are shown graphically in Figure 4.

The A-D converter uses integrating capacitor C20 connected to a positive supply and transistor T5 as a threshold comparator. Capacitor C20 is charged down to ground alternately by potentiometer VR2 + R21 of thermister 62 and parallel R27 by selecting pin D1 or D2 respectively of microcomputer 60. Capacitor C20 is reset to the positive supply by transistor T6 driven by pin L8 of the microcomputer.

The output of the threshold comparator is pulse width modulated in'accordance with the resistance value of the potentiometer or thermister. This output appears at the emitter of transistor T5 and is connected to pin L7 of microcomputer 50. Successive pulses at this pin input are indicative of the thermister and potentiometer resistance values respectively.

At the completion of a timed run microcomputer 60 switches to a "keep warm" adapted to maintain the heater at a constant 650C temperature. This is achieved by choosing a value of VR2 which corresponds to the desired keep warm temperature. When the keep warm routine is selected the A-D converter compares (fixed) resistance VR2 with the thermister 62 (and parallel R27) by selecting pins G2 or D2 respectively of microcomputer 60. If the keep warm feature is not required pin G2 may be linked to ground.

On initial warm up and after a change in temperature setting, status LED LD6 flashes to indicate that the temperature is not yet correct. While warming up or cooling down LD6 flashed at a 2Hz rate. When the temperature reaches a new setting LD6 stays on steady.

A piezo hooter (not shown) when connected to pin SK of microcomputer 60 will sound at the same rate as status LED LD6.

Microcomputer 60 incorporates a safety shut down feature. At power on if the thermister value remains at a value equal to that of R27 (i.e.

thermister open circuit) for 30 seconds, the microcomputer disables power from the heater element.

Power is also disabled if the temperature of the thermister exceeds 220 C.

In either case the triac drive is disabled and status LED LD6 flashes at two cycles of 2Hz followed by a half second delay repeated continuously. If the piezo hooter is connected it is modulated at the same rate as LD6.

Referring to Figure 7 the preferred temperature controller includes a temperature sensing means in the form of thermister 72 and a temperature setting means in the form of potentiometer VR3. Thermister 72 is connected via resistor R73 to a trigger means comprising low pass RC filter circuits. Two RC circuits are used in series to allow greater control at low conduction angles whilst minimizing the hysterysis effect.

The output of the trigger means is connected to a load control means via diac DC1. The load control means comprises a triac TR3. The triac is connected in series with the heater element 71. The gate of the triac receives trigger pulses from diac DC1. Diac DC1 is used to minimize dependance of the trigger circuit on variation in triac gate characteristics.

The trigger means derives trigger pulses directly from the mains power supply. The trigger pulses are delayed by a first delay circuit comprising potentiometer VR3 and series resistor R70 and capacitor C70 and a second delay circuit comprising resistor R71 and capacitor C71. Series resistor R70 is used to protect the potentiometer VR3 by limiting the charging current. Diac DC1 conducts when the voltage on capacitor C71 reaches the diac breakdown voltage.

The capacitor then discharges through the diac to trigger the triac. Because the triac will be triggered with either polarity of gate signal, a similar operation occurs on each half cycle of the alternating mains frequency.

The use of a double RC delay circuit extends the phase angle so that the triac can be triggered at small conduction angles. Hysterysis performance is improved because after the diac turns off, the charge on C71 is partially restored by the charge on C70.

Thermister 72 is placed in heat conducting relationship with the sole or base plate of the appliance. By utilizing a thermister 72 which exhibits a negative temperature coefficient, such as is shown in Figure 4, the thermister will shunt trigger pulses to ground wheneverthe temperature of the thermister exceeds a set level. This level may be raised by increasing the value of R73.

A pair of filter chokes L70 and L71 and capacitor C73 are included in the supply line to minimize RF injection.

Thermister 72 must be capable of operating continuously at 3000C and should have a very low resistance at approximately 250 C.

It will be appreciated that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the present invention.

Claims (45)

1. A control system for controlling supply of electrical power to a load comprising: temperature sensing means; operator setting means; and processor means to which said temperature sensing means and operator setting means are operatively connectable, said processor means providing an output signal, and load control means operated in response to said output signal to control the supply of electrical power to said load.

2. A control system according to Claim 1 wherein said operator setting means includes a temperature dial element for selecting the required operating temperature.

3. A control system according to Claim 2 wherein said temperature dial element comprises a variable potentiometer.

4. A control system according to Claim 1 wherein said temperature sensing means includes a temperature responsive element.

5. A control system according to Claim 4 wherein said temperature responsive element comprises a high temperature thermister.

6. A control system according to Claim 5 wherein said thermister exhibits a negative temperature coefficient.

7. A control system according to Claim 5 wherein said thermister is placed in heat conducting relationship with said load.

8. A control system according to Claim 1 wherein said load control means comprises a solid state switch.

9. A control system according to Claim 8 wherein said solid state switch comprises a triac device connected in series between said supply of electrical power and said load, the gate of said triac device being adapted to receive said output signal.

10. A control system according to Claim 8 where in said output signal comprises trigger pulses adapted to operate said solid state switch.

11. A control system for controlling supply of electrical power to a load comprising: temperature sensing means including a temperature responsive element; operator setting means including a temperature dial element for selecting the required operating temperature; and processor means including a microprocessor to which said temperature sensing means and operator setting means are operatively connectable, said processor means providing an output signal, and load control means operated in response to said output signal to control the supply of electrical power to said load.

12. A control system according to Claim 11 wherein said processor means comprises a microcomputer.

13. A control system according to Claim 11 wherein said processor means is adapted to control supply of electrical power to said load by comparing electrical resistance of said temperature dial element and said temperature responsive element.

14. A control system accordng to Claim 13 wherein said processor means is adapted to maintain the electrical resistance of said temperature dial element and said temperature responsive element substantially equal.

15. A control system according to-Claim 11 wherein said load control means comprises a solid state switch.

16. A control system according to Claim 15 wherein said solid state switch comprises a triac device connected in series between said supply of electrical power and said load, the gate of said triac device being adapted to receive said output signal.

17. A control system according to Claim 15 wherein said output signal comprises trigger pulses adapted to operate said solid state switch.

18. A control system according to Claim 11 wherein said output signal has an adjustable duty cycle.

19. Acontrol system according to Claim 18 wherein a 100% duty cycle corresponds to a control period.

20. Acontrol system according to Claim 19 wherein said processor means calculates the duty cycle for a control period as a function of the difference between the temperature setting of said temperature dial element and the temperature of said temperature responsive element during the preceding control period, and the difference between the temperature of said temperature responsive element during the last two preceding control periods.

21. A control system according to Claim 11 wherein said processor means includes means to disable said output signal after a preset time duration.

22. A control system according to Claim 21 wherein said operator setting means includes at least one switch element adapted to preset said time duration.

23. A control system according to Claim 21 wherein said processor means incorporates a keep warm facility.

24. A control system according to Claim 11 further including a display means operatively connectable to said processor means.

25. A control system according to Claim 24 wherein said display means comprises at least one Light Emitting diode.

26. A control system according to Claim 24 wherein said display means is adapted to provide an intermittent output whenever the temperature sensed by said temperature sensing means is above or below the temperature set by said operator setting means.

27. A control system according to Claim 24 wherein said display means-is adapted to provide a steady output whenever the temperature sensed by said temperature sensing means reaches the temperature set by said operator setting means.

28. A control system according to Claim 11 further including an audible warning means operatively connectable to said processor means.

29. A control system according to Claim 28 wherein said audible warning means comprises a hooter.

30. A control system according to Claim 11 wherein said processor means includes means to disable said output signal whenever the temperature sensed by said temperature sensing means corresponds to a very low or a very high temperature.

31. A control system according to Claim 19 wherein said processor means incorporates a shut down counter, said counter being incremented each time the change in duty cycle between successive control periods is below a threshold limit and reset whenever said change is above said threshold limit, said processor means being adapted to disable said output signal whenever said counter reaches a predetermined number of control periods.

32. A control system according to Claim 31 including at least one switch operatively connectable to said processor means adapted to alter said predetermined number of control periods.

33. An energy controller for controlling supply of Alternating current electrical power to a load comprising: A control system including; temperature sensing means, operator setting means, and trigger means to which said temperature sensing means and operator setting means are operatively connectable said trigger means providing an output signal, and load control means operated in response to said output signal to control the supply of electrical power to said load.

34. An energy controller according to Claim 33 wherein said trigger means comprises at least one delay circuit adapted to delay the alternating current pulses of said electrical power.

35. An energy controller according to Claim 34 wherein said trigger means comprises first and second delay circuits connected in series, the time constant of each delay circuit being significantly less than the period of each said alternating current pulse.

36. An energy controller according Claim 35 wherein each delay circuit comprises an RC integrator circuit.

37. An energy controller according to Claim 36 wherein the resistance element of said first RC circuit includes said temperature dial element.

38. An energy controller according to Claim 37 wherein the capacitance element of said second RC circuit is shunted by said temperature responsive element.

39. An energy controller according to Claim 38 wherein said load control means comprises a solid state switch.

40. An energy controller according to Claim 39 wherein said solid state switch comprises a triac device connected in series between said supply of electrical power and said load, the gate of said triac device being adapted to receive said output signal.

41. An energy controller according to Claim 40 including a threshold device connected between said output signal and said gate.

42. An energy controller according to Claim 41 wherein said threshold device comprises a triac.

43. Ironing orfrypan apparatus including a heater element and a control system for controlling supply of electrical power to said element comprising: temperature sensing means including a temperature responsive element; operator setting means including a temperature dial element for selecting the required operating temperature; and processor means including a microprocessor to which said temperature sensing means and operator setting means are operatively connectable said processor means providing an output signal, and heater element control means operated in response to said output signal to control the supply of electrical power to said heater element.

44. Ironing orfrypan apparatus including a heater element and an energy controller for controlling supply of Alternating current electrical power to said heater element comprising: a control system including; temperature sensing means including a temperature responsive element; operator setting means, including a temperature dial element for selecting the required operating temperature, and trigger means to which said temperature sensing means and operator setting means are operatively connectable said trigger means providing an output signal, and load control means operated in response to said output signal to control the supply of electrical power to said heater element.

45. A control system for controlling the supply of electrical power to a load, the control system being substantially as herein described with reference to the accompanying drawings.