GB2290152A

GB2290152A - Control circuit for a load

Info

Publication number: GB2290152A
Application number: GB9411147A
Authority: GB
Inventors: David Nichols
Original assignee: NOVOMEC Ltd
Current assignee: NOVOMEC Ltd
Priority date: 1994-06-03
Filing date: 1994-06-03
Publication date: 1995-12-13
Also published as: GB9411147D0

Abstract

A load control circuit which in one application is applied to control of an electric water heater, and in this case comprises at least one water level sensor (14, 16), at least one temperature sensor (18), and a microcontroller (22) having an EPROM based CMOS incorporating RAM, ROM, clock and CPU. The microcontroller performs logic operations on signals received from the sensors and generates control signals for output devices (20, 24) controlling water supply to the heater and a water heating element (12). The control signals are fed to the output devices via an optical link. Other applications are control of ventilators and/or blinds, water sprinkling and undersoil heating. <IMAGE>

Description

Control Circuit for a Load This invention relates generally to a circuit for controlling a load. One example given is a circuit for controlling water temperature and water feed in a wall mounted electric kettle.

According to one aspect of the invention there is provided a load control circuit comprising one or more sensors for sensing one or more parameters of a load, a programmable stand alone microcontroller for receiving the output of the sensor or sensors, carrying out logic operations responsive thereto, and outputting control signals for the load as a result of the logic operations carried out, output devices for receiving the control signals and controlling the load accordingly, a regulated power supply for the microcontroller, an insulated transformer through which the mains supply is connectable to the regulated power circuit, and one or more opto-isolators through which the control signals are fed to the output devices.

In this specification, the term "programmable stand gone microcontroller" refers to an EPROM based CMOS which on a single chip incorporates ROM, RAM, clock and input/output, together with a central processing unit.

Relatively small wall-mounted electric kettles are well known for use in kitchens and bathrooms in order economically to provide a ready supply of hot water.

Such kettles are commonly provided with an electric heating element, at least one water temperature sensor, circuit means for switching the heating element on and off responsively to signals received from the temperature sensor or sensors, and a water flow controller for controlling replenishment of cold water to the tank following use.

The present invention has the general object of providing a control circuit for controlling loads of many different types, some of which will be referred to hereinafter.

However, a more specific aim is that of providing an improved control circuit for an electric kettle of the above-described kind. Generally, however, the same control criteria also apply to hot water storage tanks which are able to supply larger quantities of water, and to electric shower units. More, in other disrelated fields, there exist examples of loads one or more parameters of which require to be controlled by electric signals responsively to outputs of one or more sensors which sense the parameter or parameters in question. For example, a greenhouse may have ventilators and/or blinds and/or spinkler systems and/or undersoil heaters to be controlled responsively to temperature and/or humidity sensors, times, and/or the presence or absence of the sun.

According to a second aspect of the invention with respect to the more specific aim, there is provided a control circuit for an electric water heater comprising at least one water level sensor and at least one water temperature sensor, a microcontroller (as hereinbefore defined) for receiving the outputs of the sensors, carrying out logic operations responsive thereto, and outputting control signals for the load as a result of the logic operations carried out, output devices for receiving the control signals and controlling a water heating element and a water feed to the heater accordingly, and a regulated power supply for the microcontroller.

A preferred control circuit for an electric water heater is also in accordance with the first stated aspect of the invention.

Further features of the invention, for convenience given with reference to control of an electric water heater although in many cases of wider applicability, are now described.

Two water level sensors are preferably provided, one being a low water level sensor and the other a high water level sensor. These sensors are preferably electrically conductive rods immersed in the tank of the water heater, and signal contact with the water by completing signal conditioning circuitry which incorporates two transistors respectively switched between conductive and non-conductive states, one responsive to the lower level sensor and the other to the high level sensor. The two transistors, in turn, provide output signals to the microcontroller.

It will thus be appreciated that, in other applications, the control circuit of the invention can be applied more generally to water level control by the use of three or more liquid level sensors.

Low voltage power for the conditioning circuitry, and likewise to the microcontroller, is provided by the regulated power circuit. As this power circuit is insulated from the mains supply, level sensors in direct contact with the water in the tank can safely be employed.

Temperature sensing is preferably by means of a single thermistor suitably positioned accurately to sample the temperature of the medium. The microcontroller, which operates on 16 bit logic, is able to measure the sensed temperature very accurately. It does so by comparing the time required to charge a fixed capacitor through a high stability resistance with the time taken to charge the same capacitor through the thermistor. This reduces the effects of component values and drifts, including supply voltage and system clock varieties. Water temperature can thus be maintained at a required value, typically any value between ambient temperature and boiling, within +1 degree C, with minimal risk of the occurrence of boiling. Adjustment of the controlled water temperature is enabled by adjustment of the value of the aforesaid high stability resistance.Again, in a controller of more general applicability, it will be appreciated that this method of accurately sensing a parameter of the controlled load may be employed.

The microcontroller supplies outputs through status indicator circuitry to two triacs, one controlling switching on and off the heating element immersed in the bottom of the tank, and the other controlling a solenoid which switches on and off a valve controlling water replenishment. The two triacs form part of an output circuit which is electrically isolated from the indicator circuitry, and thus from the thermistor, microcontroller and water level signal conditioning circuitry, by optoisolators. As with the level sensor rods, electrical isolation of the thermistor from the mains supply is also important.

The microcontroller is sufficiently sensitive that it can detect hum inevitably present in the measurement circuitry, particularly the output of the thermistor.

Thus, when the water temperature is at the required value, the water heating element tends to feather in and out of operation, thus minimising risk of boiling due to overshoot when the thermistor calls for more heat due to slight cooling of the water in the tank.

Overshoot when heating from cold can be minimised by programming the microcontroller so that the heating element is temporarily switched off on attainment of a water temperature below the required temperature, for example at about 70 to 75 degrees C, thus allowing the thermistor time to "catch up" the actual water temperature.

The microcontroller is also programmed so that, if the tank is very low in water, below the low level sensor, the heating element is not switched on until the water level has risen to reach the low level sensor.

A preferred arrangement of control circuit in accordance with the invention applied to control of a wall mounted electric kettle is now described by way of example with reference to the accompanying drawings, in which: Figure 1 shows in diagrammatic manner the layout of a wall mountable electric water heater; Figure 2 is a block circuit diagram of the controller shown in Figure 1; and Figure 3 is a flow chart appertaining to the micro controller program.

Referring first to Figure 1, the illustrated water heater comprises a tank 10 containing an electric water heating element 12, water level sensing rods 14, 16, one for low level and one for high level, a thermistor 18 fixed in a position on the outside of the tank providing maximum sensitivity, a mains electricity connector 20 through which power is supplied to the heating element when called for by a controller 22 to which it is connected and to which it also supplies power, a water supply solenoid valve 24, and an overflow 26 associated with an overtemperature cut-out 28.

The present invention resides more particularly in the controller 22, which will now be described with reference to Figure 2.

The controller essentially comprises: - a low voltage regulated power supply shown in the bottom right-hand corner of Figure 2; - a water level input signal conditioning circuit shown in the bottom left-hand corner; - a temperature measurement and stand alone, single chip, programmable microcontroller shown in the top left-hand corner; - status indication circuitry shown in the top centre; and - output drive circuitry shown in the top right-hand corner.

The low voltage regulated output circuitry comprises a double insulated mains transformer 30 providing a voltage rectified by rectifying circuit 32 to a conventional voltage regulating circuit 34. This supplies a regulated low voltage supply on line 36 to the water level input signal conditioning circuitry and to the microcontroller.

The signal conditioning circuitry is connected to the high and low sensing rods in the tank respectively at pins 1 and 3 of terminal block 38. The terminal pins 1 and 3 are connected to respective PNP transistor circuits 40, 42 also each connected to pin 4, which is connected to the wall of the tank and effectively to ground. The transistors are normally biassed off, but are biassed on when a circuit through the base and emitter is completed through water in the tank, thus signalling the microcontroller from the collector that the water level is below the bottom of the low level sensing rod or below or above the bottom of the high level sensing rod. The microcontroller responds logically to the signals received to try to maintain the water level at the level of the bottom of the high level sensor.The main purpose of the low level sensor is to prevent the heating element being turned on when the tank is empty or the water level is very low.

The temperature measurement circuitry receives inputs from the thermistor at pins 2 and 4 of terminal block 44, which are interconnected through a high resistance 46 to obviate risk of mis-operation if the sensed temperature is excessively low. The pins 2 and 4 connect to a capacitive/ resistive measurement circuit 48 which connects to the input side of the microcontroller 50. The programmed microcontroller operates to measure the temperature of the thermistor, and thus of the water in the tank, by comparing the time taken to charge capacitor 52 via fixed resistor 54 and adjustable resistor 56, both of high stability, with the time taken to charge the same capacitor via the thermistor. The logical measurement taken is therefore substantially independent of factors such as component drift.By virtue of the measurement taken, the microcontroller 50 endeavours to maintain the water temperature at a predetermined value, which may be as high as 98 degrees C, but adjustable by means of the resistor 56. Capacitor 57 forms part of a timing oscillator for the microcontroller. A self-explanatory flow chart of the microcontroller program is given in Figure 3.

The status indicating circuitry includes LEDs 58, 60, 52 through which control signals from the output side of the microcontroller are fed to the output drive circuitry.

The indicating circuitry is coupled to the output circuitry through opto-isolators 64, 66, thus ensuring electrical isolation of the microcontroller, thermistor etc. from the heating element, which receives current direct from the mains supply.

The output drive circuitry comprises triacs 68, 70 for controlling the heater supply, fed between terminal blocks 72 and 74, and a solenoid valve controlling the supply of replenishment water, connected between terminal blocks 76 and 74.

Referring back to the microcontroller 50, this operates on 16 bit logic and is therefore extremely sensitive to changes in the water temperature measured by the resistor.

The sensitivity is such as to minimise risk of overshoot which could lead to boiling if the preferred required temperature is as high as 98 degrees C. At this temperature, the heater feathers in and out, which is believed to be due to the fact that the microcontroller detects hum in the measurement circuitry, and particularly in the output of the thermistor. If desired, in order to minimise risk of overshoot when heating from cold, the microcontroller can be programmed to cut out the heater temporarily when a water temperature of about 70 to 75 degrees C has been attained, thus allowing the measured thermistor temperature to catch up the actual water temperature.

It will also be appreciated that the above-described control circuit can be employed in connection with hot water tanks of much larger size, i.e. supplying hot water for a complete house, and in connection with showers using instant hot water.

Moreover, a suitably programmed microcontroller will enable the electrically isolated control circuit to be used with safety for a wide variety of purposes in other fields.

For example, a greenhouse may have ventilators and/or blinds and/or sprinkler systems and/or fan and/or undersoil heating to be controlled responsively to temperature and humidity sensors, a real time clock and/or a solar panel. In such ase, the microcontroller can receive inputs from the sensors, clock and solar panel, and logically interpret the signals received appropriately to control heating, watering and ventilation. More simply, a plant propagator may be analogously controlled.

Further, it will be clear that various types of ventilation and/or refrigerating systems may be controlled by the electricelly isolated control circuit when the microcontroller is suitably programmed. Yet another example is a drinks vending machine, which includes dispensation of hot drinks.

In all such cases, it will be apparent that many features of the illustrated and above-described control circuit can be employed, not only including the means of electrical isolation, but also for example the capacitative means by which accurate parameter sensing is achieved. Amongst many load parameters which may be sensed to enable the microcontroller to control the load are velocity, acceleration, rotational speed, light intensity and many other such parameters, in addition to temperature, humidity, liquid level, and time, as mentioned in examples hereinbefore referred to.

Claims

1. A load control circuit comprising one or more sensors for sensing one or more parameters of a load, a programmable stand alone microcontroller (as herein defined) for receiving the output of the sensor or sensors, carrying out logic operations responsive thereto, and outputting control signals for the load as a result of the logic operations carried out, output devices for receiving the control signals and controlling the load accordingly, a regulated power supply for the microcontroller, an insulated transformer through which the mains supply is connectable to the regulated power circuit, and one or more optoisolators through which the control signals are fed to the output devices.

2. A load control circuit according to claim 1, applied to the control of an electric water heater.

3. A control circuit according to claim 1, applied to the control of greenhouse ventilators and/or blinds, or a sprinkler system or an undersoil heating system.

4. control circuit for an electric water heater comprising at least one water level sensor and at least one water temperature sensor, a microcontroller (as herein defined) for receiving the outputs of the sensors, carrying out logic operations responsive thereto, and outputting control signals for the load as a result of the logic operations carried out, output devices for receiving the control signals and controlling a water heating element and a water feed to the heater accordingly, and a regulated power supply for the microcontroller.

5. A control circuit according to claim 4, wherein two water level sensors are provided, one being a low water level sensor and the other a high water level sensor.

6. A control circuit according to claim 5, wherein the sensors are electrically conductive rods immersed in the tank of the water heater, signal contact with the water being effected by completing signal conditioning circuitry which incorporates two transistors respectively switched between conductive and non-conductive states, one responsive to the lower level sensor and the other to the high level sensor.

7. A control circuit according to claim 6, wherein the two transistors, in turn, provide output signals to the microcontroller.

8. A control circuit according to claim 1 or claim 4, applied to water level control by the use of a plurality of liquid level sensors.

9. A control circuit according to claim 6 or claim 7, wherein low voltage power for the conditioning circuitry, and likewise for the microcontroller, is provided by the regulated power circuit, which is insulated from the mains supply.

10. A control circuit according to claim 4 or any claim appendant thereto, wherein temperature sensing is effected by means of a single thermistor suitably positioned accurately to sample the temperature of the medium.

11. A control circuit according to claim 10, wherein the microcontroller operates on 16 bit logic.

12. A control circuit according to claim 11, wherein the microcontroller acts to compare the time required to charge a fixed capacitor through a high stability resistance with the time taken to charge the same capacitor through the thermistor.

13. A control circuit according to claim 12, wherein adjustment of the controlled water temperature is enabled by adjustment of the value of the aforesaid high stability resistance.

14. A control circuit according to any of claims 4 to 13, wherein the microcontroller supplies outputs through status indicator circuitry to two triacs, one controlling switching on and off the heating element immersed in the bottom of the tank, and the other controlling a solenoid which switches on and off a valve controlling water replenishment.

15. A control circuit according to claim 14, wherein the two triacs form part of an output circuit which is electrically isolated from the indicator circuitry, and thus from the thermistor, microcontroller and water level signal conditioning circuitry, by opto-isolators.

16. A control circuit according to any of claims 4 to 15, wherein the microcontroller is sufficiently sensitive that it can detect hum present in the measurement circuitry, particularly the output of the thermistor.

17. A control circuit according to any of claims 4 to 16, wherein overshoot when heating from cold is minimised by programming the microcontroller so that the heating element is temporarily switched off on attainment of a water temperature below the required temperature, thus allowing the thermistor time to "catch up" the actual water temperature.

18. A control circuit according to claim 17, wherein the microcontroller is also programmed so that, if the tank is very low in water, below the low level sensor, the heating element is not switched on until the water level has risen to reach the low level sensor.

19. A load control circuit substantially as hereinbefore described with reference to the accompanying drawings.