SPECIFICATIONTemperature sensing meansThis invention relates to temperature sensing means and which can be used to control any desired system operable by an electrical or electronic means, or, if required, for giving an audible or visible warning.
There are many applications where a temperature must be detected and some action instigated on its detection. Thus in a domestic application there is the control of electric water heaters such as showers and of a central heating system, and a motor vehicle there is the switching on and switching off of an electric fan in accordance of the temperature of the water in the cooling system.
In more industrial applications there is the control of large industrial boilers, the temperature of the electrolite in electro-plating and control of speed of a chemical reaction, in, say photographic development.
Hitherto, particularly in the domestic field and with motor vehicles, the temperature sensing means has usually taken the form of relatively insensitive thermostats, and there has long been the need for an accurate, faste response temperature sensor. In the industrial field such accurate sensors are available, but tend to be very expensive.
The object of a first aspect of the present invention is to provide an accurate temperature sensing means that has relatively low costs of production.
When considering electric water heaters, such as electrically heated shower units in particular, it is the case that substantially all such units employ an electric resistance heater over which the water flows on its path to a shower head, and normally, the heater has a simple ON/OFF control.
To vary the temperature of the water emitted from the shower head, valve means are normally provided whereby the flow rate of water passing across the heater can be controlled, such that the lower the flow rate the higher the temperature and the higher the flow rate the lower the temperature of emitted water. Traditionally, the unit is connected in the cold water circuit of the dwelling, and this has the inconvenience, and frequently the major disadvantage that when another outlet in the cold water supply line is opened the flow rate of water through the shower unit automatically drops with a consequent sudden, sometimes unacceptable, increase in the temperature of water emitted from the shower head.
The object of a second aspect of the present invention is to provide an electric water heater, particularly for a shower, that avoids the above mentioned disadvantage.
Again when considering electrically heated shower units in particular, there is the requirement of relatively heavy loadings in kilowatts to provide an efficient electric shower, and the need for rapid temperature sensingmeans with equally rapid response to switch onand off an electric heater to compensate for varying inlet water pressure and hence flow rates through the shower.
This problem is complicated by the provisionsof BS 5406 and EN 50.006 dealing with thelimitation of disturbances in domestic electricsupply networks, which place severe restrictions on the number of electronically controlled switching sequences in a given time, and additionally the magnitude of the switched load isrestricted.
The object of a third aspect of the presentinvention is to provide an electric water heater that avoids the above referred to disadvantages.
According to a first aspect of the present invention, a temperature sensing means comprises means for generating a temperature dependent voltage, means for generating a reference voltage, and means for comparing the temperature dependent voltage with the reference voltage to produce an output, the magnitude of which is determined by the amount by which the temperature dependent voltage is above or below the reference voltage. Thus, the invention may use an integrated circuit controlling a transistor and relay in D.C. circuits, and an integrated circuit controlling a 10 triac in A.C.
circuits. The sensing and drive power is preferably derived from the internal reference voltage of one of the integrated circuits.
In one preferred form the device makes use of the known linear temperature dependent characteristics of silicon semi-conductor junctions when a small constant or near constant current is passed through them.
The choice of the correct amount of current is very important, too large a current will cause resistive heating of the junction, too small a current can lead to pick up of unwanted signals from adjacent wiring unless costly shielding and filtering is used. A typical value of between 4080 flsA has been found suitable, and the circuit will give consistent results between -600 to + 1 800C with a voltage drop across the junction in the region of 2.5-3 MVOC, the actual value for a given semiconductor being dependent on the amount ofcurrent passing through the junction.
Preferably, the semi-conductor junction is formed by a diode, although reverse biassed zenerdiodes, transistors, or possibly temperaturesensing integrated circuits could be used. In thebasic circuit the cathode end of the diode is heldat a constant voltage, and as the resistance of thediode changes with temperature, the voltage at the anode end must increase or decrease accordingly. This voltage is compared with a preselected voltage, e.g., using a voltage comparator, and the output used to drive the control integrated circuit previously mentioned.
Although the linear temperature/resistancecharacteristics of silicon diodes have been known for some time they are not used widely due to calibration difficulties. It is therefore a further feature of the invention to isolate from each other the circuits for setting the maximum and minimum temperatures within which the invention is intended to operate so that movement of one set point has no effect on the other, considerably increasing accuracy and reducing time needed to calibrate.
According to a second aspect of the invention, an electric water heater comprises a housing in which is located an electric resistance heater over which water flows from an inlet to the housing to an outlet from the housing, there being at the inlet, or in the water supply line to the inlet, means to restrict the flow of water into the housing, and there being a means of controlling the electric resistance heating element.
Thus, a pressure reducing valve may be provided at or before the inlet to the housing and when it can be substantially guaranteed that water will flow through the housing at a rate that will not exceed a maximum predetermined rate, irrespective as to the pressure in the mains supply and irrespective of any fluctuations in the pressure in the mains supply. Alternatively a fixed rate flow control valve can be provided to achieve the same result.
The control over the electric resistance heater may be a simple thermostat, but it is preferred to provide a sensitive temperature sensing device in the outlet from the housing, connected to switch means for the heater, whereby the heater can be automatically switched on and off repeatedly and as rapidly as is required to maintain the water temperature at a constant predetermined level.
Thus should the flow rate vary the amount of heat supplied to the water is adjusted in such manner that a substantially constant temperature is maintained at the outlet.
The electric resistance heater would be selected to provide a required maximum temperature within the flow rates up to the maximum dictated by the pressure reducing valve. For the convenience of the user, further control, manually operable, can be provided to set an operating temperature below the limit available from the heater.
As a safety factor it may be desirable to include in the circuit to the heater cut-out means activated by an unacceptable and possibly dangerous rise in water temperature (and which could lead to steam being generated) to cut off current supply to the heater.
According to a third aspect of the invention, an electric water heater comprises means for predetermining a required water temperature, means for detecting any difference between the required water temperature and the actual water temperature at the outlet from the heating means, and a number of individual heating elements adapted for selective switching on or off in accordance with predetermined differences between required and actual water temperature.
Thus, with suitable rapid response temperature differential sensing means at the outlet from the heater, connected, for example, to a conventional multiple logic circuit, programme array logiccircuit or a microprocessor in which are connected the heating elements, one element canbe switched on at the onset of use of the heater,and the logic circuit or the like sequentially switches in or out the elements until the outgoing water temperature equals the demand setting.
The multiple logic circuit of the like will compensate for a change in flow by switching one or more elements on or off in accordance with detected difference between required and actual water temperature.
Thus, a considerable number of small (KW) elements can be provided, but to minimise production costs and the size of the heater, it is preferred to utilise a relatively small number of elements, the ratings of which rise in a binary series, one or more of which may have proportional control.
One embodiment of the invention will now be described by way of example only, in relation to an electrically heated shower unit, as illustrated in the accompanying circuit diagram.
In the circuit diagram, four heater elements are shown, suitably connected in a mains circuit, which heater elements 1 lie in a chamber 2 of a shower unit through which passes water from, e.g., a domestic cold water supply, to an outlet 3 normally provided with a spray head or hose. The heater elements 1 are in fixed relationship to each other in terms of power, and are preferably in the ratio 8:4:2:1. Each element is connected to a multiple logic circuit 4 by a triac 5, which logic circuit is driven by one or more of three integrated circuits 6, 7 and 8.Associated with the integrated circuits 7 and 8 is an adjustment means 9 in the form of a variable resistor to allow control of the output voltages of the integrated circuits 7 and 8, the integrated circuits 7 and 8 being connected to a fourth temperature sensing integrated circuit 10 capable of generating a temperature dependent voltage, which circuit is positioned at the water outlet from the shower unit.
At the inlet to the water chamber 2, there is provided an ON/OFF water valve 11 followed by a reduction valve 1 2 pre-set to allow the passage of water at the level below the lowest pressure that could be expected in the cold water supply, and between the reduction valve 12 and the inlet to the chamber 2 is a pressure switch 13 connected in the mains supply to the heater elements 1 and to a transformer 14 connected in the circuit to the integrated circuits 6, 7 and 8.
Thus, with the adjustment means 9 set to indicate a required temperature by the user, and with the ON/OFF valve 11 opened, water flows into the chamber 2 and the pressure activates the switch 13 to supply mains power to the heaters 1 and the transformer 14 to supply a 12 vDC drive to the integrated circuits.
With the use of appropriate resistances, the voltage outputs from the integrated circuits 7 and 8 are set at a higher and lower level respectively both dictated by the adjustment means 9, to create a "window" to be compared with the voltage output from the temperature sensing integrated circuit 10.
Initially, and with the water supplied at ambient temperature, the comparison between the voltage output from the integrated circuit 10 and the reference voltages from the integrated circuits 7 and 8, shows the water temperature at the outlet from the chamber to be below the user's requirement, and the multiple logic circuits are operated to send a buffered signal to the triacs 5 to switch in all the available heaters. The water temperature rapidly rises, and as it reaches the required temperature, the logic circuits switch in and out the individual heater elements until a single or combination of heater elements are switched on to maintain the temperature of the water at the outlet at the required level.
If, during use, the user alters the required water temperature, to a degree to take the temperature dependent voltage outside the "window" between the reference voltages, the logic circuits are immediately activated to send buffered signals to the triacs to switch in and/or out of the circuit until a further one or combination of heaters are switched on to maintain the new required temperature. Much the same sequence operates should there be an unexpected fluctuation in mains supply or water pressure.
If, due to whatever reason, the water temperature surpasses a predetermined maximum, the integrated circuit 6 signals the logic circuits to cut out all the heaters at that time in operation, and the whole system can be such that the equipment is automatically re-activated when the temperature dependent voltage falls to within the "window" between the reference voltages, or can be such as to result in a complete shut down requiring manual re-activation, after checking the cause of the unacceptable rise in water temperature.