GB2160681A - Control circuit for water heater - Google Patents

Abstract

A water heater enables small volumes of hot water to be repeatedly taken from the heater, e.g. to make cups of tea. The heater has a comparator (IC1, IC2), an inlet valve (SOL), a heater (E), a level detector (H,L,IC3,IC4) and a thermostat (S) arranged to allow water into the tank if it is not full and the water temperature is above one predetermined temperature, but to prevent further water entry if the water temperature drops below another predetermined temperature. Water is then only allowed to re- enter the heater once the water has been heated so that its temperature again exceeds the one predetermined temperature. <IMAGE>

Description

SPECIFICATION A control circuit The present invention relates to hot water heaters, and in particular, to hot water heaters which can permit hot water to be withdrawn therefrom without substantially reducing the temperature of the remaining water in the heater.

One solution to this problem is that provided in PCT Specification No. AU82/24 which discloses an "instant" heater in which water is only heated when there is a demand for hot water. However, heaters of this type suffer from the disadvantage that they are not able to directly fill a cup with hot water since there is some delay between turning on the outlet tap and receiving heated water.

An alternative arrangement is that disclosed in Australian Patent No. 476,346 in which the mechanical arrangemeng of the heating apparatus results in the desired operation of the heater.

However, the present invention seeks to provide the desired operating characteristics by means of an electrical control circuit which has the advantage that since the control circuit is of relatively low cost construction, it can be used in relation to different water heaters, and the mechanical construction of those heaters can be arranged with a view to minimum cost rather than an intended operating function.

According to one aspect of the present invention there is disclosed a control circuit for a hot water heater, said heater having a heating means, an electrically controlled inlet valve, a water level indicator, and a water thermostat; wherein said control circuit comprises a first switch means arranged to operate said heating means, a second switch means connected to said water level indicator and arranged to operate said inlet valve, and a first comparator having its inputs connected to a reference voltage and to said water thermostat respectively and its output connected to both said first and second switch means; whereby when the temperature of said water is below a first predetermined temperature the output of said first comparator operates said first switch means to activate said heating means, when the temperature of said water exceeds a second predetermined temperature the output of said first comparator operates said first switch means to de-activate said heating means, and when both the water level in said tank is below full and the water temperature exceeds said second predetermined temperature, the water level indicator and said first comparator output operate said second switch means to activate said inlet valve to allow water into said tank until said tank is full or said water temperature falls below said first predetermined temperature.

Preferably the first and second predetermined temperatures are different by several degrees celsius and the output of the first comparator modifies the reference voltage to prevent unnecessary activation of the heating means.

In addition, the water level indicator preferably includes a low level indicator to prevent the heating means from being activated until the predetermined low level of water is in the heater and activate the inlet valve until that predetermined low level of water is achieved.

It will be apparent that the present invention is applicable to water heaters having a substantially simplified construction comprising basically a tank having an inlet with an electrically controlled valve controlling the flow of water into the inlet, an outlet, a heating arrangement which preferably takes the form of an electric element even though the invention is applicable to gas and other types of water heaters, a temperature sensor, and a full and empty water indicator.

An embodiment of the present invention applicable to such a heater will now be described with reference to the drawing which is a circuit diagram of the preferred embodiment of the control circuit.

It will be seen from the drawing that the water heater is provided with an element E connected across a main supply by the normally open contacts of a relay RLY 1. The inlet to the hot water tank is controlled by a solenoid SOL whilst the temperature of the water in the tank is sensed by a sensor S.

Similarly, the water level within the tank is sensed by two resistances H and L which respectively indicate a high or full level or a low or empty level.

A mains transformer TX1 is provided with a centre tapped full wave rectifying secondary winding in order to supply a DC supply voltage for the control circuit. In addition, a low voltage AC supply is provided to the full and empty sensors H and L.

It will be apparent that the sensor resistor S forms one resistance of a bridge circuit formed from resistors S and R1 to R3 with the voltage at the junction of resistors R1 and S being compared in comparator IC1 with a reference voltage A formed at the junction of resistors R2 and R3. A variable resistance is connected in parallel with the resistor S in order to provide for some adjustment of temperature. In addition, the reference voltage A is also applied to the inverting inputs of three other comparators IC2 to IC4 respectively.

The four comparators IC1 to IC4 are provided in a single integrated circuit such as a quad operational amplifier LM324 manufactured by either National or Philips.

The operation of the circuit is as follows.

When the tank of the water heater is initially empty the resistance L is exposed and therefore is at a relatively high resistance. In conse quence a relatively high alternating voltage (typically 3.5 volts peak) is rectified via diode D4 and thereby charges capacitor C4 so as to make the output of comparatorf IC4 go high thereby turning transistor Q4 on and permitting current to flow through the solenoid SOL.

In consequence, water flows into the tank. In addition, the high output of comparator IC4 ensures that transistor Q2 is turned off thereby preventing any current being drawn via the transistors Q1 and Q2 through the coil of relay RLY 1 and thereby ensuring that the element E is also switched off.

When sufficient water has entered the tank to at least cover the resistor L, the alternating voltage appearing at the diode D4 is reduced because of the conductivity of the water in the tank effectively being in parallel with, and thus lowering the magnitude of, the resistance L. Thus capacitor C4 is prevented from being charged to a voltage sufficient to exceed the reference voltage A. In consequence, the output of comparator IC4 goes low thereby turning transistor Q4 off and preventing further water from entering the tank. In addition, since the output of comparator IC4 has gone low, base current will be drawn by transistor Q2 to turn that transistor on, if transistor 01 is permitted to conduct.

It will be apparent that at this time the water level in the tank has not yet reached full and therefore the resistance H is exposed. In consequence, there is sufficient alternating voltage at diode D3 which, when rectified thereby, charges the capacitor C3 to a voltage sufficient to make the output of comparator IC3 high. In consequence, there is base drive for the transistor 01 (through resistor R5) which therefore turns on as does transistor Q2. As a result, current passes through the coil of the relay RLY 1 thereby connecting the heating element E to the mains supply.

As the element E heats the water within the tank, so the temperature of the water rises until it reaches a first predetermined temperature, preferably approximately 97"C, at which temperature the sensor S causes the output of comparator IC1 to go low thereby causing the output of comparator IC2 to toggle low. The temperature of the sensor S will continue to rise until cooled sufficiently by any incoming water.

When the output of comparator IC2 goes low as described above, base current is able to be drawn for transistor Q3 via the resistance R6. In addition, the output of comparator IC3 is high since the water within the tank has not yet reached full and therefore the transitor Q3 turns on supplying the transistor Q4 with base current. Thus transistor Q4 in turn turns on so as to conduct current through the solenoid SOL. In consequence water flows into the tank and continues to do so until the cold water entering the tank reduces the temperature of the temperature sensor S to approximately 97"C. At that time, the output of comparator IC1 goes high thereby causing the output of comparator of IC2 to toggle high so as to cut off the supply of base current to transistor Q3.In consequence, transistor Q3 turns off thereby also turning off transistor Q4 and preventing the flow of current through the solenoid SOL thereby also preventing water flowing into the tank.

The cycle is repeated until, step by step, an amount of water has been allowed into the tank, and all water within the tank has been heated to the higher predetermined temperature.

When the water level within the tank reaches full, the resistance H is no longer exposed in air but immersed in water. The conductivity of the water reduces the effective value of the resistance H. Therefore the magnitude of the alternating voltage appearing at diode D3 is reduced thereby preventing capacitor C3 being charged sufficiently to force the output of comparator IC3 high. However, the output of comparator IC3 will remain high driven from the positive feedback path comprising resistor R7.The non-inverting input of comparator IC3 is driven low momentarily through capacitor C3 each time transistor Q3 is turned off and provided no signal is present from diode D3, the output of comparator IC3 goes low thereby preventing the supply of current to the emitter of transistor 03 which, in consequence, is not turned on when the output of comparator IC2 goes low. Since transistor Q3 does not turn on, transistor Q4 is not turned on and the solenoid SOL is therefore not energised and no further water flows into the tank.

When the output of comparator IC2 next goes low, transistor Qi is allowed to turn off preventing further current from flowing through the coil or relay RLY which therefore opens preventing further heating of the element E.

In addition, it will be observed that when the output of comparator lC2 goes low, because of the presence of resistor R6 the magnitude of reference voltage A is effectively lowered since current is conducted via diode D1 and resistance R6. This changege in magnitude of the reference voltage A introduces hysteresis into the temperature sensor S so as to provide a second predetermined temperature preferably of approximately 94"C to which the temperature of the water must drop before the output of comparator IC1 goes high again.

It will be appreciated that if a small volume of water is drawn from the tank, that water will be at an elevated temperature lying within the range of temperatures determined by the two predetermined temperatures. At that time, the step by step heating and filling cycle is recommenced, thus at all times the water within the tank is substantially maintained between the upper and lower predetermined temperatures. It will be appreciated that this cycle is particularly advantageous for those heaters where there is a demand for small volumes of hot water, for example to make a small number of cups of tea, or similar beverages.

Claims (9)

1. A control circuit for a hot water heater, said heater having a heating means, an electrically controlled inlet valve, a water level indicator, and a water thermostat wherein said control circuit comprises a first switch means arranged to operate said heating means, a second switch means connected to said water level indicator and arranged to operate said inlet valve, and a first comparator having its' inputs connected to a reference voltage and to said water thermostat respectively and its output connected to both said first and second switch means; whereby when the temperature of said water is below a first predetermined temperature the output of said first comparator operates said first switch meansto activate said heating means, when the temperature of said water exceeds a second predetermined temperature the output of said first comparator operates said first swich means to deactivate said heating means and when both the water level in said tank is below full and the water temperature exceeds said second predetermined temperature, the water level indicator and said first comparator output operate said second switch means to activate said inlet valve to allow water into said tank until said tank is full or said water temperature falls below said first predetermined temperature.

2. A control circuit as claimed in claim 1 wherein said predetermined temperatures are different, said second predetermined temperature being less than said first predetermined temperature by several degrees celsius.

3. A control circuit as claimed in claim 1 or 2 wherein the output of said first comparator is connected to said reference voltage via a series connected resistor and diode whereby when the output of said first comparator adopts one of its two logical states, current flows through said series connected resistor to modify said reference voltage the difference in reference voltages corresponding to the difference in said predetermined temperatures.

4. A control circuit as claimed in claim 3 wherein said first comparator comprises a pair of series connected comparators.

5. A control circuit as claimed in any one of claims 1 to 4 wherein said water level indicator has two outputs respectively indicating that the tank is empty or not empty and that the tank is full or not full, the latter of said outputs being connected to said second switch means and the former of said outputs being connected to said first switch means to prevent operation of said heating means with insufficient water in said tank.

6. A control circuit as claimed in claim 5 wherein said water level indicator includes two level indicator resistors each having one end thereof electrically connected to a common potential at which the water in the hot water heater is maintained, immersion of each of said level indicator resistors effectively connecting a resistance formed by the conductivity of the water in said hot water heater in parallel with that level indicator resistor thereby modifying its perceived resistance value.

7. A control circuit as claimed in claim 6 wherein the other end of each of said water level resistors is connected to a corresponding rectifying circuit including a capacitor, one input of a corresponding comparator being connected to said capacitor and the other input of said corresponding camparator being connected to said reference voltage, the outputs of the corresponding comparators constituting the outputs of said water level indicator and the perceived resistance value of each of said level indicator resistors determining the voltage to which the capacior of each rectifying circuit is charged.

8. A control circuit as claimed in claim 7 wherein said heating means is a mains voltage A.C. electric element, and each of said rectifying circuits is supplied with a low voltage A.C. input derived from a step down transformer energised by said mains voltage.

9. A control circuit for a hot water heater, said circuit being substantially as described with reference to the drawings.