GB2045980A

GB2045980A - Electromagnetic Temperature Control Arrangement for Refrigerators

Info

Publication number: GB2045980A
Application number: GB8009295A
Authority: GB
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1979-03-22
Filing date: 1980-03-19
Publication date: 1980-11-05
Also published as: IT7921201A0; FR2452075A1; DE3010345C2; ES8100473A1; GB2045980B; DE3010345A1; ES489731A0; IT1112399B; JPS55143369A; FR2452075B1

Abstract

In refrigerators having two compartments 2,3 (Fig. 1) for respectively preserving fresh and frozen foods, the frozen food compartment normally held at about -18 DEG C cannot be used to freeze foods because regulations require a considerably lower temperature (e.g. -30 DEG C) that will freeze foods within 24 hours. The temperature control arrangement includes a two-way switch 26 (Fig. 3) which can be set by the user to cause the frozen food compartment to reach such a low temperature that foods can be frozen within a prescribed or determined period. A temperature sensor 14 e.g. an NTC Thermistor is located in the coldest part of the fresh food compartment and is connected in an arm of a bridge circuit means for comparing the compartment temperature with a first or a second reference temperature value set up on pot 25 and R50 and selectable by switch 16. Means e.g. a triac 33 Fig. 3 controls the operation of the refrigeration circuit compressor 6 in dependence upon the result of the comparison. Selection of the first reference temperature signal causes the temperature in compartment 2 to be held at, for example 4 DEG C and hence the temperature in compartment 3 to be held at, for example, -18 DEG C. Selection of the second reference temperature signal causes the temperatures in the compartments 2 and 3 to be held at, for example, 1.5 DEG C and -30 DEG C respectively. A thermistor 15 responsive to evaporator temperature, is connected in a second bridge circuit coupled to amplifier C. This functions to activate the compressor 6 when a given temperature is detected indicating that defrosting by a heater M is completed. The defroster heater may be timer controlled. <IMAGE>

Description

SPECIFICATION Electronic Thermostat Device for Refrigerators This invention relates to an electronic thermostat device for a refrigerator comprising two compartments of different operating temperatures to enable both fresh and frozen foods to be preserved, and a refrigeration circuit comprising a compressor,at least one evaporator and at least one condenser.The device according to the invention is of the type comprising a first probe for continuously determining the temperature in the compartment operating at the higher temperature, and a second probe for continuously determining the temperature of the evaporator in this compartment, together with an electronic control circuit for comparing the signals from the probes with predetermined threshold signals in order to control a switching member connected in series with the compressor, in accordance with the response to this comparison.

A device of this type is described in U.S.A.

patent 3 363 429. The known device is simple and reliable, and has the advantage of being independent of the temperature of the environment in which the refrigerator is situated.

In these known devices, when the first probe detects a determined minimum temperature in the compartment in which it is disposed, it causes the stoppage of the compressor, and when the second probe detects a determined maximum (positive) temperature of the evaporator, it causes the compressor to operate. Preferably, during each halt period of the compressor, an electrical heating element is switched on to melt the frost which has formed on the evaporator. In addition, the temperature at which the compressor and defrosting a heater cut in and out can either be fixed or be adjusted within certain limits by a potentiometer.

With regard to the cutting out of the compressor, the adjustment range generally lies between 1 C and 80C, and is determined by the optimum preservation requirements for the food concerned, without wasting energy and without freezing of liquids.

A refrigerator of the two compartment type controlled by an electronic the thermostat device of the said kind preserve frozen foods at -180C, but cannot freeze foods because regulations require that this takes place at a temperature substantially below this value, so that freezing is complete within a time not exceeding 24 hours.

The object of the invention is to provide an electronic thermostat device which can be set by the user so as to enable the compartment for preserving frozen foods to reach such a temperature that foods can be frozen within a prescribed or determined time period.

This and further objects which will be more apparent from the detailed description given hereinafter are attained according to the invention by an electronic thermostat device, which is characterized in that by contact means a given threshold signal value is set, which is to be compared with the signal from the first probe, so that the switching member and consequently the compressor are deactivated at a temperature exceeding OOC, but less than the highest temperature scheduled for food preservation.

Where the food preservation temperature can be adjusted by a potentiometer the temperature which the user sets by the contact means will be substantially less than the highest temperature which can be set by the potentiometer. If the food preservation temperature is fixed, i.e. set by the refrigerator manufacturer using a fixed resistor, the temperature set by the user with the electrical contact means will be less than this higher temperature set by the manufacturer.

In this manner the user can place in the empty colder compartment a load of foodstuff substantially equal to the capacity of this compartment, and by simply pressing a pushbutton (and thus without adjusting the potentiometer, if this is provided) can freeze the load within a predetermined time, so that the compartment in question is transformed into an actual freezer.

When freezing has taken place, the electrical or electronic contact means can be returned to their initial position by the direct action of the user, who can be advised by a bell operated by a timer and/or by the automatic operation of a timer which is started when the user sets the given threshold value.

In one embodiment of the invention, the contact means are in the form of a diverter switch which replaces the potentiometer or fixed resistor of the circuit in which the first probe is connected, by a different fixed resistor.

The invention will be more apparent from the detailed description given hereinafter by way of non-limiting example, of a preferred embodiment illustrated on the accompanying drawing, in which: Figure 1 is a diagrammatic cross-section through a two door domestic refrigerator; Figure 2 is a block diagram of the electronic thermostat device; and Figure 3 shows the detailed circuit diagram of the device of Figure 2.

The domestic refrigerator of Figure 1 comprises a cabinet 1 with a first compartment 2, the operating temperature of which can either be fixed, for example 40C, or variable; for example by means of a potentiometer over a range of 1 C to 80C. This compartment is used for preserving fresh food. The cabinet also comprises a second compartment 3, the operating temperature of which is much lower, from --180C for preserving already frozen foods, down to for example -300C as in the case of the invention, if it is required to freeze foods. The refrigerator is of the cyclic defrosting type, but its defrosting can also be controlled by a timer.The compartment 2 is closed at its front by a first door 4 and the compartment 3 by a second door 5, from which the "two door" description given to this type of refrigerator derives.

The refrigerator circuit is also of the conventional type, and comprises an encased compressor 6 operated by an electric induction motor enclosed in the same casing, an evaporator comprising a first evaporating portion 7 for the "warmer" compartment 2 and a second evaporating portion 8 for the "colder" compartment 3, a condenser 9, and a capillary tube 10. The two evaporating portions 7 and 8 of the evaporator are connected in series. In Figure 1, the evaporating portions 7 and 8 are shown in the form of metal coils wound, in heat conducting relationship, about the inner surfaces 11 and 12 of the respective compartments 2 and 3.

However, it is apparent that these or even just one of them can be in the form of plates, as is well known to the expert of the art.

The electronic thermostat device according to the present invention comprises schematically a control box 13 which receives the signals originating from two probes 14 and 1 5 through the conductors 1 6 and 1 7 respectively. The control box 13 is connected to the compressor motor 6 through two further conductors 1 8 and 19. As shown in Figure 1, the first probe 14 (conveniently a negative temperature coefficient or NTC thermistor) is positioned in the coldest zone of the compartment 2 which is at the higher operating temperature, for example close to the base of this compartment.The second probe 1 5 (conveniently also a NTC thermistor) is positioned in the evaporating portion 7 for said compartment 2, and preferably at the inlet of said evaporating portion, i.e. where the temperature is lowest.

Consequently, the first probe 14 is used for continuously determining the temperature in the compartment 20f higher operating temperature, while the second probe 1 5 is used for continuously determining the temperature of the evaporating portion 7 for the compartment 2.

Examining the refrigerator control system in detail with reference to Figure 2, the preferred (but not exclusive) embodiment of the electronic thermostat device comprises the two probes 14 and 15, which are connected by the output lines 20, 21 to a comparator bridge circuit 22, whereas their inputs are connected by the line 23 to a supply device 24 in which the mains voltage applied to the terminals A and B is reduced, rectified and stabilised, for example at 12V, using well known components such as diodes, zeners, capacitors and resistors.

A potentiometer 25 is connected between the terminal B and the comparator circuit 22 by means of a diverter switch 26, and is used to set the temperature (from 1 OC to 80C) of the "warm" compartment 2. The potentiometer can be replaced by a fixed resistor if the temperature of compartment 2 is fixed by the refrigerator manufacturer.

By means of the diverter switch 26, the refrigerator user can cut out the potentiometer 25 (or the fixed resistor which replaces it) and in its place connect in a different fixed resistor R50, and thus set a given operating temperature for the compartment 2 in the comparator circuit 22, for example 1 .50C, and consequently a lower temperature in the "cold" compartment 3, for example down to -300C.

The comparator circuit 22 is also supplied by the device 24 through the line 27. The output of the circuit 22 is fed to one of the inputs of a one shot circuit 28 which emits over the line 29 a single signal of given duration each time it receives a signal from a zero crossing circuit 30 through the line 34.

The circuits 28, 30 are supplied by the device 24 through the lines 31, 32 respectively.

The output of circuit 28 is fed to the control electrode of an electrode of an electronic switch 33, for example a triac, connected in series with the compressor 6 and in parallel with the defrosting heater M. The circuit formed by the compressor 6, the defrosting heater M and triac 33 is connected to the mains terminals A, B.

A line 35 connects the zero crossing circuit 30 to an intermediate point K between the triac 33 and compressor 6.

Describing in greater detail the constructional aspect of the device according to the invention with reference to the circuit diagram of Figure 3, it should be noted that: a) the probe 1 5 constitutes an arm of a resistor bridge R10, R1 1 and R9, across the diagonal of which there is connected an operational amplifier C used as a comparator, the output of which is connected through a diode D5 and a resistor R22 to the base E of an NPN transistor Tr 4; b) the probe 14 constitutes an arm of a bridge formed by the resistors R15, R16, R1 7 and the adjustment potentiometer 25 (if connected) or the fixed resistor R50 when the potentiometer is cut out.When the potentiometer is cut out, foods can be frozen in compartment 3 without the temperature in compartment 2 equalling or falling below OOC. An operational amplifier F is connected across the diagonal of this second bridge and is also used as a comparator, its output being connected through the diode D6 to the base E of Tr4 and through the diode D4 to the positive input of the operational amplifier C. The two amplifiers F, C form part of the same integrated circuit; c) the point E constitutes the input to the one shot circuit 28, the output of which is connected by the line 29 to the control electrode of the triac 33. The circuit 28 comprises the transistor Tr4 and the NPN transistor Tr3. The collector of Tr3 is connected to the line 29 and to the voltage divider R7, R4, whose junction is connected to the base E of Tr4. The collector of Tr4 is connected to the supply source through the resistor R3 and the network formed by the resistor R5, the capacitor C3 and the resistor R6. The base of transistor Tr3 is connected to the junction of the resistor R5 and capacitor C3; d) the zero crossing circuit 30, which is connected to the collector of Tr4 through the line 34 and to the point K through the line 35 in which the capacitor C5 and resistor R1 9 are in series, comprises the NPN transistors Trl and Tr2, the collectors of which are connected to the line 34.

In addition, the emitter of one of these transistors, i.e. Tr2, is connected to said line 35, and the base of Tr2 is connected to earth; e) the two operational amplifiers C,- F are subjected to positive feedback through the resistors R49, R60 respectively.

The operation of the device is as follows: It will be assumed that the defrosting heater M is switched on and the compressor 6 is not operating. Under these conditions, the temperature determined by the probe 1 5 rises and consequently the resistance of this latter falls and the voltage at the negative input of the operational amplifier C rises. When a given temperature is reached in section 7 of the evaporator, for example +40C, at which defrosting will have been completed, a logic signal "0" will be present at the output of the operational amplifier C. This signal will also be present at the base E of Tr4, if there is a logic signal "0" present at the output of the amplifier F which has reached this state on termination of the hysteresis effect connected with the moderate positive reaction exerted by the resistor R60.

When this happens, transistor Tr4 becomes unblocked and transistor Tr3 conducts for a time equal to the time constant of C3, R5 for each passage through zero of the signal present at point K. Thus the control electrode of the triac 33 receives trigger pulses through the line 29, compressor 6 operates and the heater M is shortcircuited.

As a result of the operation of the compressor 6, the temperature in compartment 2 falls (as does the temperature in compartment 3).

Because of this temperature reduction, the resistance of the probe 14 increases and the voltage at the negative input of the operational amplifier F therefore falls. Depending on the setting of the potentiometer 25 (which is assumed to be in the circuit), a point is reached at which the output of the operational amplifier F passes to logic level "1". This level is transferred through D4 to the positive input of the operational amplifier C. Consequently, there is also a logic signal "1" at the output of amplifier C. The base E of transistor Tr4 therefore goes to this logic level.

The transistor Tr4 therefore conducts, and as the line 34 is now connected to earth, it inhibits the trigger signal for the monostable circuit 28 from Tr1 or Tr2. Consequently, the triac 33 becomes deactivated, the compressor 6 stops and the defrosting heater M becomes connected in.

Because of the positive feedback exerted on the amplifier C through the resistor R49, the logic signal "1" remains at the output of this latter until the probe 1 5 detects a temperature of for example 40C as indicated previously. This means that the hysteresis relates to such a positive feedback is greater than in the case of the amplifier F.

When the user wishes to freeze foods inserted into the "cold" compartment 3, it is necessary only to disconnect the potentiometer 25 (or the fixed resistor which replaces it) and connect the fixed resistor R50 by means of the diverter switch 26. By doing this, the user sets a particular intervention temperature for the amplifier F, namely an optimum temperature for allowing freezing within a predetermined time. In other words, the compressor 6 will be disconnected, in the manner previously indicated, when the probe 14 detects a different temperature which is greater than OOC and which will enable the compartment 3 to reach a temperature such that the foods will become frozen within the prescribed time.When this freezing has been carried out, i.e. when this time has passed, the user operates the diverter switch 26, so restoring the initial conditions. As shown in Figure 2, the switch 26 can be switched automatically, by means of a timer 40 which is activated when the contact 41 closes. This contact closes as the diverter switch 26 is operated by the user. After the set time, the timer 40 reopens the contact 41 and returns the diverter switch 26 to its initial position.

The definition of a two compartment refrigerator, and thus the scope of the invention, covers all those refrigerators which comprise two compartments at different temperatures in which foods or other items are disposed, whether these compartments lie above or to the side of each other, or whether one of these compartments, namely the colder compartment, lies within the other.

The circuit of Figures 2 and 3 is designed for triacs which require a relatively high current at their control electrode for triggering. In the case of sensitive triacs, i.e. those which require a low current for triggering, it is possible to dispense with the circuits 28, 30 and the line 35, and to connect the control electrode of such a triac to the output of the comparator circuit 22.

If provided with a potentiometer for temperature adjustment, the electronic thermostat device according to the invention can be fitted with a scale in degrees centigrade of Fahrenheit along which the potentiometer adjustment knob with its pointer is moved, as the external temperature does not influence the preservation temperature. This is an obvious advantage over conventional designs in which the scale is graduated in markings such as "high", "medium" and "low", which express relative rather than absolute values. In addition, again where temperature adjustment by means of a potentiometer is provided, the adjustment range is of considerable width, and is independent of the type of two compartment refrigerator to which the device according to the invention is fitted. It is also possible to obtain extremely fine adjustment, even of a fraction of a degree.

Claims

1. An electronic thermostat device for a refrigerator with two compartments of different operating temperatures to enable both fresh and frozen foods to be preserved, and a refrigeration circuit comprising a compressor, at least one evaporator and at least one condenser, and with a first probe for continuously determining the temperature in the compartment operating at the higher temperature, and a second probe for continuously determining the temperature of the evaporator in this compartment, together with an electronic control circuit for comparing the signals from the probes with predetermined threshold signals in order to control a switching member connected in series with the compressor, in accordance with the response to this comparison, characterized in that in order to enable foods placed in the compartment operating at the lower temperature to be frozen, by contact means a given threshold signal value is set, which value is to be compared with the signal from the first probe, so that the switching member and consequently the compressor are deactivated at a temperature exceeding OOC, but less than the highest temperature scheduled for food preservation.

2. A device as claimed in Claim 1, characterized in that the deactivation temperature of the switching member lies between about 1 OC and about 40C.

3. A device as claimed in either preceding Claim, characterized in that the switching member is a triac.

4. A device as claimed in Claim 1, characterized in that the probes are thermistors.

5. A device as claimed in any preceding Claim, characterized in that electronic control circuit comprises comparator means with resistance bridges one arm of each bridge comprising a probe.

6. A device as claimed in Claim 5, characterized in that an operational amplifier used as a comparator is connected across the diagonal of each resistance bridge.

7. A device as claimed in Claim 6, characterized in that the outputs of the operational amplifiers are connected to a logic gate.

8. A device as claimed in Claim 7, characterized in that the output of the logic gate is connected to the control electrode of the triac.

9. A device as claimed in Claim 7, characterized in that, when using a triac requiring a relatively high trigger current, the output of the logic gate is fed to pulse signal generator means associated with means for detecting the passage through zero of a physical quantity fed to the compressor, such that when a given output is present at the logic gate, the pulse signal is fed to the control electrode of the triac each time said quantity passes through zero.

1 0. A device as claimed in any preceding Claim, characterized in that the output of one operational amplifier is connected by way of a diode to an input of the other operational amplifier.

11. A device as claimed in Claim 10, characterized in that the operational amplifier having its output connected to one of the inputs of the other operational amplifier, is connected across the diagonal of the resistance bridge in which the first probe is disposed.

12. A device as claimed in Claim 6, characterized in that the operational amplifiers are subjected to positive feedback such that they present different hysteresis.

13. A device as claimed in any of the preceding Claims, characterized in that the highest temperature scheduled for preserving foods is equal to the maximum temperature obtainable when a potentiometer forming part of the resistance bridge of the first probe is in its end position, and the contact means consist of a diverter switch which cuts out said potentiometer and connects in its place a given fixed resistor corresponding to a lower temperature than the said highest temperature.

14. A device as claimed in any of the preceding Claims, characterized in that the highest temperature scheduled for preserving foods is determined by a fixed resistor forming part of the resistance bridge of the first probe, and the contact means consist of a diverter switch which cuts out said fixed resistor and connects in its place a given fixed resistor corresponding to a temperature lower than the highest preservation temperature.

1 5. A device as claimed in any of the preceding Claims, characterized in that it comprises a timer which acts on the diverter switch upon expiry of a set period of time after actuation of the diverter switch for the purpose of freezing, in order to automatically return it to its initial position.

16. A device as claimed in Claim 1, characterized in that the second probe is disposed in such a position as to determine the temperature at the coldest point of the evaporator in the compartment operating at the higher temperature.

1 7. An electronic thermostat device for a refrigerator, substantially as herein described with reference to the accompanying drawings.