HK1024065A - Electronic thermostat control unit and its use in multipoint temperature controller for refrigeration and heating systems - Google Patents

Description

Thermostat control unit and use in a multipoint temperature controller of a refrigeration heating system

The present invention relates to an electronic digital thermostat control unit and its application in a multipoint temperature controller for a refrigeration and heating system.

The electronic control in the freezing/heating system basically comprises a simple thermostat, a motor start relay, and an overload protection device that controls the motor. Larger models also include a timer and a simple heater control logic (for an automatic defrost function). Some expensive models include one or more solenoids or motors to control the blower/airflow vanes for automatic temperature control in the extra bay units.

Conventional devices for measuring and controlling temperature in a thermostat include:

a gas/liquid filled capillary tube, wherein the expansion/contraction of the gas/liquid is accompanied by a change in temperature, for determining/controlling the temperature.

A bimetal element, wherein the bending/deformation of the bimetal strip of the two metals determines the temperature it senses, since the two metals have very different coefficients of thermal expansion.

A mechanical bellows mechanically actuated by the expanding gas/liquid and sequentially moving the mechanical contacts and activating the electronic circuit at a determined set point.

The bent bimetal strip itself performs the function of a mechanical movement switch to control the electronic circuit.

These conventional methods/devices have the following disadvantages:

a. inaccurate and incomplete sensing of temperature.

b. And low reliability.

There are also electronic digital thermostats available today. Such thermostats have been described, for example, in U.S. patent nos. 5592989; 5528017, respectively; 5520327, respectively; 5329991, respectively; 5107918, respectively; 4948044, respectively; 4799176, respectively; 4751961, respectively; 4663654 and 4638850. However, these electronic digital thermostats use expensive temperature sensors, such as thermistors, thermocouples, or platinum-wire resistance thermometers. These sensors require complex and expensive interface circuitry. This has resulted in the use of thermostats which are undesirable, except for the most expensive chiller models. In addition, many of the benefits of electronic thermostats, such as improved reliability of operation, are not effectively realized when used with conventional start relays, overload protection devices, defrost timers, and the like. Replacing these components with electronic peer devices, or providing energy saving and other end consumer useful functions, has so far proven to be economically viable only on the most expensive chiller models.

The conventional overload protection mechanism is based on one of the following mechanisms:

a. bimetal element, wherein the bending/deformation of the bimetal strip of the two metals determines the temperature sensed by the bimetal element, because the two metals have very different thermal expansion coefficients. The mechanical dimensions and configuration of the bi-metal strip determine the temperature at which thermal tripping occurs to perform the overload protection function.

b. A Positive Temperature Coefficient (PTC) resistive element, the resistance of which increases sharply as the Temperature increases beyond a certain threshold Temperature, so that the resistive element effectively reduces the current in the electronic circuit to an insignificant value.

These methods all have disadvantages. The bimetal overload protection device is a mechanically moving part which interrupts the electronic circuit each time it experiences an arc, causing electronic interference and at the same time causing corrosion of the contacts.

The PTC resistive element is also exposed to constant thermal and cold cycles, causing thermal stress and reduced reliability. While the electronic and temperature characteristics of the PTC element need to be matched to the load in order to produce the correct electronic action. This limits its flexibility and is at best a compromise in efficiency, since an exact match of the PTC characteristic to the load characteristic is almost impossible.

Similarly, conventional methods for providing a starter relay function and related problems are:

a. with a conventional mechanical relay, it suffers from the standard problems of arcing and reduces its reliability due to the use of a moving mechanical contact to connect/disconnect an electronic circuit.

b. A Positive Temperature Coefficient (PTC) resistance element is used which suffers from the same problems a PTC element suffers from in the functioning of the overload protection device.

A conventional thaw timer of a refrigeration system is an electromechanical or electrical timer mechanism. Its reliability is very limited because it has a constantly moving mechanical part and an arcing contact.

In addition to the above problems, conventional electronic controls in refrigeration systems have proven cumbersome and expensive in providing multi-zone temperature control functionality, which is desirable for larger refrigeration systems. In fact, if some energy saving function is desired or a useful feature is provided to the end consumer, it is not practical to use the apparatus of this control mechanism.

It is an object of the present invention to provide an electronic digital thermostat which is cost-effective, safe and reliable to operate.

Another object of the present invention is to provide a single, multipoint, lightweight electronic control unit by using said electronic digital thermostat, which overcomes all the aforementioned drawbacks and offers the advantage of the currently available expensive electronic controls at low cost.

To achieve the object, the present invention provides an electronic digital thermostat control unit comprising:

a p-n junction temperature sensing element,

a constant current source to drive the p-n junction element,

the output of one of the p-n junction elements is connected to an analog-to-digital converter to produce a digital output,

a digital output connected to a circuit for correcting the sensitivity and offset values of the sensor by using calibration data stored in the non-volatile memory,

a modified output of the at least one digital comparator, and the other input of each digital comparator receiving a digital reference value from the non-volatile memory or from the variable control device,

a digital noise filter is used to filter the output of the comparator to eliminate spurious outputs and store them in a control latch so that the input of a control latch can be set/reset whenever the output of the digital comparator is 'true' for use in starting the device in a consumer/industrial product that is temperature corrected.

The output of the control latch is connected to an output drive and protection circuit that constantly monitors the load condition and renders the drive of the solid state switch useless if an overload condition is encountered in the consumer/industrial product. These overload conditions refer to conditions of thermal overload, overload current, and switch-on inrush current. Accordingly, the output driving and protecting circuit comprises a thermal protecting circuit, an overload current protecting circuit and a soft start circuit. The thermal protection circuit monitors the temperature of the load, and the overcurrent protection circuit monitors the current flowing by the load, and the soft start circuit provides an effectively reduced start voltage to the load during initial turn-on, and thereby reduces inrush current stress generated by the load in the case of motor and heater loads.

The temperature display unit is connected to one of the inputs of the digital comparator, which receives its input from the output of the sensitivity and offset correction circuit, and a selection switch permits selective display of the sensed temperature or reference value from the digitized output of the potentiometer/switch.

A variable control device is connected in series with the analog-to-digital converter for varying the digital value of the reference value fed to the digital comparator via a multiplexer for adjusting the control range of the temperature. The variable control device is a potentiometer or switch connected to a switch trip circuit and digital counter to remove spurious switch transitions and increment/decrement digital counters, the output of which is connected to the input of a digital multiplexer to determine whether the user control signal from the potentiometer/switch or the constant value from the non-volatile memory is used as a reference value for the digital comparator.

The output of the digital multiplexer is controlled by a signal from the selection switch through a switch trip circuit.

The digital comparator compares the corrected and sensed temperature with the reference value and generates a 'true'/'false' output after filtering through a noise filter to set/reset a control latch to eliminate false outputs.

One of the digital comparators receives a fixed reference value from the non-volatile memory and the other digital comparator receives its reference value from the non-volatile memory or from a user variable control device depending on the state of toggling the selected selector switch.

The power supply to the electronic digital thermostat control unit preferably includes a low loss capacitive voltage drop network followed by a voltage clamping device, a rectifier and a filter network to provide a DC voltage. The DC power supply supplies an output within a range of 3 to 6 volts.

A clock oscillator is connected to each circuit of the electronic digital thermostat control unit for providing timing signals for the operation of each circuit. The clock oscillator is a quartz clock oscillator having an operating frequency in the range of 4-8 mhz.

The entire control Circuit is provided as a standard Application Specific Integrated Circuit (ASIC) to provide a small and cost effective thermostat that does not include the sensor, a variable user controlled potentiometer/switch, the selector switch, temperature display unit and the solid state switch.

In other embodiments, the ASIC does not include non-volatile memory, clock circuitry, and a power supply to provide greater non-volatile memory capacity for storing temperature data and to serve as an interface to displays of different types and sizes in one embodiment, and further excludes the output drive and protection circuitry in another embodiment to use higher power solid state switches or to provide flexibility of control in multi-drop applications.

To achieve the second objective, the present invention provides an electronic multi-point temperature control unit, comprising:

multiple electronic thermostat control units, such as the one described above, having a common non-volatile memory storing reference and calibration data for controlling the temperature at desired locations in the freezing/heating system, wherein

The output from the control latch unit of the electronic thermostat unit is connected to logic circuitry which uses data separately stored in a non-volatile memory in the electronic thermostat unit to selectively connect the output to one or more output driver and protection circuits which drive and monitor the load through solid state switches,

a central control unit connected to:

each said output of the control latch unit of the electronic thermostat control unit, and the input of said output driver and protection circuit, for enabling or disabling the electronic thermostat control unit and the output driver and protection circuit, depending on the combination of the output from the electronic thermostat control unit and the user control input received from the potentiometer or digital counter, and the error occurrence condition;

a system timer unit that generates the timing signal for enabling or disabling one or more of the output drive and protection circuits during a particular mode of operation;

a start-up relay circuit which provides the required signals to control one or more output drive and protection circuits when the load is switched on.

Any one or more of the output drive and protection circuits includes a thermal protection circuit that monitors the temperature of the load, an overcurrent protection circuit that monitors the current flowing by the load, and a soft start circuit that provides an effectively reduced start voltage to the load during the initial turn-on period and thereby reduces the inrush current stress on the load in the case of motors and heaters.

The central control unit is a logic circuit for providing specific functions such as automatic thawing and quick freezing in the case of a freezer, and timed heating cycles in the case of a heating system.

A display unit is connected to an output of one of the electronic thermostat control units for displaying a temperature.

At least one switch is connected to the input of the central control unit via a switch trip circuit and a digital counter for supplying the required user control signals to operate the electronic multipoint temperature control unit.

The entire control circuit is implemented as a standard ASIC to provide a small and cost effective electronic multi-point temperature control unit that does not include the sensors, variable user control switches, selector switches, temperature display units, and solid state switches of the electronic thermostat control unit.

In another embodiment, the ASIC circuit does not include nonvolatile memory, clock circuitry, and a power supply in order to provide greater nonvolatile memory capacity for storing temperature data and to interface with different types and sizes of displays.

The invention will now be described with reference to the accompanying drawings:

fig. 1 shows an electronic digital thermostat control unit using a potentiometer according to the present invention for changing the temperature control value.

FIG. 1a shows an output driver and protection circuit in the electronic digital thermostat control unit.

FIG. 2 shows another embodiment of the unit using a switch to vary the temperature control value.

Fig. 3 shows the transformerless power supply for providing power to the electronic thermostat unit.

Fig. 4 shows an application of the electronic thermostat control unit.

FIG. 5 shows an embodiment in which the entire control circuit is implemented as an ASIC, excluding the sensor, variable user control potentiometer/switch, selector switch, temperature display unit, and solid state switch.

FIG. 6 shows another embodiment in the form of an ASIC, wherein the non-volatile memory, clock oscillator and DC power supply are external to the ASIC.

FIG. 7 shows another embodiment in the form of an ASIC, where the non-volatile memory, clock oscillator, DC power supply, and output drive and protection circuitry are located external to the ASIC.

FIG. 8 shows the use of five electronic thermostat control units and an electronic multipoint temperature control unit with a common non-volatile memory.

FIG. 8a shows an output driving and protection circuit in an electronic multi-point temperature control unit.

FIG. 9 shows the application of the electronic multi-point temperature control unit in a refrigeration device having three separate compartments.

Figure 10 shows the use of the electronic multipoint temperature control unit in a coffee vending machine.

FIG. 11 shows an embodiment in which the entire circuit, except for the sensors, switches, DC power supply, solid state switches, and display unit, is implemented as an ASIC.

FIG. 12 shows an embodiment in which the entire circuit is implemented as an ASIC, except for the sensors, switches, DC power supply, solid state switches, non-volatile memory and display unit of the electronic thermostat control unit.

Referring to FIGS. 1 and 1a, item 1 represents a sensor comprising a semiconductor p-n junction (e.g., a diode). A constant current source 2 provides bias current for the sensor 1. The signal from the sensor 1 is an analog dc voltage that decreases linearly with temperature, converted to digital form by an analog-to-digital converter 3. The digital output of sensor offset and sensitivity is adjusted by a sensitivity and offset correction digital circuit 4, which circuit 4 receives the digital form of correction factor data from a non-volatile memory 19. This digital output is provided to digital comparators 5 and 6. Each digital comparator receives a digital reference value and at its input the digital value from the sensitivity and offset correction digital circuit 4. The digital comparator 5 receives a fixed value from the non-volatile memory 19 and the further digital comparator 6 receives its reference value from the non-volatile memory 19 or from a user variable control 12, depending on the on/off state of the selector switch 16. In this case the user controlled variable is a potentiometer 12, from which the dc voltage is fed to an analog to digital converter 14 and converted into a digital value suitable for the digital comparator 6. A constant current source 13 drives the potentiometer 12 to ensure an output independent of power supply fluctuations. The output of the analog-to-digital converter 14 is fed to a digital multiplexer 15 which determines whether the user control signal from the potentiometer or a constant value from a non-volatile memory is used as a current cutoff reference for the digital comparator 6. The digital multiplexer 15 receives its control input from the output of a switch trip circuit 17 that interfaces with the selector switch 16. The outputs of the two digital comparators 5 and 6 are passed through digital squelch filters 7 and 8 to remove spurious outputs and are then provided to the input of a control latch 9 to control the output drive and protection circuit 10. The output drive and protection circuit 10 includes a soft start circuit 10A, a thermal overload protection circuit 10B, and an overload current protection circuit 10C, drives the solid state switch 11 to activate the associated devices in the consumer/industrial appliance, enables correction of the temperature and minimization of inrush current stress on the load in the case of motor and heater loads, and protects against overheating and current overload conditions. The output of the sensitivity and offset correction digital circuit 4 will also cause it to display the sensed temperature on a display unit 18. A clock circuit 20 and a power supply 21 are connected to the overall circuit, as shown in FIG. 1.

In fig. 2, the user variable control originates from a switch 22, rather than a potentiometer 12. The signal from the switch 22 is fed to a switch trip circuit 23 which feeds a pulse to a digital counter 24 at each switch depression, which represents the selected control limit value supplied to the digital comparator 6 via the digital multiplexer 15, determining whether the output of the digital counter 24 or a fixed value from the non-volatile memory 19 is supplied to the input of the digital comparator 6. The digital multiplexer 15 receives its control input from the output of a debounce switch 17 that interfaces with the selector switch 16.

Fig. 3 shows the 3-6 volt transformerless power supply 21 for powering the electronic digital thermostat control unit. A capacitive voltage drop network 25 has a voltage clamping Zener diode 26 and reduces the input high AC voltage to a low value which is rectified by a diode 27 and then filtered by a capacitor 28 to produce a low voltage DC power supply to power the circuit.

Fig. 4 shows an application of the electronic digital thermostat control unit. The sensor element 1 is placed inside the device 29 to control its temperature (for example, a freezer in the case of consumer goods, or engine coolant lines in the case of an industrial/automotive application). The sensor 1 is located remotely from the electronic digital thermostat control unit 30. Similarly, locating the device 31 remotely will activate the device 31 with the electronic digital thermostat control unit to provide temperature correction to the compressor motor in the case of a chiller or to the cooling fan/cooling pump of the chiller in the case of an air or water cooled engine.

Fig. 5 shows an implementation of the electronic digital thermostat control unit in the form of a standard ASIC 32 to provide a very compact and cost-effective solution. The sensor 1 is connected to the ASIC. Similarly, a solid state switch 11 is connected to the output of the ASIC. Two switches for the selection of the cutoff temperature and the setting of the control range are also connected to the ASIC 32, respectively. The display unit 18 is directly connected to and separately driven by the ASIC pin.

Fig. 6 shows another embodiment using an ASIC33 in which the non-volatile memory 19, clock oscillator 20, and power supply 21 are external to the ASIC to provide greater non-volatile memory capacity and interface to several different types and sizes of displays. The larger capacity of the nonvolatile memory 19 allows more temperature data to be stored.

Fig. 7 shows another embodiment of the electronic digital thermostat control unit in the form of an ASIC34 in which the disconnect drive and protection circuit 10 is also external so that higher power solid state switches are used, requiring more drive current than provided by a single chip. This enables control of significantly higher capacity loads.

A sensor includes a single semiconductor p-n junction diode 1 that generates a DC voltage at a constant bias current provided by a constant current source 2 that decreases in proportion to an increase in sensed temperature. This dc voltage is fed to the input of an analog-to-digital converter 3. The analog-to-digital converter produces a digital output that is equal to the dc voltage supplied to it at its input. This digital output is then fed to the input of a sensitivity and offset correction digital circuit 4 to correct its sensor offset and sensitivity by applying a correction factor that the circuit 4 receives in digital form from the non-volatile memory 19. This produces a corrected, sensed temperature value.

This corrected, sensed temperature value is provided to an input of each digital comparator 5 and 6. The digital comparator 5 receives at its other input a fixed reference value from the non-volatile memory 19. The corrected, sensed temperature value received by the sensitivity and offset correction digital circuit 4 is compared to the reference value by digital comparator 5 and a 'true'/'false' output is generated. The output of the digital comparator 5 is fed to a digital squelch filter 7 to eliminate spurious outputs. The filtered output from the digital squelch filter 7 is provided to a "reset" input of a control latch 9. Thus, whenever the output of the digital comparator 5 is 'true', the control latch is reset.

Depending on the state of the selection switch 16 which toggles the selection, the further digital comparator 6 receives its reference value from the non-volatile memory 19 or from a user variable control 12. In the case where the user variable control device is a potentiometer 12, the dc voltage from the potentiometer is fed to an analog to digital converter 14 and converted to a digital value suitable for the digital comparator 6. A constant current source 13 drives the potentiometer to ensure that its output is independent of power supply fluctuations. The output of the analog-to-digital converter 14 is fed to a digital multiplexer 15 which determines whether the user control signal from the potentiometer 12 or the constant from the non-volatile memory 19 is used as a reference for the digital comparator 6.

When user control is provided by a switch 22 in place of a potentiometer 12 (see fig. 2), the switch signal is first passed through a switch trip circuit 23 to remove spurious switch transitions and subsequently used to increment/decrement a digital counter 24. The output of the digital counter 24 is then provided to the input of the digital multiplexer 15 which determines whether the user control signal from the switch 22 or the constant from the non-volatile memory 19 is used as a reference value for the digital comparator 6.

The output of the digital multiplexer 15 is controlled by a signal from the selector switch 16 processed by the switch trip circuit 17 to remove spurious switch transitions. Digital comparator 6 compares the corrected, sensed temperature value with a reference value and generates a 'true'/'false' output for setting control latch 9 after filtering by digital noise filter 8 to eliminate false outputs.

The control latch 9 outputs a digital signal to enable/disable the output driver and protection circuit 10. The output drive and protection circuit 10 generates the signals required to drive the solid state switch 11 to activate the associated devices in the consumer/industrial appliance to correct the temperature. The output driver and protection circuit 10 includes a thermal overload protection circuit 10B and an overcurrent protection circuit 10C and constantly monitors the load condition and disables the driver to the solid state switch 11 if an overload or current overload condition is encountered. In the case of motor and heater loads, the output drive and protection circuit also includes a soft start circuit 10A to provide an effectively reduced start voltage to the load during the initial turn-on period and thereby reduce the inrush current stress generated on the load.

The output of the sensitivity and offset correction digital circuit 4 will also be used to display the sensed temperature on a display 18. A selection switch (not shown) is connected to the input of the display unit 18 and is also capable of selectively displaying the sensed temperature, as indicated by the output of the sensitivity and offset correction digital circuit 4, or the user selected reference temperature, as determined by the output signal of the digital multiplexer 15. A quartz crystal oscillator based clock oscillator circuit 20 in the 4-8 mhz frequency range generates all the necessary timing signals to operate each circuit component, while a power supply 21 supplies the required voltages and currents to each circuit component of the electronic digital thermostat control unit.

The electronic multi-point temperature control unit is shown in fig. 8 and 8a, where reference numerals 35 to 39 show a single p-n junction (e.g. two-pole) temperature sensor connected to the electronic thermostat control units 40 to 44, with a common non-volatile memory 75 storing reference and calibration data. The outputs from the control latch unit of the electronic thermostat control unit are connected to a logic circuit 45, which logic circuit 45 selectively connects these outputs to the inputs of one or more output driver and protection circuits 46-50 using separately stored data in the non-volatile memory 75. The output from each output drive and protection circuit is connected to solid state switches 51 to 55 which perform on/off operations on the load (e.g., compressor motor of a freezer, blower, defrosting heater, etc.). Any one or more of the output driver and protection circuits includes a soft start circuit 46A, thermal overload retention circuit 46B and overload current protection circuit 46C to provide:

an effectively reduced starting voltage is provided to the load during the initial turn-on period, and in the event of motor and heater loads, thereby reducing the inrush current stress generated on the load.

Protecting it from thermal and current overload conditions.

The central control unit 71 will selectively enable or disable the electronic thermostat control units 40-44 and the output drive and protection circuits 46-50 during error conditions and during certain modes of operation, such as in the case of 'defrost' and 'quick freeze' modes of a freezer.

User control signals are received from one or more switches 56 to 60 located on the unit control panel. The signal from each switch passes through a switch trip circuit 61-65 to remove the dummy output and then for updating a digital counter 66-70. The output from the digital counter is connected to the input of the central control unit 71 and provides the required user control data to control the operation of the electronic multiplex temperature control unit. The output of the system timer unit 72 is connected to an input of the central control unit 71 and provides signals to determine the control action for enabling or disabling the electronic thermostat control units 40 to 44 and the output drive protection circuits 46 to 50, the system timer unit comprising electronic timers for special functions such as automatic thawing and 'quick freezing', 'door open warning', 'wrong automatic reset' features in case of a freezer application or 'water pouring timer', 'milk pouring timer', 'self-opening at preset time' and 'automatic off at preset time' in case of the coffee vending machine.

The starter relay circuit assembly 73 comprises an electrical circuit that delivers a timing signal to a two-winding electric motor, such as the starter coil of the compressor motor of the refrigeration unit. This signal is transmitted to an output driving and protection circuit part through the central control unit 71.

A clock oscillator 74 at a frequency of 4-8 mhz is used to provide the timing signals required for the operation of each circuit of the electronic multipoint temperature control unit. The clock oscillator is the same as that used in the electronic thermostat unit.

A non-volatile memory 75 is used to store all control and calibration data required by the electronic thermostat control unit and the logic circuitry.

A power supply 76 is used to power the electronic multi-point temperature control unit. The power supply is connected to all internal components of the unit and is the same as the power supply used by the electronic thermostat control unit.

A display unit 77 is provided at the output of an electronic thermostat control unit.

Fig. 9 illustrates the use of the electronic multi-point temperature control unit in a three-zone freezer 78, using five electronic thermostat control units and five output drive and protection circuits. Three temperature sensors 79 to 81 located in each of the three zones measure the ambient temperature in each zone. In addition, a fourth sensor 82 located on the compressor 84 housing will monitor the temperature of the compressor to provide a thermal overload function. A fifth sensor 83 placed near the thawing heater element 85 enables precise temperature control during the thawing cycle. Each electronic thermostat control unit monitors the temperature of the compartment in which it is located and compares that temperature to a particular cut-off and cut-on temperature, and activates its corresponding output drive and protection circuit whenever the monitored compartment temperature crosses the cut-on limit temperature and deactivates the corresponding output drive and protection circuit whenever the monitored temperature crosses the cut-off limit temperature. The outputs from the five solid state switches 51-55 are connected to the 'run' winding of the compressor motor, the 'start' winding of the compressor motor, the defrost heater element, blower #186 in one compartment of the freezer, and blower #287 in another compartment of the freezer.

Fig. 10 shows the application of the electronic multi-point temperature control unit in a coffee vending machine 89, using three electronic thermostat control units and three output drive and protection circuits. A temperature sensor 90 is positioned in contact with a stainless steel container 91 containing water for the coffee and measures the temperature of the water as it heats. Second and third sensors 92 and 93 on the housing of the hot water dispensing pump 94 and milk dispensing pump 95 monitor the temperature of the pumps to provide thermal overload protection. The outputs from the three solid state switches 51-53 are connected to the electric heater 96, and the hot water and milk dispensing pumps 94, 95 to monitor the desired temperature.

Fig. 11 shows an implementation of an electronic multipoint temperature control unit in the form of a standard ASIC97, wherein the sensors 35 to 39, user controlled switches 56 to 60, dc power supply 76 and solid state switches 51 to 55 of the electronic thermostat control unit are excluded to provide a very compact and cost effective solution.

Fig. 12 shows another embodiment electronic multipoint temperature control unit using an ASIC98 in which the sensors 35-39, user controlled switches 56-60, dc power supply 76 and solid state switches 51-55 of the electronic thermostat control unit, and the non-volatile memory 75 are located outside the ASIC to provide greater data storage and interface with a variety of different types and sizes of displays.

The work is as follows:

a plurality of electronic thermostat control units 40-44 having a common non-volatile memory 75 monitor temperature at different locations of the environment where temperature is to be controlled. The output from each control latch unit of the electronic thermostat control unit is connected to a logic circuit 45, the logic circuit 45 selectively connecting the output to the input of one or more output drive or protection circuits 46-50 in accordance with data received from the non-volatile memory 75. Each electronic thermostat control unit monitors the temperature of the compartment in which it is located and compares that temperature to specific 'off' and 'on' temperatures, and will enable its corresponding output drive and protection circuit whenever the monitored temperature crosses the 'on' limit temperature and disable the corresponding output drive and protection circuit whenever the monitored temperature crosses the 'off' limit temperature. The outputs from the output driver and protection circuits 46-50 are connected to the inputs of solid state switches 51-55 and pass through the output driver and protection circuits that drive and monitor the load (blower, compressor, heater, pump or solenoid valve of the refrigeration/heating system). Any one or more of the output driver and protection circuits 46 includes a 'soft start' circuit 46A, thermal overload protection circuit 46B and overload current protection circuit 46C to provide:

an effectively reduced starting voltage is provided to the load during the initial turn-on period, and in the event of motor and heater loads, thereby reducing the inrush current stress generated on the load.

Protecting it from thermal and current overload conditions.

A central control unit 71 receives the user control values from the switches 56 to 60 after passing through the switch trip circuits 61 to 65 to eliminate spurious conversions, and passes through the digital counters 66 to 70 to generate a digital value. The central control unit also receives input from a system timer unit 72 which is supplied with control signals from one or more internal timers and from a start relay circuit 73 which, when the motor 'start' winding is turned on, generates the signals required to supply timed 'turn on' pulses to the motor. The central control circuit generates enable/disable control signals for each electronic thermostat control unit and output drive and protection circuit based on its input signal values, and thereby performs the control functions required for the operation of the overall electronic multi-point temperature control unit as well as for application-specific operating modes, such as 'defrost' and 'quick freeze' in the case of a freezer.

Claims (16)

1. An electronic digital thermostat control unit comprising:

a p-n junction temperature sensing element,

a constant current source for driving the p-n junction element,

the output of the p-n junction element is connected to an analog-to-digital converter to produce a digital output,

the digital output is connected to a circuit that corrects the sensitivity and offset values of the sensor by calibration data stored in non-volatile memory,

the modified output is connected to one input of at least one digital comparator, and the other input of each digital comparator receives a digital reference value from the non-volatile memory or from a variable control device,

the output of the comparator is filtered using a digital noise filter to eliminate spurious outputs and is stored in a control latch so that a control latch input can be set/reset whenever the output of the digital comparison is 'true' to enable the device in the consumer/industrial product for temperature correction.

2. The electronic digital thermostat control unit of claim 1, wherein the output of said control latch is connected to an output drive and protection circuit that constantly monitors the load condition and renders the drive of the solid state switch useless if an overload condition is encountered in said consumer/industrial product.

3. The electronic digital thermostat control unit of claim 2 wherein the output drive and protection circuit includes a thermal protection circuit, an overcurrent protection circuit, and a 'soft start' circuit, the thermal protection circuit monitoring the temperature of the load, the overcurrent protection circuit monitoring the current flowing by the load, and the 'soft start' circuit providing an effectively reduced start voltage to the load during initial turn-on and thereby reducing inrush current stress generated by the load in the case of motor and heater loads.

4. The electronic digital thermostat control unit of claim 1, wherein a variable control device is provided in series with the analog-to-digital converter for varying the reference digital value fed to the digital comparator via a multiplexer to adjust the control range of the temperature.

5. The electronic digital thermostat control unit of claim 1, wherein the variable control device is a potentiometer or switch connected to a switch trip circuit and digital counter to remove spurious switch transitions and increment/decrement a digital counter, respectively. The output of the digital counter or the potentiometer is connected to the input of a digital multiplexer to determine whether the user control signal from the switch or the constant from the non-volatile memory will be used as a reference value for the digital comparator.

6. The electronic digital thermostat control unit of claim 1, wherein the entire control circuit is implemented as a standard Application Specific Integrated Circuit (ASIC) to provide a small and cost effective thermostat that does not include sensors, a variable user control potentiometer/switch, selector switches, temperature display units, and solid state switches.

7. The electronic digital thermostat control unit of claim 1, wherein the ASIC does not include non-volatile memory, clock circuitry, and a power supply to provide greater non-volatile memory capacity for storing temperature data and to serve as an interface to displays of different types and sizes in one embodiment, and further does not include output drive and protection circuitry to facilitate the use of higher power solid state switches in another embodiment.

8. An electronic multi-point temperature control unit comprising:

a plurality of electronic thermostat control units of claim 1 having a common non-volatile memory storing reference and calibration data for controlling the temperature at desired locations in the freezing/heating system, wherein

The output from the control latch unit of the electronic thermostat unit is connected to logic circuitry that uses data separately stored in a non-volatile memory of the electronic thermostat unit to selectively connect the output to one or more output driver and protection circuits that drive and monitor the load through solid state switches,

a central control unit connected to:

each output from the control latch unit of the electronic thermostat control unit, and each input of the output driver and protection circuit, for enabling or disabling the electronic thermostat control unit and the output driver and protection circuit, depending on the combination of the output from the electronic thermostat control unit and the user control input received by the potentiometer or from a digital counter, and a fault occurrence condition,

a system timer unit that generates timing signals for enabling or disabling the one or more output drive and protection circuits during a particular mode of operation,

a start-up relay circuit which provides the required signals to control one or more output drive and protection circuits when the load is switched on.

9. The electronic multipoint temperature control unit of claim 8 wherein any one or more of said output drive and protection circuits includes a thermal protection circuit, an overcurrent protection circuit, and a 'soft start' circuit, said thermal protection circuit monitoring the temperature of said load, said overcurrent protection circuit monitoring the current flowing by said load, and said 'soft start' circuit providing an effectively reduced start voltage to said load during the initial turn-on period and thereby reducing the inrush current stress on the load in the case of motors and heaters.

10. The electronic multipoint temperature control unit of claim 8 wherein the central control unit is a logic circuit for providing special functions such as automatic defrosting and quick freezing in the case of a freezer and timed heating cycles in the case of a heating system.

11. The electronic multipoint temperature control unit of claim 1 or 8, wherein a clock oscillator provides the required timing signals for operating each circuit element of the electronic thermostat control unit.

12. The electronic multipoint temperature control unit of claim 1 or 8 wherein the power supply to said electronic digital thermostat control unit preferably comprises a low loss capacitive voltage drop network followed by a voltage clamping device, a rectifier and a filter network to provide a dc voltage.

13. The electronic multipoint temperature control unit of claim 1 or 8 wherein in the case of the electronic thermostat control unit, the temperature display unit is connected to one input of said digital comparator, the comparator receiving its input from the output of the sensitivity and offset correction circuit, and a selection switch permitting selective display of the temperature-sensitive or reference value of the digital output from the potentiometer/switch; and in the case of an electronic multipoint temperature control unit, the temperature display unit is connected to an output of said electronic thermostat control unit for displaying the sensed or reference temperature through a selection switch.

14. The electronic multipoint temperature control unit of claim 8 wherein at least one user controlled switch is connected to an input of said central control unit via a switch trip circuit and a digital counter for providing required user control signals to operate said electronic multipoint temperature control unit.

15. The electronic multipoint temperature control unit of claim 1 wherein the entire control circuit is implemented as a standard ASIC to provide a small and cost effective thermostat that does not include the electronic thermostat control unit's sensors, variable user control switches, selector switches, temperature display unit and solid state switches.

16. The electronic multi-point temperature control unit of claim 1, wherein said ASIC does not include non-volatile memory, clock circuitry and the power supply to provide greater non-volatile memory capacity for storing temperature data and in one embodiment serves as an interface to different types and sizes of displays, and in another embodiment further excludes the output drive and protection circuitry to facilitate the use of a high power solid state switch.