Analog Air Conditioner Design using LTSpice
Abstract--- Air conditioning is a technique that can change the temperature of the air by cooling and heating it. Air conditioner systems,which have become increasingly popular in recent years, are systems that can perform these operations. A more developed system would have ability to perform these operations automatically using a feedback system to ensure that system is always trying to set ambient temperature towards the set temperature or staying in idle if set temperature is reached with a given margin on temperature. This project is about a micro analog air conditioner system design with automatic functionality given a target temperature, and this report includes the design of the air conditioner system as well as various simulations which are run using different numerical inputs.
Keywords--- air conditioner, analog, feedback, design.
The goal of this project is to design a micro air conditioning system. This system will be composed of four main subsystems which can be seen in Fig. 1: a sensing unit, a display unit, a control unit, and an operation unit. Also, there will be a temperature adjustment unit (set unit) to obtain target temperature from users. The temperature adjustment unit will send the intended temperature to the control unit and display unit, which is a voltage value determined using a system uses a potentiometer, and the sensing unit will provide the ambient temperature to both control unit and the display unit, which is also a voltage value. The control unit will then, compare these to voltage values and drive the operation unit. The control unit's main goal is to turn on the cooling system if the ambient temperature is higher than the desired temperature when the difference is larger than 1 degree Celsius the heating function will be activated. If the temperature difference between the targeted and ambient temperatures is less than 1 degree Celsius, the system will enter idle state, which means neither cooling nor heating will operate.
Fig. 1. Micro Air Conditioner System Diagram
In the following parts of this report, firstly requirements of the desired system will be investigated, after that the design approach used in each unit, their designs and simulation results obtained using a SPICE software, namely LTSpice, will be investigated.
The control unit must be able drive the operation unit until the desired temperature is reached. The ambient temperature and set value obtained from adjustment unit must always be checked and if they deviate 1 degree from each other, necessary operation unit must be driven to reach target temperature.
The operation unit must be consisting of a cooler and a heater. Heating element will be an electrical resistor. Cooling element will be an electric fan. Their driver circuits must be implementing according to used circuit elements and devices while constructing them.
The sensing unit must sense the ambient temperature. Then the sensor must inform the display and control units about increase or decrease in temperature by a variable parameter level.
The display unit must contain a RGB led and a switch to show the set or the ambient temperature level whenever they are needed. The display unit must cover the visible spectrum continuously. Required color scale and temperature ranges can be seen in Fig. 2.
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The temperature adjustment unit (set unit) must be able to allow users to set a temperature between 24 and 40 degrees
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Celsius and fed this target temperature to both display and controls units by a variable parameter level.
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The system must be testable and designed using a modular approach. Temperature range will be 24-40 degrees Celsius. RGB led indicator must cover all visible spectrum regarding this temperature range. Set value must be adjusted by a potentiometer, and system must be autonomous when there is a 1-degree Celsius difference between ambient and target temperature. The heater and cooler operations must not be working simultaneously.
Fig. 2. RGB Led Color Scale and Temperature Range
A system which would meet these requirements will be designed in following manner. A determined voltage range will be used the represent the ambient and target temperature. Determining this temperature range, temperature sensor will be used in sensing unit and its output will be the deciding factor. After these sensed ambient temperature and target temperature are obtained as voltage levels, they will be normalized to this determined voltage range. 0 degree Celsius is represent with 0 potential difference between these to voltages, positive and negative values will be considered as the resultant to drive the necessary operation unit. Heater will run with positive voltage outputs and cooler will run with negative voltage outputs of the control unit. The normalized voltage outputs from sensing and adjustment unit will be fed to the display unit and a RGB Led driver circuit will be designed to meet the required color scale depending on these voltage values.
The sensing unit is the subsystem which is responsible from the sensing of ambient temperature and providing this information to control and display units. LM35 Temperature Sensor is considered as the sensor will be used in the system which will be constructed. Regarding to its datasheet, this temperature sensor can work in a temperature range of -40 and 110 degrees Celsius and, will provide a voltage output with a 10mV/Celsius voltage step within this range [1]. It outputs 0 V when the sensed environment is 0 degree Celsius and 240mV when the ambient temperature is 24 degrees Celsius.
Considering these specifications, voltage will be fed into the control unit is determined to be in the range of 240-400mV for 24-40 degrees Celsius while every 10mV is representing 1 degree Celsius from the sensing unit. This determined voltage range will also be the key factor of designing the control, display, and temperature adjustment units.
The control unit is the subsystem which is responsible of comparing the sensed ambient temperature with the target temperature set by the user and driving the necessary operation unit depending on this comparison. As mentioned above, a decision unit to output positive or negative voltage depending on the necessary operation to reach target temperature, will be designed.
Normalized voltage values from both the sensing unit and the adjustment unit will be fed into a difference amplifier circuit. The system will subtract the voltage value of ambient temperature from the target temperature value coming from the adjustment unit. Every 10mV voltage difference between these two voltage values is considered as 1-degree Celsius temperature difference between ambient and target temperature.
Design specifications require this system to be operating if there is temperature difference greater than 1 degree Celsius, otherwise none of the operation units must be working. Difference amplifier is designed to amplify the signal almost 70 times to obtain voltage output around 700mV if there is a 10mV voltage difference between voltage values coming from sensing and adjustment units. Using diodes which would have 0.7V threshold voltage levels, would provide the required idle state when there is a voltage difference lower than 10mV which means a temperature difference less than 1 degree Celsius. A diode model is used to provide this characteristic is used in SPICE designs. The control unit is designed in this manner and can be seen in Fig. 3.
Fig. 3. Control Unit Design
Using a modular design approach, in this design, voltage values which will be obtained from sensing and adjustment units representing ambient temperature and target temperature are represented as voltage sources namely "ROOM_TEMP" AND "SET_TEMP". U1 Op-Amp is the difference amplifier, and it amplifies these signals 67 times. After using an Op-Amp as voltage buffer this output voltage is fed into decision unit using two oppositely directed diodes. After these diodes output signals are amplified 100 times to obtain a voltage level around 10~12V. After this amplification operations units are driven. A voltage bias with a voltage level of 5V is connected at the other ports of the operation unit resistances to obtain a ~5V voltage level to operate. Operation units are represented with 1kOhm resistors in SPICE designs.
With the voltage levels given as input in Fig. 3, system is expected to be driving only the cooler. 0.32V coming from sensing unit which represents ambient temperature being 32 degrees Celsius and 0.28V coming from adjustment unit representing 28 degrees Celsius, system must be operating as a cooler to cool the environment down to target temperature level. Simulation result showing a -5mA current is only passing through the cooler resistance in this configuration can be seen in Fig. 4.
Fig. 4. Cooler ON Configuration Simulation Result
A configuration to allow system to perform as a heater is provided in Fig. 5. Ambient temperature voltage level is still 0.32V and target temperature voltage level is increased to 0.36V. Using this configuration, a -5mA current is only passing from the heater as expected in the simulation result which can be seen > Fig. 6.
Fig. 5. Heater ON Configuration
Fig. 6. Heater ON Configuration Simulation Result
System is expected to be in idle state, none of the operation units are running, when there is a voltage difference level lower than 10mV. A configuration for this is shown in the Fig. 7., SET_TEMP voltage value is set as 0.32V and ROOM_TEMP voltage value is set as 0.315V. There is a 5mV voltage difference between these two inputs and amplifed voltage output is not sufficient to exceed diode threshold voltage value and thus, to drive none of the operating units as expected. The simulation result regarding this configuration can be seen in Fig. 8.
Fig. 7. Idle Mode Configuration
Fig. 8. Idle Mode Simulation Result
The operating unit is the subsystem that contains the elements which perform heating and cooling functions of the system. A stone resistor as heater, and a computer case fan (5-12V) can be used in the system constructed. These elements are represented as 1kOhm resistors SPICE designs. Amplified voltage coming from the control unit and the biasing voltage provides a 5~6V voltage value to operate these units.
The display unit is the subsystem responsible for giving visual output to users about the ambient temperature and target temperature set using the temperature adjustment unit. The display unit would consist of a RGB Led and a switch to change visual output between ambient and target temperature. A color scale representing temperature levels between 24 and 40 degrees Celsius is required in display unit.
This requirement would be met designing and constructing an approach as follows. Normalized voltage levels representing ambient and target temperatures where every 10mV voltage value corresponding to a 1-degree Celsius temperature interval, would be provided to this display unit. Display unit would use these voltage levels to determine the duty cycles to derive each leg of a common cathode or anode RGB led to obtain required color output. A common cathode or anode RGB led depending on which type is used, require a positive or negative voltage being supplied to its legs, to provide various amounts of glowing of each red, green, and blue colors. Resulting a color with various illumination levels of red, green, and blue in visible spectrum depending on these determined duty cycles.
Color LEDs will need 3~3.2V to work (considering a grounded common cathode RGB LED). A decision unit for this display unit would apply duty cycle outputs using the adjustment and sensing unit voltage outputs to apply 3~3.2V signal with necessary duty cycle to each leg separately to obtain desired color output. In further detail this decision subunit would provide following
function, considering the normalized determined voltage level (240 -- 400mV) coming from input, voltage levels lower than 240mV would only power blue LED with constant 100% duty cycle. Starting from 240mV up to 320mV duty cycle of blue LED will start to decrease from 100% pulse width modulation (PWM) and would be completely off after 320mV and higher voltages. Green LED will start with 0% duty cycle at 240mV and would reach 100% at 320mV. After 320mV green LED would start to decrease and will be completely off at 400mV. Red LED would start from 0% duty cycle at 320mV and reach 100% cycle at 400mV and stay there with 100% PWM for voltage levels higher than that.
Since normalized voltage levels are already obtained and will be supplied to this display unit, same circuitry to drive RGB led and determining duty cycles of color legs would used for displaying both ambient and target temperatures. A switch used for changing between the voltage provided to this duty cycle deciding unit would be used to displaying ambient and target temperatures.
Aim of this temperature adjustment unit is to allow users to set a target temperature between 24 and 40 degrees Celsius with help of a potentiometer. Sensing unit is providing a voltage output between -400mV - 1.1V corresponding to from -40 to 110degrees Celsius temperature where voltage output is altered 10mV for every 1 degree Celsius in LM35 temperature sensor. Design specifications require a temperature interval of 24 to 40 degrees Celsius. This adjustment unit is designed to be provide a voltage output between 240 and 400mV. Design of the temperature adjustment unit can be seen in Fig. 9.
Fig. 9. Temperature Adjustment Unit Design
A 100k potentiometer represented with the "POT1" and "POT2" naming for each of its resistance between different legs used in the design. Changing the resistance using this potentiometer which will work as a voltage divider with the positive DC voltage input which is being used throughout the system would provide a voltage deviation. This voltage divider followed by a voltage amplifier of factor 1/20 and 1/3.3, a positive voltage between ~1-2mV to ~160-170mV is obtained. This 160mV voltage interval obtained using this circuitry constructed with the potentiometer would allow changing the voltage in a 160mV voltage interval (240mV 400mV range). To further normalize this temperature adjustment output voltage. Using another voltage divider circuitry to DC offset the system with a ~240mV voltage value is providing a voltage value output between ~240 and ~400mV. Second voltage divider using two resistors for dividing the DC input voltage (12V) to obtain this DC voltage value. The DC offset voltage and potentiometer circuitry outputs are summed using a voltage divider Op-Amp configuration and resulting negative voltage output with a range of ~ -400mV (maximum output on one end of the potentiometer, design and simulation results can be seen in Fig. 11 and Fig. 12 respectively) and ~ -240mV (minimum output on other end of the potentiometer, design and simulation results can be seen in Fig. 9 and Fig. 10 respectively).
Fig. 10. Temperature Adjustment Unit Maximum Output Simulation Result
Fig. 11. Configuration for Minimum Output of Temperature Adjustment Unit
Fig. 12. Temperature Adjustment Unit Minimum Output Simulation Result
This negative output voltage of the temperature adjustment unit is provided to the control unit using an inverting amplifier. Overall diagram of this designed system including this temperature adjustment unit, control unit and representative operation units can be seen in Fig. 13. Used configuration in Fig. 13 showing a state of 240mV voltage input from sensing unit (24 degrees Celsius) whereas the potentiometer set to its maximum resistance to obtained maximum voltage from the adjustment unit. Resulting voltage output of adjustment unit is fed into the control unit as a voltage level of ~400mV as can be seen in the Fig. 10. In this configuration target temperature is 40 degrees Celsius and heater is expected to perform. As can be seen in the simulation results in Fig. 14 only heater operation is running since target temperature is greater than the ambient temperature.
Fig. 13. Overall System Design Diagram
Fig. 14. Simulation Result Showing Heater Performing
When ambient temperature is higher than 24 degrees Celsius, providing a voltage level greater than 240mV to control unit while target temperature is set to 24 degrees Celsius with minimum configuration of adjustment unit giving 240mV output, only the cooler operation is run as expected. Configuration and simulation results can be seen in Fig. 15 and Fig. 16 respectively.
Fig. 15. Configuration to Run The Cooler
Fig. 16. Simulation Result Showing Cooler Performing
When temperature adjustment unit and sensing unit voltage output difference is less than 10mV corresponding to a temperature difference less than 1-degree Celsius system is expected to be in idle mode where none of the operation units are working. To obtain an idle state, sensing unit output voltage is set to 235mV corresponding to 23.5 degrees Celsius and temperature adjustment unit is set to its minimum state where it is providing a voltage level of ~240mV. This configuration can be seen in Fig. 17. As expected, since voltage difference between sensing unit and adjustment unit outputs which is corresponding to a temperature difference less than 1-degree Celsius, thanks to diodes used after the control unit while driving the operation units that threshold voltage provides system an idle state when the voltage difference is less 10mV. Simulation results can be seen in Fig. 18 showing the idle mode of the system.
Fig. 17. Idle Mode Configuration
Fig. 18. Idle Mode Simulation Result
To conclude, a micro air conditioner system which automatic functioning given a target temperature input, can be constructed using the design approaches, subsystems designed and numerical values for the circuit elements mentioned in this report. Further designs approaches can be developed for example to provide system a resting interval after passing the target temperature with a determined margin to ensure system won't turn on and off continuously at edge of a 1-degree Celsius temperature difference between target and ambient temperatures. Dynamic circuit elements for allowing system to be keep performing its cooling or heating operations after control unit output is set off to create this mentioned margin can be used in these applications.
Even though some realistic models for circuit elements are using in SPICE designs, practical application of this micro air conditioner would result in errors in expected outputs from subsystems. Further investigation and fine tuning of resistances used in the design would be required to obtain desired voltage levels and intervals for given design specifications.
[1] Texas Instruments. (2017, December). LM35 Precision Centigrade Temperature Sensors Datasheet. Texas Instruments Symlink. Retrieved February 10, 2022, from https://www.ti.com/lit/ds/symlink/lm35.pdf
Some parts of this markdown file is generated using Vertopal PDF to MD Converter.
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