The temperature sensor allows you to maintain the right amount of cold in the refrigerator. It activates the compressor when required, which fills the equipment with cold. And with such “emissions”, he creates clearly programmed conditions inside the refrigerator. Too warm? The compressor turns on. Cold? Turns off.
This is based on the fact that the pressure in the part varies with temperature. With it, it connects or disconnects the contacts that regulate the operation of the compressor. So there is a constant maintenance of the required amount of cold. However, a malfunction may allow the compressor to freeze too much or too little.
Signs that it's time to change the temperature sensor
The detail is important, so the signs of damage will be more than serious and noticeable. They will definitely leave no doubt that it is time to replace it. Therefore, checking and subsequent replacement of the refrigerator thermostat will not cause you any problems. Damage signs:
- The technique began to turn products into ice. Yes, it's very easy to see. We are sure that this will definitely not pass by your eyes and will instantly make you think about repairs. Also, such a breakdown can manifest itself in the formation of ice on the walls of the equipment.
- Food is not getting enough cold. It is clear that this will lead to their deterioration.
- You hear the compressor running too often or not often enough. Yes, during the operation of the equipment, you probably got used to the noise, but a change in frequency may indicate a breakdown.
- Leaks also show a malfunction of the equipment. The ice melts due to the lack of cold, which the broken thermostat does not allow to add.
Instructions for replacing the thermostat
1. Check the location of the part
For older refrigerators, it is inside, and for new ones, it is outside. We will analyze the case with its location on the door, however, the method is suitable for any refrigerator.
2. Loosen the screws and remove the door
Perhaps they are closed with a rubber pad. First, dismantle it, then - the door.
3. Remove the rear door cover
Most likely, it rests on hex screws.
4. Remove the temperature control knob
To do this, simply unscrew it from the main body
5. Remove the bracket and pull out the thermostat
Remember the type of connection! You have to connect a new one in exactly the same way.
Compliance temperature regime is a very important technological condition not only in production, but also in Everyday life. Having so great importance, this parameter must be regulated and controlled by something. A huge number of such devices are produced, having many features and parameters. But making a thermostat with your own hands is sometimes much more profitable than buying a ready-made factory analogue.
Create your own thermostat
General concept of temperature controllers
Devices that fix and simultaneously regulate the set temperature value are more common in production. But they also found their place in everyday life. To maintain the necessary microclimate in the house, thermostats for water are often used. With their own hands they make such devices for drying vegetables or heating an incubator. Such a system can find its place anywhere.
In this video, we will learn what a temperature controller is:
In fact, most thermostats are only part of the general scheme, which consists of the following components:
- A temperature sensor that measures and fixes, as well as transmits the information received to the controller. This happens due to the conversion of thermal energy into electrical signals that are recognized by the device. A resistance thermometer or a thermocouple can act as a sensor, which in their design have a metal that reacts to temperature changes and changes its resistance under its influence.
- The analytical block is the regulator itself. It receives electronic signals and reacts depending on its functions, after which it transmits a signal to the actuator.
- An actuator is a kind of mechanical or electronic device that, when receiving a signal from the unit, behaves in a certain way. For example, when the set temperature is reached, the valve will shut off the coolant supply. Conversely, as soon as the readings fall below the set values, the analytical unit will give a command to open the valve.
These are the three main parts of the system for maintaining the set temperature parameters. Although, in addition to them, other parts, such as an intermediate relay, may participate in the circuit. But they perform only an additional function.
Principle of operation
The principle by which all regulators work is the removal of a physical quantity (temperature), the transfer of data to the control unit circuit, which decides what needs to be done in a particular case.
If you make a thermal relay, then the simplest option will have a mechanical control circuit. Here, with the help of a resistor, a certain threshold is set, upon reaching which a signal will be given to the actuator.
To get additional functionality and the ability to work with a wider temperature range, you will have to embed a controller. This will also help increase the life of the device.
In this video you can see how to make a thermostat for electric heating yourself:
Homemade temperature controller
There are actually a lot of schemes for making a thermostat yourself. It all depends on the area in which such a product will be used. Of course, creating something too complex and multifunctional is extremely difficult. But a thermostat that can be used to heat an aquarium or dry vegetables for the winter can be created with a minimum of knowledge.
The simplest circuit
The most simple circuit the do-it-yourself thermal relay has a transformerless power supply, which consists of a diode bridge with a zener diode connected in parallel, stabilizing the voltage within 14 volts, and a quenching capacitor. You can also add a 12 volt stabilizer here if you wish.
Creating a thermostat does not require much effort and money investments The whole circuit will be based on a TL431 zener diode, which is controlled by a divider consisting of a 47 kΩ resistor, a 10 kΩ resistance and a 10 kΩ thermistor acting as a temperature sensor. Its resistance decreases with increasing temperature. It is better to select a resistor and resistance in order to achieve the best operation accuracy.
The process itself is as follows: when a voltage of more than 2.5 volts is formed on the control contact of the microcircuit, it will make an opening, which will turn on the relay, applying a load to the actuator.
How to make a thermostat for an incubator with your own hands, you can see in the video below:
Conversely, when the voltage becomes lower, the microcircuit will close and the relay will turn off.
To avoid rattling of the relay contacts, it is necessary to select it with a minimum holding current. And in parallel with the inputs, you need to solder a 470 × 25 V capacitor.
When using an NTC thermistor and microcircuits that have already been in use, you should first check their performance and accuracy.
Thus, turns out to be the simplest device temperature control. But with the right components, it performs excellently in a wide range of applications.
Indoor device
Such thermostats with a do-it-yourself air temperature sensor are optimally suited for maintaining the specified microclimate parameters in rooms and containers. It is fully capable of automating the process and controlling any heat emitter from hot water and ending with tenami. At the same time, the thermal switch has excellent operational data. And the sensor can be both built-in and remote.
Here, the thermistor, indicated in the diagram R1, acts as a temperature sensor. The voltage divider includes R1, R2, R3 and R6, the signal from which is fed to the fourth pin of the operational amplifier microcircuit. The fifth pin DA1 receives a signal from the divider R3, R4, R7 and R8.
The resistance of the resistors must be selected in such a way that at the lowest possible temperature of the measured medium, when the resistance of the thermistor is maximum, the comparator is positively saturated.
The output voltage of the comparator is 11.5 volts. At this time, the transistor VT1 is in the open position, and the relay K1 turns on the actuator or intermediate mechanism, as a result of which heating begins. The ambient temperature rises as a result, which lowers the resistance of the sensor. At input 4 of the microcircuit, the voltage begins to rise and, as a result, exceeds the voltage at pin 5. As a result, the comparator enters a negative saturation phase. At the tenth output of the microcircuit, the voltage becomes approximately 0.7 Volts, which is a logical zero. As a result, the transistor VT1 closes, and the relay turns off and turns off the actuator.
On the LM 311 chip
Such a do-it-yourself thermocontroller is designed to work with heating elements and is able to maintain the set temperature parameters within 20-100 degrees. This is the safest and most reliable option, since its operation uses galvanic isolation of the temperature sensor and control circuits, and this completely eliminates the possibility of electric shock.
Like most similar circuits, it is based on a DC bridge, in one arm of which a comparator is connected, and in the other - a temperature sensor. The comparator monitors the circuit mismatch and reacts to the state of the bridge when it passes the balance point. At the same time, he also tries to balance the bridge with the help of a thermistor, changing its temperature. And thermal stabilization can occur only at a certain value.
Resistor R6 sets the point at which a balance should be formed. And depending on the temperature of the environment, the thermistor R8 can enter into this balance, which allows you to adjust the temperature.
In the video you can see an analysis of a simple thermostat circuit:
If the temperature set by R6 is lower than required, then the resistance on R8 is too high, which reduces the current on the comparator. This will cause current to flow and open the sevenstor VS1 which will turn on the heating element. This will be indicated by an LED.
As the temperature rises, the resistance R8 will decrease. The bridge will tend to the point of balance. On the comparator, the potential of the inverse input smoothly decreases, and on the direct input it increases. At some point, the situation changes, and the process occurs in the opposite direction. Thus, the do-it-yourself thermocontroller will turn on or off the actuator, depending on the resistance R8.
If there is no LM311 available, then it can be replaced with a domestic chip KR554CA301. It turns out a simple do-it-yourself thermostat with minimal cost, high precision and reliability.
Necessary materials and tools
By itself, the assembly of any circuit of the electric temperature controller does not take much time and effort. But to make a thermostat, you need minimal knowledge in electronics, a set of parts according to the diagram and a tool:
- Pulse soldering iron. You can use the usual, but with a thin sting.
- Solder and flux.
- Printed circuit board.
- Acid to etch the tracks.
Advantages and disadvantages
Even a simple do-it-yourself thermostat has a lot of advantages and positive aspects. It is not necessary to talk about factory multifunctional devices at all.
Temperature controllers allow:
- Maintain a comfortable temperature.
- Save energy resources.
- Do not involve a person in the process.
- Follow the technological process, improving the quality.
Among the shortcomings can be called the high cost of factory models. Certainly, homemade appliances it doesn't apply. But the production ones, which are required when working with liquid, gaseous, alkaline and other similar media, have a high cost. Especially if the device must have many functions and capabilities.
Temperature controllers are widely used in modern household appliances, automobiles, heating and air conditioning systems, in production, in refrigeration equipment and during the operation of furnaces. The principle of operation of any thermostat is based on turning on or off various devices after reaching certain temperature values.
Modern digital thermostats are controlled by buttons: touch or conventional. Many models are also equipped with a digital panel that displays the set temperature. The group of programmable thermostats is the most expensive. Using the device, you can provide for a change in temperature by the hour or set the desired mode for a week in advance. You can control the device remotely: via smartphone or computer.
For a complex technological process, for example, a steel-smelting furnace, making a thermostat with your own hands is a rather difficult task that requires serious knowledge. But collect small device for a cooler or an incubator, any home master can do it.
In order to understand how a temperature controller works, consider a simple device that is used to open and close the damper of a shaft boiler and is activated when the air is heated.
For the operation of the device, 2 aluminum pipes, 2 levers, a return spring, a chain that goes to the boiler, and an adjusting unit in the form of a crane box were used. All components were mounted on the boiler.
As you know, the coefficient of linear thermal expansion of aluminum is 22x10-6 0C. When an aluminum pipe with a length of one and a half meters, a width of 0.02 m and a thickness of 0.01 m is heated to 130 degrees Celsius, an elongation of 4.29 mm occurs. When heated, the pipes expand, due to this, the levers are shifted, and the damper closes. When cooling, the pipes decrease in length, and the levers open the damper. The main problem when using this scheme is that it is very difficult to accurately determine the threshold for the operation of a thermostat. Today, preference is given to devices based on electronic elements.
Scheme of operation of a simple thermostat
Typically, relay-based circuits are used to maintain the set temperature. The main elements included in this equipment are:
- temperature sensor;
- threshold scheme;
- actuating or indicating device.
As a sensor, you can use semiconductor elements, thermistors, resistance thermometers, thermocouples and bimetallic thermal relays.
The thermostat circuit reacts to the excess of the parameter above the set level and turns on the actuator. by the most simple option such a device is an element on bipolar transistors. The thermal relay is based on the Schmidt trigger. The thermistor acts as a temperature sensor - an element whose resistance changes depending on the increase or decrease in degrees.
R1 is a potentiometer that sets the initial offset on R2 thermistor and R3 potentiometer. Due to the adjustment, the actuator is activated and relay K1 is switched when the resistance of the thermistor changes. In this case, the operating voltage of the relay must correspond to the operating power supply of the equipment. To protect the output transistor from voltage pulses, a semiconductor diode is connected in parallel. The load value of the connected element depends on the maximum current of the electromagnetic relay.
Attention! On the Internet, you can see pictures with thermostat drawings for various equipment. But quite often the image and description do not match. Sometimes the illustrations may simply represent other devices. Therefore, production can only begin after a thorough study of all the information.
Before starting work, you should decide on the power of the future thermostat and the temperature range in which it will work. For the refrigerator, some elements will be required, and for heating, others.
Thermostat on three elements
One of the elementary devices, on the example of which you can assemble and understand the principle of operation, is a simple do-it-yourself thermostat designed for a fan in a PC. All work is done on a breadboard. If there are problems with the palnik, then you can take a solderless board.
The thermostat circuit in this case consists of only three elements:
- power transistor MOSFET (N channel), you can use IRFZ24N MOSFET 12V and 10A or IFR510 Power MOSFET;
- potentiometer 10 kOhm;
- NTC thermistor in 10 kOhm, which will act as a temperature sensor.
The temperature sensor reacts to an increase in degrees, due to which the whole circuit is triggered, and the fan turns on.
Now let's move on to the settings. To do this, turn on the computer and adjust the potentiometer, setting the value for the off fan. At that moment, when the temperature approaches the critical one, we reduce the resistance as much as possible before the blades rotate very slowly. It is better to make the adjustment several times to make sure the equipment is working efficiently.
The modern electronics industry offers elements and microcircuits that differ significantly in appearance and technical specifications. Each resistance or relay has several analogues. It is not necessary to use only those elements that are indicated in the scheme, you can take others that match the parameters with the samples.
Temperature controllers for heating boilers
When adjusting heating systems, it is important to accurately calibrate the device. This will require a voltage and current meter. To create a working system, you can use the following scheme.
Using this scheme, you can create outdoor equipment for controlling a solid fuel boiler. The role of the zener diode here is performed by the K561LA7 microcircuit. The operation of the device is based on the ability of the thermistor to reduce resistance when heated. The resistor is connected to the electricity voltage divider network. The required temperature can be set using a variable resistor R2. The voltage is supplied to the inverter 2I-NOT. The resulting current is fed to capacitor C1. A capacitor is connected to 2I-NOT, which controls the operation of one trigger. The latter is connected to the second trigger.
Temperature control is as follows:
- when the degrees decrease, the voltage in the relay increases;
- upon reaching certain value the fan connected to the relay turns off.
Soldering is best done on a mole rat. As a battery, you can take any device that operates within 3-15 V.
Carefully! Installing home-made appliances for any purpose on heating systems can lead to equipment failure. Moreover, the use of such devices may be prohibited at the level of services that provide communications in your home.
Digital thermostat
In order to create a fully functioning thermostat with accurate calibration, digital elements are indispensable. Consider a temperature control device for a small vegetable store.
The main element here is the PIC16F628A microcontroller. This chip provides control of various electronic devices. The PIC16F628A microcontroller contains 2 analog comparators, an internal oscillator, 3 timers, SSR comparison and USART data exchange modules.
When the thermostat is operating, the value of the existing and set temperature is fed to the MT30361 - a three-digit indicator with a common cathode. In order to set the required temperature, the buttons are used: SB1 - to decrease and SB2 - to increase. If you carry out tuning while pressing the SB3 button, you can set the hysteresis values. Minimum value The hysteresis for this circuit is 1 degree. A detailed drawing can be seen on the plan.
When creating any of the devices, it is important not only to solder the circuit itself correctly, but also to think about how best to place the equipment. It is necessary that the board itself be protected from moisture and dust, otherwise it cannot be avoided. short circuit and failure individual elements. You should also take care to isolate all contacts.
Video
Device
The thermostat consists of:
- A corrugated cylinder (bellows) filled with freon, from which a capillary (bellows) tube comes out, which is a sensitive element.
- A lever that changes its position depending on the pressure inside the bellows.
- Contacts opening and closing with a lever.
The principle of operation of the thermostat
The bellows tube is attached to the surface of the evaporator, and when the temperature in the evaporator drops, the pressure in the bellows tube and the bellows itself drops, the bellows is compressed, and the lever opens the contact of the motor-compressor power circuit.
The refrigerator is turned off, the temperature on the surface of the evaporator begins to rise, the pressure in the bellows tube and bellows increases, and the bellows, expanding, presses on the lever, thus closing the contacts.
Schematic diagram of the thermostat
Here we will consider three main types of thermostats. Outwardly, they look the same, the differences are in the temperature of opening and closing the contacts.
1. On single-chamber refrigerators thermostats of the following designations were installed:
T-110; T-111; T-112. Thermostat T-112 may have the designation TAM-112, or TAM-112-1M. In terms of temperature parameters, all these thermostats are the same. They differ in appearance - the diameter of the handle shaft and bellows tube, the presence of a transverse bar for mounting the thermostat. The end of the thermostat bellows tube is usually attached directly to the evaporator via a plastic gasket. The length of the bellows tube is indicated on the thermostat housing and has the form of two numbers separated by a comma. Example: a) 0.6 - tube length - 60 cm; b) 1.3 - tube length - 1 meter 30 cm.
There are three terminals on the end of the thermostat housing. Dual - this is the "land", i.e. thermostat housing. The other two, numbered 3 and 4, are the contacts through which the motor-compressor is powered.
Switching temperature - 12°С
Switch-off temperature −14°С
To install the thermostats of the new TAM-112, instead of the T-110, an installation kit is provided, consisting of a crossbar, a nut and a nylon adapter that increases the diameter of the adjusting rod.
2. On two-chamber refrigerators and refrigerating chambers two-engine two-chamber refrigerators were installed thermostats of the following designations: T-130; T-132; T-133; TAM-133 and TAM-133-1M.
Temperature parameters are the same. They differ in appearance, the diameter of the handle shaft and bellows tube, the presence of a transverse bar for mounting the thermostat.
Switch-on temperature +4°С
Switch-off temperature −14°С
3. On freezers, Basically, thermostats T-144 and T-145 were installed.
The T-144 thermostat does not have a temperature control rod, this value is set at the factory.
Switch-on temperature -20°С
Switch-off temperature −24°С
There are four terminals on the end of the thermostat housing. Dual - this is the "land", i.e. thermostat housing. The other two, numbered 3 and 4, are the contacts through which the motor-compressor is powered. The red alarm lamp is energized through pin 6, which means that the temperature in the freezer is too high. The opening temperature of this contact is −15°С.
4. Separately, we will consider thermostats for refrigerators "Stinol":
These can be thermostats K-57 and K-59 from RANCO, as well as domestic thermostats TAM-133-1M and TAM-145-1M. They differ from other thermostats in that they have a bellows tube that is covered with a vinyl sheath. In addition, they are equipped with a third contact at number 6, from which the motor-compressor is powered.
ATTENTION! The on-off temperature of thermostats is given as an average for each thermostat model and cannot be a guide for diagnostics or repair.
We present appearance thermostats manufactured by various companies:
Thermostat manufactured by RANCO
- Temperature range adjusting screw;
- Operating differential adjustment screw.
Thermostat manufactured by DANFOSS
- Adjustment screw for differential actuation;
- Adjusting screw for temperature range.
View from the end of the thermostat
View with the group of contacts removed.
Domestic
- The bottom screw adjusts the temperature range
thermostat device
The temperature regulator is designed to maintain the set temperature in the refrigerator by automatically turning off and on the compressor motor (in compression refrigerators) or the heater in (in absorption refrigerators).
When adjusting the cooling capacity by periodically stopping and starting the unit, the temperature in the refrigerator will fluctuate somewhat, which to a certain extent depends on the sensitivity of the thermostat.
According to the principle of operation, the thermostats of household refrigerators belong to manometric type devices, the operation of which is based on a change in the pressure of the working filler when its temperature changes (currently, electronic temperature controllers are used in certain models of foreign-made refrigerators).
The temperature regulator of a domestic refrigerator is a lever mechanism with a power lever and a contact system, in electrical circuit refrigerator. The power lever is acted upon by the elastic element (bellows) of the temperature-sensing system and the main spring, which is adjustable by a screw. The electrical insulating gasket isolates the electrical circuit of the device from its mechanical parts. The temperature-sensitive system of the manometric type consists of an elastic element - a bellows (a metal container with corrugated walls) or a membrane with a tube soldered to them. The system is filled with a small amount of freon or chloromethyl and carefully sealed.
Under operating conditions, freon is in a state of saturated steam, the pressure of which, as you know, varies in a certain dependence (for a given steam) on its temperature. The liquid phase of freon is located at the end of the tube. This part of the tube, especially at the point where the liquid and freon vapor separate, reacts to temperature changes, and it is placed in the controlled environment of the cooled object.
The operation of the thermostat.
As the temperature of the tube decreases, the saturated vapor pressure in the thermal system will decrease. Under the influence of the main spring, the corrugations of the bellows will be compressed and the power lever will turn on its axis, as a result of which the contacts will open. As the temperature rises, the vapor pressure will rise correspondingly. Overcoming the resistance of the spring, the corrugations of the bellows will expand, and the lever will turn in the opposite direction, while the contacts will close.
It follows from this that the set temperature at which the contacts will open depends on the force of the spring. So, with a smaller force of the main spring, the contacts will open at a correspondingly lower vapor pressure in the temperature-sensitive system and, consequently, at a lower temperature.
On the contrary, to obtain a higher temperature, the spring force must be large. In this case, the spring must overcome the relatively greater resistance of the bellows, since at a higher temperature there will be a greater freon vapor pressure in the temperature-sensitive system. Thus, to change the set temperature, it is necessary to change the force of the main spring. In practice, this is carried out with a thermostat knob, which, when turned, changes the spring tension.
The main elements of the thermostat.
In domestic refrigerators, thermostats of various designs are used, but their individual elements perform quite specific functions that are the same for all designs.
Knot of sharp opening contacts protects the thermostat contacts from burning when opening. In the above circuit diagram temperature controller, in order to simplify, the movable contact is placed on the power lever, which is directly affected by the bellows and the main spring. With such an arrangement of the movable contact, severe burning of the contacts and their rapid failure are inevitable. This is explained by the fact that the break in the electrical circuit when the contacts are opened will occur slowly in accordance with the movement of the lever, which, in turn, is determined by a slow change in temperature and, accordingly, the pressure of freon vapor in the temperature-sensitive system. In addition, with such an arrangement of the moving contact, a slight rotation of the power lever will immediately open or close the contacts, i.e. often break the chain. The knot of sharp opening of contacts eliminates these shortcomings. In this case, the movable contact is located on another lever (plate) connected to the power lever by a special toggle spring. When turning the power lever to certain positions, the lever with the contact will remain stationary, and then the toggle spring will sharply change its position and the contacts will abruptly open (or close).
Temperature change unit is a device by which the tension of the main spring is changed. In some thermostats, the spring tension is changed by rotating the screw, which moves the nut resting against the end of the spring, in others, by rotating the roller with a profile cam pressed onto it, acting on the spring. The screw (roller) is rotated with a handle that has a pointer to set it to a certain position on the scale of the device.
The temperature sensitive system is a sensor that reacts to temperature changes in the controlled object and acts on the contact system of the device.
The end part of the tube, which is sensitive to temperature changes, may differ slightly for different thermostats, which depends mainly on the level of the freon liquid phase in it. With a small internal diameter of the tube or relatively in large numbers freon in the tube, when the level of its liquid phase exceeds 80 ... .100 mm, it is difficult to ensure a snug fit of the tube to the evaporator wall at such a length. In these cases, the end of the tube is twisted into a spiral, bent into a knee, or a can is soldered with an inner diameter larger than that of the tube.
Differential adjustment unit serves to regulate the magnitude of the differential. The thermostat differential is the difference between the opening and closing temperatures of the contacts (at a certain tension of the main spring). The smaller the differential of the device, the more within narrow limits the set temperature will be maintained. In the thermostats of household refrigerators, this unit is used only for the factory installation of the device. It is missing in many designs.
The differential is changed using a screw, which, being a limiter for moving the power lever, brings closer or removes the moment of throwing the lever with a moving contact by the toggle spring.
The semi-automatic defrosting unit of the evaporator creates convenience when removing the snow cover. The unit is used in separate designs of temperature controllers. The principle of its operation and the device depends on the method of removing snow cover, adopted in a particular refrigerator.
TAM 133
1 – thermosensitive system; 2, 7 - levers, 3-case, 4.5 - springs, 5-slider, 6-nut, 7,10,14- adjustment screw, 8-block, 9-additional contacts, 11- main contacts, 12 lever, 13-spring, 16-axle, 17-lever
To maintain the required temperature range in a modern refrigerator, special device thermostat, thermostat for short. The refrigerator thermostat switches the compressor on and off. Sometimes a situation arises when it fails, and there is nothing to replace it, then you can find correct solution and make it with your own hands, consider the scheme of such a device.
The thermostat has a galvanic isolation from the supply voltage and allows you to maintain the temperature inside the refrigerator chamber with a fairly good accuracy.
Refrigerator thermostat on OS TLC271
The temperature sensor is LM335. In fact, as follows from the description, this is a voltage regulator, the parameters of which are sensitive to temperature changes. The LM335 is connected with only two contacts. The cathode is connected to the positive through load resistor R1, and the anode to minus.
The voltage from the LM335 is fed to the direct input of the TLC271 comparator, at its inverse input there is a potential from the voltage divider across the resistances R3, R4, R5.
The temperature range in the internal chamber of the refrigerator is regulated by the variable resistance R4. If the temperature rises above this range, then the voltage at the direct input of the comparator will decrease compared to the inverse input. This will create a logic one signal at the output of the comparator, which will open the transistor.
Two optothyristors are connected in the collector circuit of the KT3102 transistor. Their LED parts are connected in series, while their thyristor parts are connected in parallel and in opposite directions. Therefore, it appears interesting opportunity manage alternating current(The first thyristor of the optocoupler operates on the first half-wave, and the second on the second half-wave. The refrigerator compressor turns on.
As soon as the temperature inside the refrigerator chamber drops below the set range, a logic zero level will form at the output of the comparator and the compressor will turn off.
At this option circuit, the compressor turns on when the temperature reaches + 6 degrees and turns off when it drops to + 4 degrees Celsius.
This temperature range is quite enough to maintain the required temperature for storing food, and at the same time, comfortable operation of the compressor is ensured, preventing its heavy wear. This is especially true in older models that use a thermal relay to start the engine.
Refrigerator thermostat on LM35
The thermostat reads the temperature with an LM35 sensor, whose resistance varies with the temperature in the refrigerator compartment, linearly calibrated with a factor of 10 mV per 1 degree Celsius.
Since the output voltage is clearly not enough to open VT1, the LM35 sensor is connected according to the current source circuit. Its output is loaded with resistance R1 and therefore the current strength varies in proportion to the temperature in the chamber. This current causes a drop across the resistance R2. The voltage drop controls the operation of the first bipolar transistor VT1. If the voltage drop is above the threshold voltage level of the emitter junction, both transistors open, relay K1 is activated, and its front contacts start the motor.
Resistance R3 creates a positive circuit feedback. This provides a hysteresis to prevent the compressor from starting too often. The winding of the electromagnetic relay must be five volts, and its contacts must withstand the current and voltage flowing through them, see.
The LM35 temperature sensor is located inside the refrigeration unit in the correct location. The resistance of the resistor R1 is soldered directly to the sensor so that you can connect the LM35 to the board with just two wires.
If you need to slightly adjust the temperature level, then this can be done by selecting the resistance value of the resistors R1 or R2. Resistor R3 sets the amount of hysteresis.
The basis of the design is the K157UD1 operational amplifier with an output current of 300mA, which makes it possible to connect an optothyristor directly to the output of the op-amp without using a buffer transistor. The op-amp is included as a comparator. The refrigerator compressor shutdown temperature is set by the resistance R1. The difference between the on and off temperatures is set by the resistance R4.
Instead of an electronic key on an optotriac and a powerful triac VS1, you can use a conventional relay with a switching current of 10 Amperes. In this case, the relay winding is connected to the sixth pin of the DA1 chip and the third pin of DA2. A damping diode is connected to the same pins. In the case of using a relay, it will be necessary to increase the value of the capacitance of the capacitor C5 to 1 microfarad. If an electronic key is used in the design, then the diodes VD1 and VD2 can be eliminated by connecting the second output of DA2 directly to the case.
After all, no one can forbid us to use one of them for a possible replacement.
