Hello to all fans of electronic homemade products. Recently I quickly made an electronic thermostat with my own hands; the circuit diagram of the device is very simple. An electromagnetic relay with powerful contacts that can withstand current up to 30 amperes is used as an actuator. Therefore, the homemade product in question can be used for various household needs.
According to the diagram below, the thermostat can be used, for example, for an aquarium or for storing vegetables. Some may find it useful when used in conjunction with an electric boiler, while others may use it for a refrigerator.
As I already said, the circuit is very simple and contains a minimum of inexpensive and common radio components. Typically, thermostats are built on a comparator microcircuit. Because of this, the device becomes more complicated. This homemade product is built on an adjustable zener diode TL431:
Now let's talk in more detail about the parts that I used.
Device details:
A burnt Granit-1 electronic meter was used as a housing. The board on which all the main radio components are located is also from the meter. Inside the case there is a power supply transformer and an electromagnetic relay:
As a relay, I decided to use a car one, which can be purchased at any auto store. Coil operating current is approximately 100 milliamps:
Since the adjustable zener diode is low-power, its maximum current does not exceed 100 milliamps, it will not be possible to directly connect the relay to the zener diode circuit. Therefore, we had to use a more powerful transistor KT814. Of course, the circuit can be simplified if you use a relay whose current through the coil is less than 100 milliamps, for example, or SRA-12VDC-AL. Such relays can be connected directly to the zener diode cathode circuit.
I'll tell you a little about the transformer. The quality I decided to use was non-standard. I have a voltage coil lying around from an old induction meter for electrical energy:
As you can see in the photo there is free space for the secondary winding, I decided to try winding it and see what happens. Of course, the cross-sectional area of the core is small, and therefore the power is small. But for this temperature controller this transformer is sufficient. According to calculations, I got 45 turns per 1 volt. To obtain 12 volts at the output, you need to wind 540 turns. To fit them I used a wire with a diameter of 0.4 millimeters. Of course, you can use a ready-made one with an output voltage of 12 volts or an adapter.
As you noticed, the circuit contains a 7805 stabilizer with a stabilized output voltage of 5 volts, which powers the control pin of the zener diode. Thanks to this, the temperature controller has stable characteristics that will not change due to changes in the supply voltage.
As a sensor, I used a thermistor, which at room temperature has a resistance of 50 Kom. When heated, the resistance of this resistor decreases:
To protect it from mechanical influences, I used heat-shrinkable tubes:
A place for the variable resistor R1 was found on the right side of the thermostat. Since the resistor axis is very short, I had to solder a flag onto it, which is convenient to turn. On the left side I placed the manual control switch. Using it, it is easy to check the operating status of the device, without changing the set temperature:
Despite the fact that the terminal block of the former electric meter is very bulky, I did not remove it from the housing. It clearly includes a plug from some device, such as an electric heater. By removing the jumper (yellow on the right in the photo) and using an ammeter instead of the jumper, you can measure the current supplied to the load:
Now all that remains is to calibrate the thermostat. For this we need. You need to connect both sensors of the device together using electrical tape:
Use a thermometer to measure the temperature of various hot and cold objects. Using a marker, mark the scale and markings on the thermostat, indicating the moment the relay turns on. I got from 8 to 60 degrees Celsius. If someone needs to shift the operating temperature in one direction or another, this can be easily done by changing the values of resistors R1, R2, R3:
So we made an electronic thermostat with our own hands. Externally it looks like this:
To prevent the inside of the device from being visible through the transparent cover, I closed it with tape, leaving a hole for the HL1 LED. Some radio amateurs who decided to repeat this scheme complain that the relay does not turn on very clearly, as if it was rattling. I didn't notice any of this, the relay turns on and off very clearly. Even with a slight change in temperature, no chattering occurs. If it does occur, you need to select more precisely the capacitor C3 and resistor R5 in the base circuit of the KT814 transistor.
The assembled thermostat according to this scheme turns on the load when the temperature drops. If, on the contrary, someone needs to turn on the load when the temperature rises, then you need to swap sensor R2 with resistors R1, R3.
Modern refrigerators are considered reliable household appliances. There are practically no complex electronics in them, therefore, there are a minimum of parts that fail. The most common refrigerator breakdown is the failure of the thermostat. In the mechanical control circuit of the refrigerator, it participates in the operation of the motor-compressor. The thermostat is mounted in the chamber or on the front panel of the unit.
In the latest generation of refrigeration units, the thermostat has replaced a device that more accurately copes with its responsibilities. In this article we will try to figure out how to check the refrigerator thermostat.
As you know, refrigeration units run on freon. So far, this is the only gas that is not dangerous and, due to its special properties, is capable of changing its state of aggregation. It moves through the cooling system using a motor-compressor. First, increased pressure is created on the rear wall of the unit, while a reduced pressure is formed on the evaporator. As a result, the freon located on the back of the cooler is liquefied, and evaporation begins on the evaporator, which is confirmed by the refrigerator diagram attached to the instructions.
A thermostat is a fairly simple device. Even in modern refrigerators and refrigerators this is a simple contact group. It is controlled by a pressure gauge with a capillary tube, the end of which is located in the chamber and measures the temperature. Today there are two types of temperature regulators in refrigerators: mechanical and electronic.
A modern thermostat has two main elements. This is a box in which there are control and actuator mechanisms, and a capillary elongated into a tube. The box is a bellows (hermetically sealed tubular spring). The accuracy of the determined indicators depends on its tightness. The compression and expansion of the bellows is controlled by a spring, optimizing it with pressure indicators. Modern ones may have several springs. It depends on the destination: refrigerator or freezer.
An electronic thermostat for a refrigerator is more reliable and allows smooth regulation of the operation of the entire refrigeration system. The price of this device is significantly higher than mechanical ones and ranges between two thousand rubles (while a mechanical one costs up to a thousand). In an electronic thermal relay, a thyristor or sometimes a resistor is responsible for sensitivity.
In refrigerators with high energy consumption, such thermostats quickly fail. In class A+ cooling units with linear compressors, electronic temperature controllers require replacement much less frequently. Therefore, most manufacturers of such equipment are now switching to linear compressors with electronic thermostats.
The direct purpose of the thermostat in a refrigeration unit is to maintain the temperature set by the consumer. In compression refrigeration units, the thermostat turns the compressor motor on and off, and in absorption refrigeration units, the heater is turned on and off. The device that regulates the temperature in the cooling chambers is classified as a manometric design. This means that the operation of the unit depends on the instability of the pressure of its filler (usually gas) as the temperature fluctuates.
A mechanical thermostat is a lever device that contains a power lever and a contact circuit. The elastic element (tubular bellows) of the temperature control system and the spring exert an impact on the power lever. The electrical part of the device is separated from the mechanical part by an electrically insulating gasket.
The operating conditions for freon are concentrated steam, the pressure of which depends on temperature conditions. At the end of the tube, liquid gas accumulates. The section of tube in which the separation of vaporous freon and liquid occurs reacts to temperature fluctuations. This segment is located in the cooling zone.
The mode is always associated with a knob that switches temperature modes. In previous generation models, the thermostat is located under a plastic cover inside the refrigerator compartment. To replace it, you need to use a flat screwdriver to pry up the mode switch, remove it, then remove the plastic cover.
In models of recent years, from the attached instructions (refrigerator diagram) you can find out where the thermostat is located in the refrigerator. Most often it is placed above the door. To get to it, you need to remove the mode switch and the plastic structure covering the thermal relay.
There may be several problems with the thermostat. For example, the refrigerator freezes, but very weakly. In this case, you need to try adjusting the temperature controller or replacing it. Before checking the refrigerator thermostat, you need to make sure that the door closes tightly enough and the compressor is operating at the specified power.
It happens that the device begins to leak or the compressor works without stopping. It is not necessary that in each of these cases it is the thermostat that fails. It is likely that there may be another reason, but the temperature regulator needs to be checked first.
The most common reason for thermostat failure is physical wear and tear. Why is this happening? Refrigerator thermostat malfunctions may be due to loss of seal, swelling, or oxidation. There have been cases of defective devices, but this is rare. Therefore, it makes no sense to repair such a system. It will be cheaper to replace the thermostat in the refrigerator.
There are several ways to check the refrigerator thermostat:
This brand of refrigerators is very popular in our country. Almost the only disadvantage of such units is that the thermostat becomes faulty very quickly (after 5-6 years of operation). The cause of the breakdown is the short working life of this device, supplied by the German company RANCO (5 years). The tightness of the bellows in the thermostat, which is sensitive to temperature fluctuations, is broken.
Defects indicating that the refrigerator thermostat is faulty:
At home, it is impossible to accurately determine the malfunction of the thermostat of the Stinol refrigerator. But if the compressor turns on when the contacts are closed by a jumper, then there is a high probability that the temperature controller is faulty, and therefore it is necessary to contact a company that carries out urgent repairs of refrigerators.
A refrigerator breakdown due to a thermostat failure, especially in the hot season, feels like the end of the world. Food disappears, there is no way to cool drinks, and a leak may occur that damages the flooring. Naturally, you need to call a specialist.
Please note that urgent refrigerator repairs are always carried out at home. But a professional technician with extensive experience will easily identify the malfunction based on the symptoms mentioned and will come to the call with the necessary set of spare parts.
After replacing the temperature controller or during long-term operation, minor changes may occur in the operation of the refrigerator. There may be several reasons, but most often it is an incompletely adjusted thermostat. How to fix it?
Setting up a refrigerator thermostat is a labor-intensive and time-consuming process. The time spent depends on the duration of the cycles between turning the device on and off. If time is limited, you can debug the thermostat by measuring the temperature in the freezer or refrigerator compartment. In this case, there is no necessary correction for ambient temperature.
The adjustment involves tightening or loosening the power spring. To do this, you need to find out where the power spring screw is located, in which direction turning will reduce the temperature, and in which it will increase for a specific refrigerator model. Typically, rotating the screw on the spring clockwise increases the temperature, and counterclockwise decreases it (one turn is approximately equal to 5-6 °C).
Before starting work, you need to pull out the gasket between the bellows and the chamber wall (after completing the adjustment, the gasket should return exactly to its place). Then the temperature on the evaporator shelf is measured with the motor-compressor running and average temperature conditions. After 3-3.5 hours, the temperature is measured again. After comparing the initial and final temperatures, it is necessary to relax or tighten the power spring (after disconnecting the refrigerator from the electrical network).
The electronic thermostat for refrigerator will help in cases where your own (factory) thermostat is faulty or its operating accuracy is no longer sufficient. Older refrigerators use a mechanical temperature thermostat using a liquid or gas that fills a capillary.
When the temperature changes, the pressure inside the capillary also changes, which is transmitted to the membrane (bellows). As a result, the thermostat turns the refrigerator compressor on and off. Of course, such a temperature control system has low accuracy, and its parts wear out over time.
As you know, the temperature of food storage in the refrigerator should be +2...8 degrees Celsius. The operating temperature of the refrigerator is +5 degrees.
An electronic thermostat for a refrigerator is characterized by two parameters: start and stop temperatures (or average temperature plus hysteresis value) of the compressor. Hysteresis is necessary to prevent the refrigerator compressor from turning on too often.
This circuit provides a hysteresis of 2 degrees at an average temperature of 5 degrees. Thus, the refrigerator compressor turns on when the temperature reaches + 6 degrees and turns off when it drops to + 4 degrees.
This temperature range is sufficient to maintain the optimal temperature for storing food, and at the same time it ensures comfortable operation of the compressor, preventing its excessive wear. This is especially important for older refrigerators that use a thermal relay to start the engine.
The electronic thermostat is a suitable replacement for the original thermostat. The thermostat reads the temperature using a sensor whose resistance changes depending on the change in temperature. A thermistor (NTC) is often used for these purposes, but the problem is its low accuracy and the need for calibration.
To ensure accurate setting of the controlled temperature and eliminate the need for hours of calibration, in this version of the thermostat for the refrigerator, . It is an integrated circuit linearly calibrated in degrees Celsius, with a gain of 10 mV per degree Celsius. Due to the fact that the threshold temperature is close to zero, the relative change in output voltage is large. Therefore, the signal from the sensor output can be controlled using a simple circuit consisting of only two transistors.
Since the output voltage is too low to turn on transistor VT1, sensor LM35 is included as a current source. Its output is loaded by resistor R1 and therefore the current on it changes in proportion to the temperature. This current causes a drop across resistor R2. The voltage drop controls the operation of transistor VT1. If the voltage drop exceeds the threshold voltage of the base-emitter junction, transistors VT1 and VT2 open, relay K1 turns on, whose contacts are connected instead of the old thermostat contacts.
Resistor R3 creates positive feedback. This adds a small current to R2, which shifts the threshold and thereby provides hysteresis. The coil of the electromagnetic relay must be designed for 5...6 volts. The relay contact pair must withstand the required current and voltage.
The LM35 sensor is located inside the refrigerator in a suitable location. Resistor R1 is soldered directly to the temperature sensor, which in turn allows you to connect the LM35 to the circuit board with just two wires.
The wires connecting the sensor can introduce noise into the circuit, so capacitor C2 is added to suppress interference. The circuit operates from a 5 volt power supply built by . Current consumption mainly depends on the type of relay used. must be reliably isolated from the network.
The big advantage of this circuit is that it starts working immediately upon first startup and does not require calibration or configuration. If there is a need to slightly change the temperature level, this can be done by selecting resistances R1 or R2. Resistance R3 determines the amount of hysteresis.
Portable USB oscilloscope, 2 channels, 40 MHz....
The described electrical circuit of the electronic thermostat for the refrigerator changes the duration of the pause in the operation of the compressor, which depends on the internal temperature.
The electrical circuit (Fig. 1.35) contains a generator on the DD1 microcircuit, switches on the radio elements DD2.2, DD2.3 and an inverter on the DD2.1 element.
The generator on the K176IE5 chip has switchable RC circuits (Rl, R3, Cl and R2, R4, C2). modification of timing circuits is carried out using keys on the K561KT3 microcircuit. Key control begins with signals from the output of the fifteenth digit (pin 5) of the signal divider DD1.
At high voltage, the output is 5 k internal log. One RC circuit (R2, R4, C2) is connected to the elements of the DD1 microcircuit. At low voltage, the electrical signal is reversed by the inverter on element DD2.1 and, through the key DD2.2, another electrical circuit is connected (Rl, R3, Cl). To change the refrigerator thermostat, resistance R4 can have a value of 100 kiloOhms or more.
When the temperature in the refrigerator dropped to 0 degrees, the MMT4 thermistor with a resistance of 220 kiloohms had a resistance of 400 kOhms. Since the thermistor is connected in the circuit that determines the duration of the pause, the lower the temperature in the refrigerator compartment, the greater the pause time in the operation of the refrigerator compressor.
Consequently, the temperature is adjusted by changing the duration of the pause in the operation of the refrigerator compressor with resistance R3. The control pulse, through the switch on transistor VT1, turns on the intermediate electric relay Kl, which turns on a more powerful relay. Intermediate electric relay brand RES6, RES49.
The K561KT3 microcircuit can be replaced with a K176KT1. Switch SA1 is needed to enable continuous operation of the compressor after the refrigerator has defrosted. The printed circuit board of the electric relay is shown in Figure 1.36, and from the installation side of the radio components, Figure 1.37.
The dimensions of the board are limited by the size of the 220 V electric relay. The board contains rectifier diodes and filter capacitors. Thermistor R3 is soldered to a thin MGTF wire and placed in the freezer.
Resistor R4 and switch SA1 are placed close to the plastic side cover of the relay. The alternating voltage going to the electrical circuit must be such that the rectified voltage does not exceed 9 V. At a lower voltage. The K176IE5 chip can still work, but at voltage. more than 9V it may not work.
If you need an extremely low frequency generator with separate adjustment of the duration of high and low levels, then resistance R3 can be replaced with a potentiometer up to 3 MOhm. The frequency is approximately calculated using the formula F =0.7/RC.
When calculating the duration, it should be remembered that the moment of work or pause will be equal to half the calculated one, since only part of the period is taken - either a high level or a low one.
A simplified thermostat diagram was considered. Now let's look at its second option.
After checking the electrical circuit on the breadboard, I felt dissatisfied with the fact that I had to use an entire microcircuit package to use one inverter. Of course, it was possible to replace the inverter with a transistor, but I wanted to make do with two cases. Therefore, the electrical circuit shown in Fig. 1.31.
It eliminates the inverter, and the keys for the RC pause circuit are controlled from the output of the 14th bit of the DD1 divider. The timing diagrams of the operation of two adjacent counter bits are shown in Fig. 1.32a. If the frequency being divided does not change, then the time intervals tl, t2, t3, t4 are the same and equal to half the pulse period of the low-order digit of the counter.
When switched on according to the proposed scheme, the timing diagram will look approximately like in Fig. 1.32b.
When one appears at the output of the 14th bit of the counter (state 01), the RC generator operates with the time-setting pause elements - Rl, RЗ, Cl - turned on. The next state of the counter is 10. One at the output of the 15th digit on the timing elements of the work - R2, C2 and resistors Rl, RЗ, R4 are connected in parallel with R2.
The generator operates at a different frequency and therefore the period tl is not equal to the period t2. When the counter state is 11, time-setting elements and pauses and work are included in parallel.
Moreover, if, when connected in parallel, the capacitances Cl, C2 are summed up, then the resistor values are calculated using a well-known formula and will always be less than the smaller value due to the parallel connections (with the values indicated in the diagram, the difference between the maximum and minimum influence on the value of the electrical circuit of operation will be 1 kOhm ).
The period t3 will differ from the interval t2, but their sum will be the operating life of the refrigerator. State 00 is interesting in that the values of the capacitances C l, C2 are not only summed up with each other, but also with small values of the capacitances of the public key transitions in series connection. That is, the total capacity of the timing electrical circuit will be very small.
Even with a large resistance Rl + RЗ+ R4 included in the RC circuit, the frequency of the generator will be high, and the period t4 will be a fraction of a second (maximum - 0.8 s, minimum - 0.2 s). The moment t4 is added to the interval tl and constitutes the pause period. The operating interval, with the ratings indicated on the diagram, is 20...23 minutes. the pause time varies from 3 to 30 minutes. It is practically determined that any refrigerator mode can be set by changing only the pause duration.
If you need other intervals of work and pause periods, then you need to follow a simple rule. To reduce the influence of timing circuits on the calculated frequency when they are connected together, you should increase the capacitance rating in the RC circuit connected to the high-order digit of the counter (pin 5 of the DD1 microcircuit). And in the RC circuit connected to the low-order digit of the counter (pin 4) - increase the resistor values.
One from the output of the 15th digit of the counter through resistance R5 and the switch on transistor VT1 turns on the intermediate electric relay Kl. The intermediate electric relay was chosen in order to reduce the size of the power supply. To quickly return the refrigerator to mode after defrosting, it is possible to place a toggle switch in the gap of the transistor base. One contact of the toggle switch will go to the power supply plus, and the second to pin 5 of the DD1 microcircuit. After approximately an hour of continuous operation, the refrigerator will reach temperature and the toggle switch can be switched to temperature control mode.
Note: as an alternative, you can use the previously described one.
The electric relay is used brand RES6 passport RF0.452.145. A more powerful 220 V electric relay can be any with contacts that can withstand switching current of at least 10 A. In Fig. 1.33 shows the topology of the board from the side of the printed conductors, and in Fig. 1.34 - view from the installation side of the elements. Resistors MLT0.125, R3 - SP00.5.
Capacitors: Cl - KM5B, C2 - K7317. The K561KTZ microcircuit can be replaced without changing the printed circuit board with a K176KT1. Electric relay Kl and filter capacity C3 are located together with the power supply.
