Shutdown thermostat. Thermal relay with remote temperature sensor: principle of operation. The main parameters of the thermal relay

A thermostat on the farm is sometimes an indispensable thing that helps control the thermal regime on a home incubator or vegetable dryer. Built-in mechanisms for this purpose often deteriorate quickly or do not differ in decent quality, which forces you to invent a simple thermostat with your own hands.

If you are among those who urgently needed homemade device with the function of thermoregulation, stay here, because all the suitable and tested schemes, combined with theory and useful tips, are listed below.

What is applicable for?

A temperature controller or thermostat is a device capable of restarting and stopping the operation of heating or cooling units. For example, it allows you to maintain the optimal mode in the incubator, and is also able to turn on the heating in the basement, fixing the low temperature.

How it works?

Before you make a thermostat with your own hands, you need to understand the accompanying theory. Principle this device is identical to the operation of simple measurement sensors capable of changing resistance depending on ambient temperature conditions. A special element is responsible for changing the indicator, and the so-called reference resistance remains unchanged.

In the thermostat device, an integrated amplifier (comparator) reacts to a change in the resistance value, switching microcircuits when a certain temperature is reached.

What should be the scheme?

On the Internet and in regulatory documentation, it is easy to find diagrams of thermostats for various purposes, which you can assemble with your own hands. In most cases, the basis of a schematic drawing is the following elements:

Control zener diode, designated TL431;
Integrated amplifier (K140UD7);
Resistors (R4, R5, R6);
Quenching capacitor (C1);
Transistor (KT814);
Diode bridge (D1).

The circuit is powered by a transformerless power supply, and an automotive relay designed for a voltage of 12 volts is perfect as an actuator, provided that the current entering the coil is at least 100 mA.

How to do?

Instructions for making a thermostat with your own hands are based on strict adherence to the chosen scheme, according to which it is necessary to connect all the components into a single whole. For example, electronic circuit for the incubator is assembled according to the following algorithm:

Examine the image (it is better to print and put in front of you).
Find the necessary parts, including the case and the board (the old ones from the meter will do).
Start with the "heart" - the integrated amplifier K140UD7 / 8, connecting it with a positively charged reverse action, which will give it the functions of a comparator.
Connect the negative resistor MMT-4 in place of “R5”.
Connect the remote sensor using shielded wiring, and the length of the cord can be no more than a meter.
To control the load, include a thyristor VS1 in the circuit by installing it on a radiator small size to ensure proper heat dissipation.
Set up the rest of the chain.
Connect to power supply.
Check functionality.

By the way, by adding a temperature sensor, the assembled device can be safely used not only for incubators, dryers, but also for maintaining the thermal regime in an aquarium or terrarium.

How to properly install?

In addition to high-quality assembly, it is necessary to pay attention to the conditions of its operation, which should include:

Location - Bottom part rooms;
Dryness of the room;
The absence of a number of “knocking down” units: radiating heat or cold (electrical equipment, air conditioning, opened door with draft).

Having figured out how to connect the thermostat with your own hands, you can proceed to it regular use. The main thing is that the power of the manufactured device is designed for the relay contacts. For example, with a maximum load of 30 Amps, the power should not exceed 6.6 kW.

How to repair?

A factory or homemade thermostat can be repaired so as not to buy a new one and not waste time searching for and assembling the necessary parts. First of all, the device must be found (if you did not install it), because the photo of the thermostat shows that its dimensions are small, which makes the search somewhat difficult.

Advice will help: the thermostat is located next to the temperature mode button.

Signs of a device failure may include the following:

The device has ceased to perform the main function: the temperature has dropped significantly or increased without the reaction of the mechanism;
The connected machine works without going into standby or saving mode;
The unit turned off spontaneously.

Depending on the cause of the malfunction, the following steps must be taken to repair the thermostat with your own hands:

Disconnect the repaired device from the network.
Remove the protective case from the device.
Check the quality of contacts and connections.
Disconnect and pull out the capillary tube.
Get the relay.
Change bellows tube, fix.
If necessary, replace other parts.
Connect wiring back.
Put the relay in place.

Many household and household appliances are equipped with thermostats, and knowing how to fix them, reassemble them with your own hands and install them will significantly save your money, time and effort.

DIY thermostat photo

A thermal relay with an external temperature sensor is a device for maintaining the temperature within the specified limits. It is impossible to do without it in heating systems, microclimate and greenhouses. Such devices differ, both in characteristics, in price and in reliability. Do right choice possible after obtaining general information about such devices.

What does a thermostat do

Devices of the considered type belong to the class of thermostats. For example, a thermal relay with an external temperature sensor is considered to be such. This means that the relay maintains the temperature within the specified limits. When the temperature goes beyond these limits, the relay switches the heating device: boiler, underfloor heating, heater or heating element. Switching is done in such a way that the temperature returns to the specified limits.

In the simplest case, the thermostat turns on the heater when the temperature drops below the desired temperature and turns it off when the temperature rises above the desired one. Sophisticated thermostats can connect and disconnect several sections of heaters or smoothly adjust the power.

Thermal relays consist of two mandatory parts: a temperature sensor and an actuator - this is the part that closes the contacts in the power circuit. These parts are combined in one device or connected with a cable. In each of these cases, the relay only works correctly when the sensor is located where the set temperature is maintained.

In addition to the sensor and output contacts, thermal relays often also contain a device for setting the desired temperature. In older devices, such a device looked like a rotary knob or a disk with a scale applied along the radius of the indicating beak or mark. New, modern devices are mostly digital and contain a few keys and a display. But in some models, the temperature is set as before, with a rotary knob, which is preferred by consumers, mostly elderly people with established habits. The choice on the market is sufficient.

The main parameters of the thermal relay

There are several such parameters. Here are the most important ones:

Operating temperature range;
setting accuracy;
hysteresis;
load power.

The temperature set for the operation of the relay is called the setpoint. The setting lies within the range of operating temperatures with which the temperature relay operates.

Hysteresis is the temperature interval of the steady state of the relay, when the relay maintains the load in the on state. The setpoint can take any position on this interval, but belongs to this interval. The hysteresis is not a bad quality of the relay, it is often normalized, even separately regulated and helps to avoid too frequent switching in the heater circuit, which will reduce the life of the heaters.

In domestic devices, the setpoint position is characterized as “plus or minus”. It's easier to count that way. For example, room temperature is comfortable for a person within 18-20 degrees Celsius. If the hysteresis of the regulator is 1 degree, then the setpoint in this case will be 19 degrees. If at the same time the accuracy of the thermal relay is 0.5 degrees, then the temperature will be maintained within 17.5 ... 20.5 degrees. More precisely, the thermal relay will work, and the true temperature will be determined by the power of the heater that works in conjunction with this relay.

The load power is expressed in terms of the current that the relay is capable of switching. It is known that electric heaters consume the highest power among other energy consumers. This means that such heaters need sufficient current and the relay must provide this current with its contacts. If the current load is too high for the relay contacts, then an intermediate relay is used: a magnetic starter or an electronic power switch. Otherwise, the relay contacts will quickly burn out and the relay will fail.

Types of thermal relay and its device

These types of thermal relays are used, which differ from each other in the principles of operation:

Relay with bimetallic plate.
Relay with thermal resistance.
Relay with thermocouple.
Relay with digital sensor.

Let's consider each of these types of relays in more detail. Such thermal relays are sold and the consumer should have a sufficient understanding of them.

With bimetal plate

These relays were among the first to be used and were the best for their time. In a relay with a bimetal plate, the temperature sensor and the contacts of the external circuit are located side by side. As the main part, a bimetallic plate is used. It is made of two metals with different coefficients of thermal expansion. When heated, a metal with a large coefficient expands more than another. This leads to the fact that the part made from such a plate begins to bend with a constant dependence on temperature.

The bending plate acts on a mechanical part with contacts that close and open under the influence of temperatures. To add hysteresis to the mechanical part, an elastic rocker has been added, giving the mechanism a trigger effect for clear shifting. This mechanism is also controlled by a screw connected to a scale marked in degrees or symbols.

The figure above shows an example of a bimetal switch (valve) for a water heating system. In a real relay, instead of a stem or plunger, the force is transmitted to electrical contacts. A similar device was used in old electric irons, thermal relays of magnetic starters, and is still used (in an unregulated version) to protect electric kettles from turning on without water. But not only. He found application in industry. The best samples achieved good accuracy, but at the expense of complexity and high price.

Why do we need a thermostat for a heating battery, how it works, how to use it and how to mount it ourselves - we will talk about all this in

In bimetallic relays, in order to control the setpoint and hysteresis simultaneously, two relays were often used at once, the changeover contacts of which were connected in accordance with the required logic. Such a relay is shown in the figure above. It shows one of the two bimetallic plates, which is coiled for greater sensitivity. One scale was used for the on setpoint and the other for the off setpoint, and the hysteresis was chosen arbitrarily.

The advantages of bimetallic thermal relays are their cheapness and reliability, and the disadvantages are sensitivity to shocks and shocks, and also not high accuracy and the inability to use a remote sensor.

RTD relay

A thermal relay with resistance uses the dependence of the electrical resistance of a conductor or semiconductor on the ambient temperature. This type of relay proliferated in the 1970s with the industrial use of op amps. The sensor in such a relay can be taken out at a sufficient distance, and the sensor itself can have miniature dimensions.

As a sensor in industrial thermal relays, standard copper or platinum resistances were used, enclosed in a sealed housing made of of stainless steel. These sensors are interchangeable. In simple and cheap models, in particular household ones, where high accuracy and stability of control are not required, a thermistor sensor is used.

Note! The thermistor (semiconductor thermistor) has a good response to temperature changes, but the disadvantage of the thermistor is the non-linearity of the dependence of resistance on temperature. Because of this, each device can only work with one type of sensor and even one instance. When replacing with the same type, re-calibration may be required.

The electronic part of the thermostats of the described type consists of a voltage divider, one arm of which uses a thermistor, and the other a resistance with a small temperature coefficient. The received signal is amplified and controls the electromagnetic relay. Improved circuits use a bridged sensor, a signal amplifier from the bridge and a comparator with an adjustable reference (comparison) voltage. The setting is set by the value of the reference voltage, and the hysteresis is either by selecting the signal amplification (in cheap devices) or by using two comparators.

Relay with thermocouple

This type of device is close to the previous one, operating on thermal resistances. The difference lies in the fact that to register the temperature, it is not the change in the resistance of the sensor that is used, but the thermal emf. (electromotive force). emf occurs in an alloy (junction) of two insulated wires from different metals. Such sensors have good performance, but require compensation for the second junction. Since in practice it usually does not exist, this compensation is created artificially, and the “cold junction” is considered to have a temperature of 20 degrees Celsius, the standard normal (room) temperature.

Note! The “cold junction” is named so not for its temperature, but because, in contrast to the “hot” junction, it does not participate in measurements.

Note! For appliances made in the USA for the domestic market, the normal temperature is 27 degrees Celsius.

Thermocouples are standardized and interchangeable, but only to the original type, which is set to the instrument being used. The connection of thermocouples can sometimes use three terminals, one of which is connected to a compensating thermistor. This is used with increased requirements for accuracy and a small operating range.

Note! The correct polarity must be observed when connecting thermocouples. This is important to consider when repairing after breaks!

Relay with digital sensor

This is the most modern type of thermal relay for the temperature range from -50 to +100 degrees, that is, close to the field of human activity and the environment.

The sensor uses a semiconductor chip of a large integrated circuit (smaller than a match head) containing a semiconductor sensor and a microprocessor for signal data processing. Three wires are used to communicate with the rest of the relay: ground, power, and a single-wire interface.

A feature of such sensors is that they can be connected in parallel with a “garland”, up to 64 sensors and work independently in one network on one bus. To work with them, a special protocol has been developed: the controller transmits the address of the sensor, after which it receives a response from it. This allows you to get advanced temperature control devices with flexible configurations and minimum consumption wires and cables.

The figure above shows a single-channel thermal relay board with a display. Three buttons are designed to control the operating mode. One button puts the relay in setpoint mode, and the other two buttons are used to “scroll” the values on the display. The device then enters the keep warm mode. This is an example of the simplest digital thermostat for budget applications.

A digital thermostat does not necessarily use digital temperature sensors. Such a relay can be made for analog sensors with digitization of the input signal in the relay itself, but the sensor will be remote. The device may include a sensor that measures its internal temperature.

Video - Temperature controllers with an air temperature sensor

Relay on DIN rail

Modules assembled on a DIN rail have now completely replaced the old panel mounting of equipment in cabinets, which is very inconvenient for maintenance and repair. Snapping onto the rail takes seconds. The wires are laid in cable trays within the cabinet and clamped with screw terminals at the connection points with their full accessibility for installation and illumination.

In this way, electrical equipment for industrial, municipal and domestic purposes is assembled. Thermal relays are no exception, which are also produced in a housing for mounting on a DIN rail.

When installed in a cabinet or box, there is no need to spoil the walls and the appearance of the premises. The relay sensors are displayed in the controlled area, and the relays themselves stand with the rest of the equipment in the cabinet.

In the DIN rail version, most types of thermostats are available. Online stores offer consumers big choice. Some models contain an interface for cable connection, for example, for a wireless communication device, if there is a need to control the relay remotely from a mobile phone or smartphone.

Do-it-yourself thermostat

For those who know how to tinker: work with a soldering iron, have a sufficient minimum knowledge in the field of electrical engineering, there are options for self-manufacturing of a thermal relay. From the available variety, it is better to choose not archaic schemes of past decades, but an option close to the present. It is easier to find modern components that are reliable in operation and more accurate than old ones. Circuits have also become simpler, thanks to the high degree of integration of the new chips. Here is a variant with a solid-state analog sensor:

The U1 sensor is available in the TO-92 or TO-220 package. In the first case, it is suitable only for measuring air temperature. The second housing is suitable for fastening to metal plates, for example, for measuring the temperature of batteries or pipes. The variable resistor R5 must be linear, since the LM35 sensor itself has good linearity. Comparator U2 compares the reference voltage from the slider of the resistor R5 and from the sensor.

The output of the comparator is current amplified by transistor T1 and then fed to the base of transistor T2, the switch that turns on relay K1. Diode D1 must be used to protect transistor T2 from electrical breakdown during self-induction of the relay coil. The load contacts must be rated for a current of 2-5 A. If the load power is more than 400-1000 W, which corresponds to the selected relay, then an intermediate magnetic starter or triac should be used.

Table 1. Replacing transistors and diodes

BC549C	KT315V, KT315G
BD139	KT815B, KT805B
1N4002	KD105B, KD212A

The sensor can be taken out of the board of the device at a distance of 5-10 meters. But in this case, the wire from pin 2 must be in a metal braid (shielded). The braid is connected to pin 3 (ground), and the power is supplied by a separate wire. Resistor R1 and capacitor C2 also need to be taken out with the sensor and placed in its own case. The device is powered by a voltage source direct current 12 V.

The scale must be calibrated according to the readings of a reference thermometer, which is placed close to the sensor. When changing the temperature, it is necessary to wait 2-3 minutes for the readings of the sensor and the thermometer to equalize with each other.

Conclusion

Thermal relay - thermostat, temperature relay, thermostat, device synonyms. Evolving from simple electromechanical, with bimetal or bellows, to modern digital devices, thermal relays have undergone great improvements in performance towards accuracy and reliability. At the same time, their price remains low, affordable for consumers, and the devices themselves remain necessary for domestic air conditioning, microclimate, kitchen appliances and greenhouse farming.

Before purchasing a thermal relay, it is advisable to familiarize yourself with the parameters listed here in order to select the appropriate device, and also take into account the features of the purchased device for reliable and durable operation. And we must also remember that the thermal relay is a control device, the serviceability of which affects other equipment or property, and it must always be operational and in good order.

Video - Wireless thermostat for a heating boiler

Nowadays, a person makes his life easier with the help of various devices. These units make it possible to transfer the heating, hot water supply and ventilation systems to automatic mode. This type of device also includes a thermostat. Thermal relay for heating systems to turn on off - in addition to convenience, it is a very useful device. This unit allows the owner to save on energy consumption.

The main advantage is that the parameters are set by the owner, after which his participation for the operation of the device is not required. It is only necessary to choose the appropriate model. We will analyze what models of thermal relays are designed to control temperature, and also in what places you can use thermostats with a remote sensor, and how to make such a unit yourself.

The principle of operation of this device depends on the temperature in the room, the closing or opening of the electrical contacts of the boiler depends on the increase or decrease in the temperature inside the room. Thanks to this, the house is constantly optimum temperature and no extra energy is wasted.

The thermal switch with the possibility of temperature control is an electromechanical device whose task is to control the temperature in a non-aggressive environment. The temperature is controlled by the ability to close and open the contacts of the electrical circuit, based on changes in temperature. This feature allows you to turn on devices only when necessary.

For many modern boilers, a variety of sensors are included in the design, the purpose of which is to control operating modes. But in fact, if you figure it out, then the owner has to constantly monitor these devices. Based on this, we can conclude that once a day the owner needs to inspect the boiler and check how it works. But most people place the boiler in a separate room and jogging back and forth cause some inconvenience. Despite the fact that these sensors monitor the temperature of the coolant, and not the climate in the house.

To solve this problem, engineers created a room thermostat. Its design includes a sensor that monitors the temperature of the environment where it is located. As soon as the temperature falls below the set value, the unit is activated and continues to operate until the temperature reaches the set parameters. Based on the conditions, the thermal relay gives commands to the boiler to turn on or off.

For example, a thermostat with external heat-sensitive sensors can be used to adjust the operation of the heating system, based on what the weather conditions are. The regulator will give a command to start the heaters as soon as the outside temperature drops below the set parameters.

Moreover, the thermostat can be used for:

control of units for heating water in hot water supply systems and autonomous heating;
water heating boiler and battery life"warm floor";
automation of air conditioning systems in greenhouses;
in automatic systems heating of the cellar and other storage and utility rooms.

For the device to work properly, it must be positioned so that it does not have any thermal influence - batteries, fireplaces, stoves, etc. Otherwise, you should not expect the correct operation of the thermal relay.

Types of thermostats with a temperature sensor

There are several types of aggregate data that perform certain tasks. And therefore, before you purchase a device, you should study its types in more detail.

Thermal relays are divided into groups:

Room. The name itself suggests that the installation of this type of device is carried out directly in the room. The parameters of the room in no way affect the operation, so this type of device can be installed both in a residential area and in others. BUT! It is worth considering that they monitor the temperature of the external environment, and from this it follows that the wrong installation location may affect the correct operation of the device. Units of this type are installed in open spaces, but in such a way that there are no foreign objects or heating devices in front of them. Otherwise, the natural air circulation is disturbed, which will lead to the fact that the sensor will not be able to correctly monitor the ambient temperature. This type of thermostat is successfully combined with outdoor sensors.
TRV. This type of thermostat is necessary not so much for the boiler, as in order to regulate the valve devices installed on the heating pipes. Thanks to this, it is possible to control each circuit separately, which is very convenient and economical if there are rooms that are not used for some reason.
Cylinder thermostat. This type of relay is suitable for double-circuit boilers with simple electronics. This type of device prevents too hot coolant from entering the system. What is it for? The whole trick is that heating can be used different types pipes - somewhere there may be old cast-iron elements, and somewhere polypropylene. What most people don't think about is that high temperatures cause PP and PE pipes to deform, which entails the risk of rupture or leakage. The cylinder thermostat allows you to set a specific temperature limit for the coolant, and if it rises due to something, then the unit will simply automatically turn off the boiler for a while. When the boiler is turned off, the coolant cools down.
Zone thermostat. Such devices serve for large areas, which is why it is quite rare to find them in private homes. This type the relay works in conjunction with fans and makes it possible to regulate the current of the coolant, breaking it literally into “threads”. This process occurs, starting from the temperature regime in each section.

When purchasing a relay for turning on and off, you need to pay special attention to what heating system is installed, what type of boiler it has, how much is the area of \u200b\u200bthe house, is there a need to heat the entire area of \u200b\u200bthe house and so on. Based on these facts, you can choose the right device for you.

What parameters to pay attention to when choosing?

Thermal relays are configured for specific temperature characteristics or adjustable. There are, in addition, devices for both simultaneous closing / opening of contacts, and for separate performance of these functions.

There are some specifications that you need to study before purchasing such a device:

the temperature at which the device is triggered - the parameters during which the opening or closing of the contacts occurs;
temperature return indicator - at the moment this parameter is reached, the device takes its original position;
differential - represents the difference during which the device is in a state of "rest", that is, from the moment of operation to return;
switched current and voltage - are indicators of "durability", because of this, starting from the parameters of the current in the home network, it is necessary to choose a device with a slightly larger value;
contact resistance;
time indicator of operation;
error - this characteristic may have a value of ± 10% of the specified value.

These are the main parameters that each thermal relay has. But based on the modification, their meaning may change.

If we consider prices, then it all depends on the device:

Mechanical thermostats. Most simple options bottom type will cost about $ 20, while its payback is measured literally by the end of its first heating season.
Programmable thermostat. Prices for this type of relay start at $ 30, the disadvantages of this type of device include the presence of batteries, which must be changed periodically.

The range of choice of thermostat is quite large, and naturally their prices can vary quite a lot. But this does not mean that it is necessary to chase the cheapness of the device in order to mount it into the system. More or less high-quality devices cost from 2000 rubles, you should not pay attention to everything that is cheaper.

How to assemble a thermostat with your own hands?

A relay that will be similar in principle of operation can be assembled independently. Often, homemade air temperature controllers can be powered from a 12 V battery. Power can also be supplied using power cable from electrical wiring.

Before you start making a thermostat, you need to prepare the body of the device and other tools that will be required for work in advance.

To independently make a reliable thermostat with a sensor, you must:

Prepare the body of the device. For this task, a case from an old electric meter or circuit breaker is perfect.
Connect a potentiometer to the comparator input (marked with a “+”), and LM335 temperature sensors to the negative inverse input. The principle of operation of the device is quite simple. As soon as the voltage rises at the direct input, the transistor transfers power to the relay, which subsequently to the heater. At the moment when the voltage on the reverse stroke becomes higher than on the forward stroke, the level at the output of the comparator approaches zero and the relay turns off.
A negative relationship must be created between direct input and output. This will allow you to set the limits for turning on and off the thermostat.

In order to power the thermostat, a coil from an old electromechanical electric meter is suitable. In order to get a voltage of 12V, you need to wind 540 turns on the coil. To solve this problem, a copper wire with a cross section of at least 0.4 mm is best suited.

After the installation of the regulator, it must be powered from a separate machine, which is installed in the switchboard. For these purposes, a two-wire cable is used, connected to the input terminals of the regulator "zero" and "phase".

In the event that the amount of current that is switched by the device corresponds to the power of the heater, then the wires from it must be connected to the input terminals "+" and "-". It is better to use wires with a section margin in order to avoid heating them when the maximum current passes through them.

If the current that the heater uses exceeds the limiting characteristics of the thermal relay, a magnetic starter with the required load current must be connected to the output terminals. It is also required to connect multiple heaters to one controller. It is extremely necessary to establish grounding on the heater body. For this, a separate wire is used, which has a low resistance. After all the conditions and recommendations are met, the regulator can be put into operation.

If there is not even a minimum experience for working with electrical equipment, then in order to prevent various sad misunderstandings, it is better to seek help from a qualified specialist.

At home and subsidiary farm often it is required to maintain the temperature regime of a room. Previously, this required a rather huge circuit made on analog elements, we will consider one such circuit for general development. Today, everything is much simpler, if it becomes necessary to maintain the temperature in the range from -55 to +125 ° C, then the programmable thermometer and thermostat DS1821 can perfectly cope with the goal.

Scheme of a thermostat on a specialized temperature sensor. This DS1821 temperature sensor can be bought cheaply from ALI Express (click on the picture just above to order)

The temperature threshold for switching on and off the thermostat is set by the values TH and TL in the sensor's memory, which must be programmed into the DS1821. If the temperature exceeds the value recorded in the TH cell, the level of a logical unit will appear at the sensor output. To protect against possible interference, the load control circuit is implemented in such a way that the first transistor is locked into that half-wave of the mains voltage when it is zero, thereby applying a bias voltage to the gate of the second field-effect transistor, which turns on the opto-triac, and it already opens the VS1 smystor that controls the load . The load can be any device, such as an electric motor or a heater. The reliability of locking the first transistor must be adjusted by selecting the desired value of the resistor R5.

The DS1820 temperature sensor is capable of detecting temperatures from -55 to 125 degrees and operating in thermostat mode.

The scheme of the thermostat on the sensor DS1820

If the temperature exceeds the upper threshold TH, then the output of the DS1820 will be a logical unit, the load will turn off the network. If the temperature drops below the lower programmed level TL, then a logical zero will appear at the output of the temperature sensor and the load will be turned on. If there are any unclear points, homemade construction was borrowed from #2 in 2006.

The signal from the sensor goes to the direct output of the comparator on the operational amplifier CA3130. The inverting input of the same op-amp receives a reference voltage from the divider. Variable resistance R4 set the required temperature.

Thermostat circuit on the LM35 sensor

If the potential at the direct input is lower than that set at pin 2, then at the output of the comparator we will have a level of about 0.65 volts, and if vice versa, then at the output of the comparator we will get a high level of about 2.2 volts. The signal from the output of the op-amp through transistors controls the operation of the electromagnetic relay. At high level it turns on, and when low, it turns off, switching the load with its contacts.

TL431 is a programmable zener diode. Used as a voltage reference and power supply for low power circuits. The required voltage level, at the control output of the TL431 microassembly, is set using a divider on resistors Rl, R2 and a negative TCR thermistor R3.

If the voltage on the TL431 control pin is higher than 2.5V, the microcircuit passes current and turns on the electromagnetic relay. The relay switches the control output of the triac and connects the load. As the temperature rises, the resistance of the thermistor and the potential at the control contact TL431 drops below 2.5V, the relay releases its front contacts and turns off the heater.

Using resistance R1, we adjust the level of the desired temperature to turn on the heater. This circuit is capable of driving a heating element up to 1500 watts. The relay is suitable for RES55A with an operating voltage of 10 ... 12 V or its equivalent.

The design of the analog thermostat is used to maintain the set temperature inside the incubator, or in a box on the balcony for storing vegetables in the winter. Power is provided by a 12 volt car battery.

The design consists of a relay in the event of a temperature drop and turns off when the set threshold rises.

The temperature of the thermostat relay operation is set by the voltage level on pins 5 and 6 of the K561LE5 microcircuit, and the relay off temperature is set by the potential at pins 1 and 21. The temperature difference is controlled by the voltage drop across resistor R3. In the role of the temperature sensor R4, a thermistor with a negative TCR is used, i.e.

The design is small and consists of only two blocks - a measuring unit based on a comparator based on an op-amp 554CA3 and a load switch up to 1000 W built on a power regulator KR1182PM1.

The third direct input of the op-amp receives a constant voltage from a voltage divider consisting of resistances R3 and R4. The fourth inverted input is supplied with voltage from another divider at the resistance R1 and the thermistor MMT-4 R2.

The temperature sensor is a thermistor located in a glass flask with sand, which is placed in the aquarium. The main node of the design is the m / s K554SAZ - voltage comparator.

From the voltage divider, which also includes a thermistor, the control voltage goes to the direct input of the comparator. The other comparator input is used to adjust the desired temperature. A voltage divider is made of resistances R3, R4, R5, which form a bridge sensitive to temperature changes. When the temperature of the water in the aquarium changes, the resistance of the thermistor also changes. This creates a voltage imbalance at the comparator inputs.

Depending on the voltage difference at the inputs, the output state of the comparator will change. The heater is made in such a way that when the water temperature drops, the aquarium thermostat automatically starts, and when it rises, on the contrary, it turns off. The comparator has two outputs, collector and emitter. To control a field effect transistor, a positive voltage is required, therefore, it is the collector output of the comparator that is connected to the positive line of the circuit. The control signal is obtained from the emitter terminal. Resistors R6 and R7 are the load output of the comparator.

The IRF840 field effect transistor is used to turn the heating element on and off in the thermostat. To discharge the gate of the transistor, there is a diode VD1.

The thermostat circuit uses a transformerless power supply. Excess alternating voltage is reduced due to the reactance of capacitance C4.

The basis of the first design of the thermostat is the PIC16F84A microcontroller with a DS1621 temperature sensor with an l2C interface. At the time of power-up, the microcontroller first initializes the internal registers of the temperature sensor, and then configures it. The thermostat on the microcontroller in the second case is already made on the PIC16F628 with the DS1820 sensor and controls the connected load using the relay contacts.

DIY temperature sensor

The dependence of the voltage drop on p-n junction semiconductors on temperature, is the best suited for creating our homemade sensor.

A thermostat with temperature control can be purchased at a store or you can make it yourself Today, devices are being actively introduced into the life of a modern person to automate the operation of heating and ventilation systems, hot water supply. These devices include thermostats. What types of thermostats for temperature control exist today, where you can use thermostats and how to make a device yourself - read below.

What is a thermostat with temperature control

The temperature control thermostat is an electromechanical device designed to control the temperature in a non-aggressive environment. Temperature control through the device occurs due to the ability of the relay to open and close the contacts of the electrical circuit, in accordance with changes in the temperature regime.

This allows you to use heating devices only when they are actually needed.

So, for example, a thermal relay with external heat-sensitive sensors can be used to control the operation of the heating system depending on weather conditions. The regulator will turn on the heaters when the temperature in the street drops below the set one.

In addition, the thermostat can be used for:

Management of equipment for heating water in autonomous heating and hot water supply systems;
Autonomous operation of "warm floor", water-heating boiler;
Automation of air conditioning systems in greenhouses;
In automatic heating systems of the cellar and other storage and utility rooms.

There are several types of thermostats. Basically, the devices differ in execution. At the same time, their device remains practically unchanged. The main structural elements of the thermal relay include a temperature-sensitive sensor and a thermostat, which gives a signal to turn on or off heating and air conditioning devices. Information about the actual and set temperature conditions is usually displayed on the digital display of the device, and the LED indicator signals the operating status of the relay.

What is the hysteresis of the thermostat for?

Today, most temperature control devices have the function of both setting the desired temperature and adjusting the hysteresis. What is thermostat hysteresis? This is the temperature at which the signal reverses. By setting the hysteresis, the relay turns on or off the equipment connected to it.

The main function of the thermostat hysteresis is to turn off and on the equipment that is connected to it.

That is, hysteresis is the difference between the on and off temperatures of devices that provide heating or cooling of the medium.

So, for example, if the hysteresis of the thermostat is 2 °C, and the device itself is set to 25 °C, then when the ambient temperature drops to 23 °C, the thermostat will start the equipment that controls the heating of the room. Such equipment can be electric heater or gas boiler heating. At the same time, the greater the hysteresis, the less often the thermal relay will start. This should be taken into account if the main purpose of installing an automatic thermostat is to save energy.

Types of thermal relays for on-off

A conventional on/off thermostat is a compact electronic unit that is mounted on a wall at a suitable location and connected to the controlled equipment. The simplest, and therefore the most affordable temperature controller has a mechanical control.

In addition, the entire thermal relay is divided into:

Programmable control devices. Such regulators are connected to the equipment both wired and wireless. The relay is configured through a special program or LCD display. Thanks to software you can configure the relay to operate at certain times of the day and year.
Thermal relay with GSM wireless programming module. Such devices can be either with one or two temperature sensors.
Autonomous regulators powered by batteries. Such installations are most often used to control the operation of household appliances (for example, refrigerators), incubators.

Separately allocate wireless devices with an external sensor. Such devices are considered the most effective. They differ in speed, because the temperature sensor reacts to temperature changes even before it has time to affect the temperature inside the room.

How to make a thermostat with your own hands

A thermal relay suitable for the mode of action can be ordered in the online store, or you can assemble it yourself. Most often, home-made air temperature controllers are designed to be powered by a 12 V battery. You can also power the thermal relay to the electrical wiring through the power cable.

In order to make a thermostat, you must first prepare the body of the device and other tools for work.

In order to assemble a reliable thermostat with a sensor, you should:

Prepare the body of the device. For these purposes, you can choose a case from the old electric meter, automatic switch.
Connect a potentiometer to the comparator input (marked with a “+”), and LM335 temperature sensors to the negative inverse input. The scheme of operation of the device is quite simple. When the voltage at the direct input rises, the transistor supplies power to the relay, and it, in turn, to the heater. As soon as the voltage at the reverse input becomes higher than at the direct input, the level at the output of the comparator will approach zero, and the relay will turn off.
Create a negative relationship between direct input and output. This will create limits for turning the thermostat on and off.

To power the thermostat, you can take a coil from an old electromechanical electric meter. To obtain the required voltage of 12 V, it will be necessary to wind 540 turns on the coil. For this it is best to use copper wire with a diameter of at least 0.4 mm.

How to make a thermostat for an incubator with your own hands

An incubator is an indispensable thing in agriculture, which allows you to breed chicks at home. The temperature of the incubator can be controlled using a thermostat. The thermostat for the incubator can be purchased, or you can assemble it yourself from improvised materials.

There are two ways to make a thermostat for an incubator:

Using a zener diode, a thyristor and 4 diodes with a power of at least 700 watts. Temperature control is carried out through a variable resistor with a resistance in the range from 30 to 50 kOhm. The temperature sensor in this device will be a transistor installed in glass tube and placed on a tray with eggs.
Using a thermostat. Using a soldering iron, you will need to attach a screw to the thermostat housing and connect it to the contacts. Turning the screw will adjust the temperature readings.

The second method is considered the simplest and most affordable. Regardless of the type of thermostat, before laying eggs, the incubator must be warmed up, and the home-made thermostat must be adjusted.

A temperature-controlled thermostat is a simple device that allows you to automate the operation of heating, heating and air conditioning equipment. Thanks to the thermal relay, electrical appliances can be automatically used for their actual purpose, reducing energy consumption. The recommendations above will help you choose a thermostat. And if you couldn’t find the most suitable device, you can always assemble a thermostat on your own!

Thermal relay with temperature control: do-it-yourself thermostat, temperature sensors for turning on and off

Thermal relay with temperature control: where you can use thermostats, ways to make a thermostat with a sensor with your own hands.

Do-it-yourself thermostat

The device and principle of operation of the thermal relay
Typical thermal relay circuit
How the finished scheme works
A simple device diagram

thermostat or thermostat living conditions used for refrigerators, irons and other appliances. Often there are situations when it is necessary to set a certain temperature in the room or connect underfloor heating. For this purpose, you can use factory products, or you can make a thermostat with your own hands with the parameters necessary for specific conditions.

The device and principle of operation of the thermal relay

For amateur designs, the most common practice is to use thermistors, diodes or transistors. Based on them, a simple electrical circuit is obtained.

The set temperature is maintained by periodically turning on or off the heating element. When the temperature approaches the set level, the comparator is activated, which turns off the heating element. However, despite the apparent simplicity, in practice there are certain difficulties.

The greatest difficulty is setting up and adjusting the required temperature. The characteristic points of the temperature scale are determined by alternately immersing the sensor in a container with melting ice and boiling water. Thus, it is possible to calibrate the temperature of zero degrees and the boiling point. Based on the data obtained, the necessary intermediate temperature for the operation of the thermal relay is adjusted.

In the thermal relay circuit, it is recommended to use temperature sensors that have already been calibrated at the factory. They are produced in the form of sensors that work with microcontrollers. The transfer of information is carried out in digital form. Most often, the designs use the LM335 device and its modifications 135 and 235. The first digit of the marking means the purpose of the device. The sensor with the number 1 is used in the military field, with 2 - in the industry, and 3 is intended for household appliances. It is the 335th model that is used in the household relay circuit. The device is intended for operation in the temperature range from -40 to +100 degrees.

Typical thermal relay circuit

The basis of the design is the LM335 temperature sensor or its a log, as well as the LM311 compramator. The thermal relay circuit is supplemented by an output device, to which a heater with an installed power is connected. There must be a power supply, if necessary, indicators can be used.

A more complex circuit includes transistors, a relay, a zener diode and capacitor C1, which smooths out voltage ripples. Current equalization is carried out using a parametric stabilizer. In this case, the device can be powered by any source whose parameters match the voltage of the relay coil in the range from 12 to 24 volts. The power supply can be stabilized using a conventional diode bridge with a capacitor.

How the finished scheme works

With the help of a transistor, a relay is turned on, which, in turn, ensures that the magnetic starter is turned on. Through its contacts, the heater is connected to the network with two of its own contacts. In this case, there is no phase left on the load when the starter trips. If the humidity in the room is increased by I, it is recommended to use an RCD for connection.

As a heater, in addition to heating elements, are used oil coolers, 100 W incandescent lamps and household heaters with a built-in fan. Avoid direct access to live parts.

After the thermal switch for switching on and off with your own hands is assembled, you should check the quality and correctness of installation. All connections must be well soldered. After that, you can configure the device in accordance with the specified parameters.

Do-it-yourself thermostat

After the thermostat is assembled with your own hands, you should check the correct installation. All connections must be well soldered. After that, you can configure the device

Temperature sensors, thermistors, thermal relays.

Temperature sensors are sensors that convert a temperature value into other physical parameters, such as resistance or voltage.

Thermistors

Thermistors are temperature sensors that convert a temperature value into resistance. Any conductor has resistance, which also changes with temperature. The value that shows how much the resistance changes when the temperature changes by 1 0 C is called the temperature coefficient of resistance -TCR, and if the resistance increases with increasing temperature, then the TCR is positive, and if it decreases, then it is negative.

The main characteristics of thermistors:

Range of measured temperatures;

Maximum power dissipation (meaning thermal characteristic);

Thermistors- these are thermistors with negative TCS (NTC - negative temperature characteristic). They are made from oxides. various metals, ceramics and even diamond crystals.

NTC resistors are used as temperature sensors, in household appliances and in industrial applications, from -40 to 300 0 С.

Another area of application is the limitation of the inrush current in various electronic devices, for example, in switching power supplies, which are in absolutely all devices powered by the mains. When connected to the network, the thermistor has room temperature and a resistance of the order of several ohms. At the moment of charging, the capacitor experiences a current surge, but the thermistor does not allow it to rise above the limit, which depends on the resistance of the thermistor. When the current passes, the thermistor heats up and its resistance drops to almost zero, and in the future it does not affect the operation of the device.

posistors- thermistors with positive TCR (PTC - positive temperature characteristic). All metals, for example, have a positive TCR, they are also made from ceramics and semiconductor crystals.

Posistors are also used as temperature sensors, but their scope is not limited to this, they are used:

As protective elements in transformers, electric motors and other electronic devices where there is a risk of overheating. To do this, the posistor is connected in series with the load - the motor winding or the electronic circuit, and the posistor itself directly into the heating zone - is glued with hot glue to the winding or healed with a clamp or simply pressed using thermal paste. At the same time, such overheating protection is quite effective and has no limits on the on / off cycle, since there are no breaking contacts, just a protective thermistor acquires a high resistance and a residual current passes through it, the value of which is absolutely not dangerous for the load. But the posistor can still be disabled - with a sharp voltage surge, since the current will exceed the rated one. For example, if instead of 220 V 380 V comes, its resistance will be quite low, since the temperature is normal, but the current that passes through it will exceed the nominal one and it will simply burn out, opening the load.

Another application is the starting of compressor motors. This scheme is used in low-power refrigerating machines– refrigerators, freezers ah, in which single-phase electric motors with a starting winding are installed. In modern air conditioners, such a scheme is no longer used, using two-phase electric motors with working phase-shifting capacitors.

In this case, the working winding is connected directly to the network, and the starting winding through a posistor. After starting the compressor, the posistor heats up from the current passing through it and increases its resistance, turning off the starting winding. By the way, because of this, during a short-term loss of supply voltage, the compressor may not start, since the thermistor will not have time to cool down and will fail due to overheating of the main winding.

PTC resistors are used in fluorescent lamp start-up circuits.

In this circuit, when the lamp is turned on, the posistor has a small resistance and current flows through it, while the filaments in the lamp and the posistor itself heat up, after heating, the posistor circuit opens and the lamp turns on already with heated electrodes. This scheme significantly extends the life of energy-saving lamps.

These thermistors have also been used as liquid level sensors. The control scheme is based on the different properties of the liquid and air - the heat capacity and heat transfer of the liquid significantly exceeds these parameters in air.

Also, posistors are used as heating elements - in household appliances, the automotive industry. These are just those very advertised ceramic heaters that “do not burn oxygen”

A thermocouple is a thermoconverting element, which is a “junction” of dissimilar metals.

In a circuit with two such junctions, with a temperature difference between them, a thermo-EMF will appear in the circuit, the value of which will depend on the nature of the metals and the temperature difference between the junctions. The thermoelectric effect was first discovered in the first half of the nineteenth century.

Applications for thermocouples are very different - in industry, in medicine, for research purposes. Thermocouples can measure quite high temperatures, such as the temperature of liquid steel (about 1800 0 C).

The material for the manufacture of thermocouples is copper, chromel, alumel, platinum, and semiconductor materials.

Used and reverse effect- when passing electric current in the circuit, a temperature difference appears between the two junctions, refrigerators were produced in the middle of the last century, the working element was a semiconductor-based thermocouple. But due to the lower efficiency compared to compressor refrigerators, they were no longer produced.

Semiconductor temperature sensors

Although I also make thermistors from semiconductor materials, here we are talking about the effect of temperature changes on the p-n junction of transistors and diodes. These devices are characterized by a voltage temperature coefficient - TKN. This is the change in applied voltage with a change in temperature. For all semiconductors, it is negative, approximately 2mV / 0 C.

On the basis of semiconductor temperature sensors, specialized microcircuits are produced, in which a temperature-sensitive element, signal amplifiers and stabilization circuits are placed on one crystal. Currently, such microcircuits are widely distributed and are produced in millions of pieces by many manufacturers. And the consumer receives a finished calibrated product with an output signal of the desired value and the error (accuracy) he needs. They use such microcircuits as temperature sensors in a wide variety of devices.

Another application of semiconductor thermal sensors is as stabilization and compensation elements in electronic circuits. For example, when current flows through powerful power elements, it heats up, x resistance changes and, accordingly, the parameters, in order to compensate for this effect, a thermotransistor is attached to its case and included in the thermal compensation circuit.

Thermal relays are devices for turning on or off the load when a certain temperature is reached, they convert thermal energy into a mechanical one, which goes to make / open electrical contacts.

The scope of these products is automation and protection of devices in everyday life, at work, in cars. For example, they are used in irons, thermal curtains, electric fireplaces. Their main advantage is low price and simplicity.

They produce adjustable thermal relays and tuned to a specific response temperature. With make and break contacts, as well as with groups of contacts for making/breaking at the same time.

Thermal relay technical parameters:

Response temperature - the temperature at which the relay contacts close / open

Return temperature, respectively, at which a return to its original state occurs

Hysteresis (differential) - the difference between the response and return temperatures

Switched current and voltage, the durability of the device depends on this parameter, it is worth choosing a device with a current margin

Instrument error, e.g. +/- 10%

Bimetal thermal switches

In such relays, operation occurs due to the bending of platinum or a disk made of bimetal (that is, from two metals), due to different volumetric expansion of dissimilar metals. They are fairly simple and trouble-free.

There are two varieties of these types of relays - temperature controllers and temperature limiters. The first type regulates the temperature within certain limits, automatically turning the load on and off, while the second ones are used for protection and require a reset after a special button is triggered.

Manometric type temperature sensors

The temperature measurement with these sensors is based on the effect of volumetric expansion by various liquids.

They are used, for example, in water heaters or air conditioners to turn on crankcase heating and drainage. They are a flask with a liquid that is in contact with the measured medium and is connected to the contacts with a metal tube. As a working substance, a mixture based on alcohol or ethylene glycol is usually used.

Electronic thermal relays

It's already quite complex electronic devices which switch the load using electromagnetic relays, contactors, almost all of the above types can serve as temperature sensors. The signal is processed by a microcontroller or a specialized electronic circuit. Such devices can have several channels, for example, four, that is, they can control four points and control four loads, and display information on an electronic display. For installation in an electrical panel, a thermal relay is produced in a DIN rail housing.

Temperature sensors, thermistors, thermal switches

AT refrigeration use absolutely all types of temperature sensors and thermal relays, let's take a closer look at their types

Thermostat with many adjustments. W1209 DC 12V.

Measurement accuracy:

- 0.1 ° C - in the range from -9.9 to +99.9 ° C

— 1 °C ranging from -50 to -10 and from +100 to +110

- 0.1 °C - in the range from -9.9 to +99.9 °C

— 1 °C in the range from -50 to -10 and from +100 to +110 °C

Hysteresis: 0.1 to 15 °C

Hysteresis accuracy: 0.1 °C

Refresh rate: 0.5 seconds.

Circuit supply voltage: 12V DC (DC12V).

Power consumption: static current: 35mA; current with relay closed: 65mA

Thermistor: NTC (10K+-0.5%).

The length of the sensor extension is 50 cm.

Output: 1 channel relay output, power = 10A

Humidity 20%-85%

Size: 48*40*14mm.

Digital two-threshold, two-mode, unpackaged, 12V power supply temperature controller XH-W1209 is designed to maintain the required air temperature in incubators, greenhouses, terrariums, in heating systems, to control the temperature of underfloor heating, pools, freezers, systems for not freezing drains, etc.

The temperature controller is controlled by the STM8S003F3P6 microcontroller, which analyzes the temperature measured by the digital sensor, compares it with the set value, takes into account the set operating mode, and, based on these data, turns the load on and off. Switching is carried out by an electromagnetic relay.

Temperature regulator - contact (a relay power element is used in the temperature regulator). thermostat two-threshold- upper and lower thresholds(possibility of setting the upper value (threshold) of the switch-on (switch-off) temperature and the lower value (threshold) of the switch-on (switch-off) temperature.

set - selects the installation mode and parameter settings

AND - change the value of the setting and parameters

while the temperature is below the setpoint, the relay contacts are open, when the set temperature is reached, the relay contacts close and remain in this position until the temperature drops by the value of the set hysteresis (by default, 2ºС).

If you press the “SET” button, then using the “+” and “-” buttons you can set the temperature for switching on the relay (if the current temperature is LOWER than this value, then the contacts of the power terminals are closed.)

The thermostat must be paired with a heater or cooler.

To set the control temperature, press the SET button, then use the "+" or "-" buttons to set a new temperature, and press the SET button again.

To enter the programming mode, hold the SET button for 5 seconds, then use the "+" or "-" buttons to select a menu item from the list below. To save the settings, press and hold the SET button, or do not press any buttons for 10 seconds. To return to the default settings, press and hold the "+" button.

Instructions for use, with a detailed description of the programming modes, in Russian, included.

Control controller STM8S003F3P6. The reference voltage for the temperature sensor and the controller power are stabilized 5.0 V on AMS1117 -5.0.

Current consumption of the thermostat in the mode of the disabled relay 19 mA, enabled 68 mA (with a supply voltage of 12 V).

Versatility
Socket sensor included
Possibility of calibration
Small dimensions, weight and cost

The control relay is 12 V with NO contact, switches current up to 20 A (14VDC) and up to 5 A (250VAC).
Sensor type ― waterproof: NTC (10K/3435). The temperature sensor is a 10 kOhm thermal resistance hermetically sealed in a protective metal cap. The length of the temperature sensor wire is 50 cm, but if necessary, it can be extended.
Range of measured and controlled temperature: -50

110 degrees.

Measurement accuracy: ± 0.1 °C.

Control accuracy: 0.1 °C.

Hysteresis: 0.1°C - 15°C.

Refresh rate: 0.5 sec.

Supply voltage: 12 volts DC.

Power consumption:< 1W.

The temperature setting and display range is -50ºС +110ºС, which is quite enough for domestic use.

Red LED 3-digit indicator 22×10mm shows temperature up to tenths of a degree, temperatures below -10ºС (up to -50ºС) and above 100ºС (up to 110ºС) are displayed without decimal parts, because indicator digits are missing. The setpoint increment is set according to the same principle.

The red LED on the board simply duplicates the relay being turned on.

3 control buttons: set, +, - .

set - selects the setpoint mode and parameter settings

AND - change the value of the setpoint and parameters

It was more logical to put the + button on the right, and not in the center, because according to common sense, the increase should be on top or on the right

In mode C (cooling) it works like this:

In H (heating) mode, it works the other way around

The control relay is 12V with NO contact, switches current up to 20A (14VDC) and up to 5A (250VAC)

It would be better if the relay was installed with a switching contact and all 3 outputs were brought to the connection connector, while the scope of the thermostat is slightly expanded

The temperature sensor is a 10 kOhm thermal resistance hermetically sealed in a protective metal cap. The length of the cable is 30cm (50cm is declared), but if necessary, it can be extended.

Setting parameters with decryption:

Set point temperature -50°C 110°C, default 28°C

P1 switching hysteresis 0.1 - 15.0ºС, default 2.0ºС

Asymmetric (minus the setpoint), allows you to reduce the load on the relay and the actuator to the detriment of the accuracy of maintaining the temperature.

P2 maximum temperature setpoint -45ºC 110ºC, default 110ºC

Allows you to narrow the setpoint range from above

P3 minimum temperature setpoint -50ºC 105ºC, default -50ºC

Allows you to narrow the setpoint range from below

P4 measured temperature correction -7.0ºC 7.0ºC, default 0.0ºC

Allows you to perform a simple calibration to improve the accuracy of the measurement (only a characteristic shift).

P5 response delay in minutes 0-10min, default 0min

Sometimes it is necessary to delay the operation of the performer, for example, it is critical for a refrigerator compressor.

P6 limitation of the displayed temperature from above (overheating) 0ºС-110ºС, default is OFF

It is better not to touch it unless necessary, because. if the setting is incorrect, the display will constantly display “-” in any mode and you will have to reset the settings to the default state, for this you need to hold down the + and - buttons the next time you turn on the power.

Operating mode C (cooler) or H (heater), default C

In fact, it simply inverts the logic of the thermostat.

All settings are saved after power off.

No additional and tricky settings (PID, slope, processing, signaling) were found, but they are not needed by a simple user.

At temperatures below -50ºС (or when the sensor is disconnected), the indicator displays LLL

At temperatures above 110ºС (or when the sensor is shorted), the indicator displays HHH

An interesting feature is that the rate of updating the temperature readings depends on the rate of temperature change. With fast temperature changes, the indicator updates the readings 3 times per second, with slow changes - about 10 times slower, i.e. there is a digital filtering of the result to increase the stability of the readings.

The measurement accuracy is claimed to be 0.1ºС, but this is simply not possible for a conventional non-linear thermistor without individual multi-point calibration, which 100% did not do, and 10-bit ADC does not allow such luxury. At best, you can count on an accuracy of 1ºС

Real thermostat circuit

Control controller STM8S003F3P6

Reference voltage to the temperature sensor and controller power - stabilized 5.0V on AMS1117 -5.0

Current consumption of the thermostat in the mode of the disabled relay 19mA, enabled 68mA (with a supply voltage of 12.5V)

It is undesirable to connect the supply voltage below 12V, because. the relay is energized by 1.5V less than the supply voltage. It is better that it be a little more (13-14V)

The current-limiting resistors on the indicator are in the chain of discharges, not segments - this leads to a change in their brightness depending on the number of burning segments. It does not affect normal operation, but it catches the eye.

The RESET input (4 pin) is connected to contacts for programming, it has only an internal high-resistance pull-up (0.1mA) and the controller is sometimes falsely reset from strong spark interference nearby (even from a spark in its own relay), or if the contact is accidentally touched by hand.

Easily fixed by installing a 0.1uF blocking capacitor on the common wire