To maintain the required temperature range, a modern refrigerator uses a special thermostat device, abbreviated as a thermostat. The refrigerator thermostat turns the compressor on and off. Sometimes a situation arises when it breaks down, and there is nothing to replace it with, then you can find the right solution and make it yourself; let’s look at the diagram of such a device.

The thermostat is galvanically isolated from the supply voltage and allows you to maintain the temperature inside the refrigerator chamber with fairly good accuracy.

Refrigerator thermostat on TLC271 op-amp

The temperature sensor is LM335. In fact, as follows from the description, this is a voltage stabilizer, the parameters of which are sensitive to temperature changes. The LM335 is connected with only two contacts. The cathode is connected to positive through load resistor R1, and the anode to negative.

The voltage from LM335 is supplied to the direct input of the TLC271 comparator; at its inverse input there is potential from the voltage divider across resistances R3, R4, R5.

The temperature range in the internal chamber of the refrigerator is controlled by variable resistance R4. If the temperature increases above this range, the voltage at the forward input of the comparator will decrease compared to the inverse input. This will create a logical one signal at the output of the comparator, which will open the transistor.

Two optothyristors are connected in the collector circuit of the KT3102 transistor. Their LED parts are connected in series, and their thyristor components are parallel and counter-directional. Therefore, there is an interesting opportunity to manage alternating current(the first thyristor of the optocoupler operates on the first half-wave, and the second on the second half-wave. The refrigerator compressor turns on.

As soon as the temperature inside the refrigerator chamber drops below the set range, a logical zero level is formed at the output of the comparator and the compressor turns off.

With this circuit option, the compressor turns on when the temperature reaches + 6 degrees and turns off when it drops to + 4 degrees Celsius.

This temperature range is quite sufficient to maintain the required storage temperature of products, and at the same time ensures comfortable operation of the compressor, preventing its severe wear. This is especially true in outdated models, using a thermal relay to start the engine.

Refrigerator thermostat on LM35

The thermostat reads the temperature with an LM35 sensor, the resistance of which varies depending on the temperature in the refrigerator compartment, linearly calibrated with a coefficient of 10 mV per 1 degree Celsius.

Since the output voltage is clearly not enough to open VT1, the LM35 sensor is connected according to the current source circuit. Its output is loaded with resistance R1 and therefore the current varies in proportion to the temperature in the chamber. This current causes a drop across resistance R2. The voltage drop controls the operation of the first bipolar transistor VT1. If the voltage drop is higher than the threshold voltage level of the emitter junction, both transistors open, relay K1 is activated, and its front contacts start the electric motor.

Resistance R3 creates a positive chain feedback. This provides hysteresis to prevent the compressor from turning on too often. The winding of the electromagnetic relay must be five volts, and its contacts must withstand the current and voltage flowing through them, see.

Temperature sensor LM35 is located inside the refrigeration unit in the correct location. Resistor R1 is soldered directly to the sensor so that the LM35 can be connected to the board with just two wires.

If you need to slightly adjust the temperature level, this can be done by selecting the nominal value of resistors R1 or R2. Resistor R3 sets the hysteresis value.

The basis of the design is the K157UD1 operational amplifier with an output current of 300 mA, which makes it possible to connect an optothyristor directly to the op-amp output without using a buffer transistor. The op-amp is included as a comparator. The shutdown temperature of the refrigerator compressor is set by resistance R1. The difference between the on and off temperatures is set by resistance R4.

Instead of an electronic switch based on an optosimistor and a powerful triac VS1, you can use a conventional relay with a switching current of 10 Amps. In this case, the relay winding is connected to the sixth pin of the DA1 chip and the third pin of DA2. A damping diode is also connected to the same terminals. If a relay is used, it will be necessary to increase the capacitance value of capacitor C5 to 1 µF. If an electronic switch is used in the design, then diodes VD1 and VD2 can be eliminated by connecting the second pin DA2 directly to the housing.

After all, no one can forbid us to use one of them for a possible replacement.

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.

DIY electronic thermostat, device diagram

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:

• 12 volt step down transformer
• Diodes; IN4007, or others with similar characteristics 6 pcs.
• Electrolytic capacitors; 1000 microns, 2000 microns, 47 microns
• Stabilizer chip; 7805 or another 5 volt
• Transistor; KT 814A, or other p-n-p with a collector current of at least 0.3 A
• Adjustable zener diode; TL431 or Soviet KR142EN19A
• Resistors; 4.7 Kohm, 160 Kohm, 150 Ohm, 910 Ohm
• Variable resistor; 150 Kom
• Thermistor as a sensor; about 50 Kohm with negative TCS
• Light-emitting diode; any with the lowest current consumption
• Electromagnetic relay; any 12 volt with a current consumption of 100 mA or less
• Button or toggle switch; for manual control

How to make a thermostat with your own hands

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 place 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 with right side 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.

Simple thermostat for refrigerator

With your own hands

Make a Simple Refrigerator Thermostat Circuit

Want to make an accurate electronic thermostat for your refrigerator? The solid state thermostat circuit described in this article will surprise you with its cool performance.

Introduction

The device, once built and integrated with any related device, will instantly begin to demonstrate improved system control, saving energy, as well as increasing the life of the device. Conventional refrigeration thermostats are expensive and not very accurate. Moreover, they are subject to wear and therefore are not permanent. A simple and effective electronic refrigerator thermostat is discussed here.
A thermostat, as we all know, is a device that is capable of sensing a certain set temperature level and turning off or switching the external load. Such devices may be electromechanical types or more complex electronic types.
Thermostats are usually associated with air conditioning, cooling and water heating devices. For such applications, the device becomes an important part of the system, without which the device can reach and operate under extreme conditions and ultimately become damaged.
Adjusting the control switch provided in the above devices ensures that the thermostat will cut off power to the unit once the temperature crosses the required limit and switches over once the temperature returns to the lower threshold.
In this way, the temperature inside the refrigerators or the room temperature through the air conditioner is maintained in favorable ranges.
The refrigeration thermostat circuit idea presented here can be used externally above a refrigerator or any similar device to control its operation.
Control of their operation can be accomplished by attaching the thermostat sensing element to an external heat sink typically located behind most cooling units that use freon.
The design is more flexible and wider than built-in thermostats and can provide better efficiency. The circuit can easily replace conventional low-tech designs and is also much cheaper in comparison.
Let's figure out how the scheme works:

Description of the scheme
Simple Refrigerator Thermostat Diagram

The diagram shows simple circuit, built around IC 741, which is basically configured as a voltage comparator. It uses a transformer with lower power consumption to make the circuit compact and solid state.
The bridge configuration, containing R3, R2, P1 and NTC R1 at the input, forms the main sensing elements of the circuit.
The inverting input of IC is clamped to half the supply voltage using a voltage divider network of R3 and R4.
This eliminates the need to provide dual power to the IC, and the circuit can provide optimal results even with a single-pole supply voltage.
The reference voltage to the non-inverting input of the IC is fixed via a given P1 with respect to the NTC (Negative Temperature Coefficient).
In case the temperature under control tends to drift above the desired levels, the NTC resistance drops and the potential at the non-inverting input of the IC crosses the set point.
This instantly switches the output of the IC, which in turn switches the output stage containing the transistor, the triax network, turning off the load (heating or cooling) until the temperature reaches a lower threshold.
Feedback resistance R5 helps to some extent to induce hysteresis in the circuit, an important parameter without which the circuit may spin rapidly in response to sudden temperature changes.

Once the assembly is complete, setting up the circuit is very simple and is done with the following points:

REMEMBER THE EXTERNAL CIRCUIT IS BASED ON A CONSTANT SOURCE POTENTIAL, CAUTION IS WARNED AGAINST TESTING AND INSTALLATION PROCEDURES. THE USE OF WOODEN BLANK OR ANY OTHER INSULATING MATERIAL ALONG YOUR FOOT IS STRICTLY RECOMMENDED; ALSO USE ELECTRICAL TOOLS, WHICH MUST BE INSULATED NEAR THE SITE.

How to Adjust This Electronic Refrigeration Circuit Thermostat You will need a sample heat source precisely adjusted to the desired thermostat circuit cutoff threshold level.
Turn on the circuit and introduce and attach the above heat source to the NTC.
Now set the preset so that the output simply switches (output LED lights up). Remove the heat source from the NTC, depending on the circuit hysteresis the output should turn off within a few seconds.
Repeat the procedure many times to confirm proper functioning.
This completes the setup of this refrigeration thermostat and is ready to integrate with any refrigerator or similar device to accurately and continuously regulate its operation.

Parts List

R2 = Preset 10KR3,

R9 = 56 OHM / 1watt

C1 = 105 / 400V

C2 = 100uF / 25V

Z1 = 12 V, 1 W Zener diode

*option via optocoupler, added switch and diode bridge to the power supply

How to Create an Automatic Refrigerator Temperature Controller Circuit

The idea of ​​this scheme was suggested to me by one of the keen readers of this blog, Mr. Gustavo. I have published one similar circuit for an automatic refrigerator thermostat, however the circuit was intended to determine more high level temperature available at the rear of the refrigerator grille.

Introduction

Mr. Gustavo didn't quite understand the idea and he asked me to design a refrigerator thermostat circuit that could sense cold temperatures inside the refrigerator rather than hot temperatures at the back of the refrigerator.
So with some effort I could find a real CIRCUIT DIAGRAM for a refrigerator temperature controller, let's explore this idea with the following points:
How circuits function
The concept is not very new nor unique, it is a common comparator concept that has been included here.

IC 741 was rigged in standard comparator mode and also as a circuit without an inverting amplifier.
The NTC thermistor becomes the main sensing component and is specifically responsible for sensitivity to cold temperatures.
NTC stands for Negative Temperature Coefficient, which means the thermistor's resistance will increase as the temperature around it drops.
It should be noted that the NTC must be rated according to these specifications, otherwise the system will not function properly.
The preset P1 is used to set the IC trip point.
When the temperature inside the refrigerator drops below a threshold level, the thermistor resistance becomes high enough to reduce the voltage at the inverting pin below the non-inverting pin voltage.
This instantly makes the IC pin high, activating the relay and turning off the refrigerator compressor.
P1 should be set such that the op amp output goes high at zero degrees Celsius.
The slight hysteresis introduced by the circuit comes as a boon, or rather a blessing in disguise, because it causes the circuit to not switch quickly at threshold levels, but to respond only after the temperature has risen about a couple of degrees above the shutdown level.
For example, suppose if the trip level is set to zero, the IC will turn off the relay at that point and the compressor of the refrigerator will also be turned off, the temperature inside the refrigerator will now start to rise, but the IC will not switch immediately, but maintains its position until the temperature will not rise to at least 3 degrees Celsius above zero.


If you have any further questions regarding this Automatic Refrigerator Temperature Controller Circuit, you can express the same through your comments

Regulation RP1, RP2 can be temperature control set points, 555 timing inverting Schmitt circuits using relays to achieve automatic control.

Updated 01 Apr 2018. Created March 29, 2018

A refrigerator often contains two thermostats (temperature regulators), they are designed differently and perform different functions. The first monitors compressor overheating, the second monitors evaporator temperature. Why are relays always used? Simple, reliable. Today we see mechanical and electrical varieties. The refrigerator thermostat acts as a bell that triggers a complex mechanism. The signal will not sound, the system will remain dead, forget the frost!

Where to look for a refrigerator thermostat

The owners of refrigerators with mechanical regulators touched the thermostat with their hands. Not everyone guessed. The handle with which the temperature is set, the mode switch, is mounted on swivel mechanism thermostat. It is formed by two main parts, thanks to which it is difficult to confuse the components:

1. A box containing actuators and control mechanisms.
2. Long thin capillary (metal tube with an internal diameter of 0.5 mm).

There is a bellows inside the box in a sealed casing. A cylindrical metal accordion that tracks changes in ambient pressure by changing its linear dimensions. To better visualize the shape, imagine a short length of metal corrugated hose. The difference between the measuring bellows: it is sealed at both ends, therefore, hermetically sealed. When the pressure outside increases, the sensing element contracts. The design contains a spring that changes the response of the bellows to the applied pressure.

To better understand the purpose, let's take a brief excursion into the production processes. Bellows are considered measuring elements of refrigerators. Many uses have been found for the element. In pipelines, the bellows serves as a damping element. The ambient temperature rises, the oil pumping line begins to expand in length. A rupture poses a fire hazard. Bends the line into an arc. The bellows segment comes to the rescue. The accordion contracts, nothing special happens to the pipeline as the temperature rises. The situation repeats itself, sensing frost.

Giant bellows (a few meters in diameter) are made from high-quality steel. First, a cylindrical segment is pulled through. Something interesting happens next. The cylinder is inserted into a special machine of impressive size; a press equipped with a gripper compresses the accordion several times and straightens it with a controlled force. The platform rises, the podium exposes a bellows that does not have pronounced elastic properties, like a spring. You can stretch it, compress it like the press did, or deform it.

Refrigerator thermostat

To balance the external pressure force applied to the bellows, gas is pumped inside for use in instrumentation. External, external influences are considered factors that lengthen and compress the bellows. Obviously, a thermal relay equipped with a sensitive element will operate at one temperature. Also used in refrigerators simple models. But it is much more convenient to see a device with a regulator that changes the response threshold, making the temperature in the refrigerator chambers correspond to the program.

A spring appears on the scene. The spiral encloses the bellows and is attached to both sealed ends. The spring tension determines the response threshold of the sensing element. Some bellows are equipped with one fixed actuation moment, others are designed to provide two ranges (chambers). It is clear that different models are used for the freezer and refrigerator compartments.

Refrigerator thermostat operation

We examined in detail the principle of operation of the bellows for a reason. Despite the dominance of electronics, thermal relays continue to be equipped with a proven element. There is no need to install power supplies that generate reduced voltages.

Repair of the thermostat of the Stinol refrigerator has to be done approximately 5 years after purchasing the equipment. This is the lifespan of a sensitive element produced by one German company.

The durability is questionable, perhaps the matter is determined by accuracy and reliability. We believe the answer concerns the area of ​​unification. The refrigerator operates by generating four phase states of freon:

1. Compression;
2. Condensation;
3. Extension;
4. Evaporation.

Helps get low temperatures. The refrigerator thermostat device uses freon. Why? Once freon becomes a gas inside the evaporator of the cooling circuit, it will easily change the state of aggregation inside the capillary tube of the thermal relay, which, as mentioned, is formed by two components (see above). We took a moment to indicate that the system is filled with refrigerant and is completely sealed. The tube is sealed at the free end, inside there is freon under pressure, allowing it to become a liquid, only the evaporator temperature drops below the response threshold. Causes a shock decrease in system pressure, the bellows straightens.

The necessary contacts are closed, the control voltage of the compressor engine start relay is removed. As a result, the refrigerator stops and the temperature stops decreasing. The state is maintained until the threshold for switching on the thermal relay is passed. The freon inside becomes steam, the pressure on the bellows increases, the corrugation compresses, and the contacts of the control winding of the compressor engine starting device close. The refrigerator turns on and runs until the specified parameters are reached.

Now a couple of notes about the operation of the thermal relay. As mentioned above, the temperature of the evaporator is measured. How does this happen? We are amazed at the length of the sensing tube. Incredible length, if necessary it reaches the floor. Is all the freon involved in the process? The change in the state of aggregation occurs at the very tip, involving a relatively small area directly adjacent to the evaporator. Reliable contact is ensured. Usually glue is used and the top is sealed with sealant. Excess turns of the sealed tube are placed in the space between the walls. A new refrigerator thermostat is being installed to replace the broken one.

Replacing the thermostat for a refrigerator is within the power of most craftsmen, but a nuance has been noticed. The new refrigerator thermostat is similar to the old type. Otherwise, the result will be very different from what was expected. Separate thermostats for refrigerators provide the possibility of adjustment. Experienced masters manage to resolve the situation with honor. A breakdown of the thermostat is often indicated by the fact that the temperature of the refrigerator does not closely match the set temperature. Having turned the regulator knob to the Off position, we wait in vain to hear the characteristic click emitted by a working thermal relay. However, the factor is completely uncharacteristic electronic devices discussed below.

The control knob, which we twist and flip to adjust the temperature, directly acts on the spring of the refrigerator thermostat. The disadvantage of mechanical bellows is the difficulty of ensuring fine regulation. The modes are set in steps. For example, domestic thermal relays for TAM brand refrigerators support one or two modes. Caused by difficulties in adjusting the spring.

Electronic thermal relays

They mentioned the difficulty of setting up bellows thermostats for refrigerators. Old proven developments have served more than one generation quite well. The electronic thermostat of refrigerators will allow you to flexibly monitor the behavior of the structure and provide ample opportunities for adjusting modes.

The sensitive element is a special resistor, a thyristor. The keys are formed by power transistors; it is possible to use ordinary relays. The disadvantage of electronic thermostats for refrigerators is limited by exorbitant energy consumption, but we believe that durability is much more important.

Electronic thermostats are convenient in refrigerators equipped with linear (piston) compressors. This is not a separate type of engine, rather a way management. The pursuit of secondary parameters of refrigerators has been going on for a long time:

1. Energy consumption.
2. Noise level.
3. Dimensions.

New models began to be equipped initially with inverter compressors, then linear ones were introduced. They work without interruption, maintaining the temperature at a given level. Theoretically, the mode turns out to be noisy, but in practice it turns out: the compressor works at half capacity and behaves incomparably quieter.

The thermostat adjustment in the refrigerator is fine, the sensor is sensitive so that the linear compressor works. Electronics provides such opportunities.

We will discuss the refrigerator compressor thermostat later.

Here is the design of a thermostat for a refrigerator that has been working for more than 2 years. It all started when I returned from work and opened the refrigerator to find it warm. Turning the thermostat control did not help - the cold did not appear. So I decided not to buy new block, which is also rare, and make an electronic thermostat yourself on the ATtiny85. The difference with the original thermostat is that the temperature sensor is on the shelf and not hidden in the wall. In addition, 2 LEDs appeared - they signal that the unit is turned on or the temperature is above the upper threshold.

Refrigerator thermostat diagram on MK

Photo of the original thermostat and the homemade one

To connect, it was necessary to run a second 220 V wire (taken from a lighting lamp) to power the transformer.
The connector to which the potentiometer is connected is also the ISP programming connector.

The board is protected from moisture with a special varnish for printed circuit boards.

The thermostat currently works without problems, and most importantly, it costs about 10 times less than the original one.

The transformer here is 6 V. This was chosen to minimize losses on the 7805 chip.

The relay here can be set to 12 V. If you take the voltage to it before the stabilizer. To reduce costs, it would be possible to create a transformerless power supply, although there will be supporters and opponents of such a solution (electrical safety). Another cost reduction is the elimination of the AVR microcontroller. There are Dallas thermometers that can also operate in thermostat mode.