The thermostat is the “heart” of climate control in your refrigeration unit, responsible for maintaining the set temperature inside the chambers. It is this small unit that makes the decision to start or stop the compressor by analyzing the current temperature of the evaporator or the air inside the refrigeration compartment. Without a working thermostat, the device will not be able to keep food fresh, turning either into a useless cabinet or into a freezer that freezes all the contents.
Understanding this how the thermostat on the refrigerator worksis necessary not only for professional craftsmen, but also for every owner of household appliances who wants to independently carry out an initial diagnosis of the malfunction. Modern models contain both classic mechanical regulators and complex electronic control systems, but the basic principle of their operation remains similar. We will analyze in detail the internal structure, the differences between the types of sensors and methods for identifying breakdowns so that you can clearly understand the processes occurring inside your refrigerator.
It is important to note that mechanical thermostat and the electronic thermostat are fundamentally different devices in terms of design, although they perform the same function. Mechanics relies on the physical properties of gases and metals, while electronics uses changes in the resistance of semiconductors. Differences in design directly affect testing methods: if a mechanic can be “ringed” with a multimeter at home, then specialized equipment is often required to accurately diagnose electronics.
The service life of thermostats varies depending on the intensity of use and build quality, but on average it is 5-10 years. However, sudden surges in network voltage or temperature changes can significantly shorten this period. Knowing the design of the unit will help you quickly respond to the first signs of unstable operation, such as prolonged failure to turn off the compressor or, conversely, too infrequent engine starts.
The principle of operation and the main function of the thermostat
The main task of any thermostat is to open and close the electrical power circuit of the compressor depending on the temperature of the controlled environment. When the temperature inside the chamber rises above the set threshold, the contacts close and the motor starts working, cooling the evaporator. As soon as the required low temperature is reached, the circuit opens and the compressor stops until the next heating cycle.
The classic one is based on a bellows tube filled with refrigerant (freon or its analogues). This tube fits tightly to the evaporator or is located in the air flow of the refrigerator compartment. As the temperature rises, the gas inside the tube expands, creating pressure on the walls of the bellows, which, in turn, acts on the lever system, closing the contacts. Cooling leads to compression of the gas and opens the circuit. mechanical thermostat there is a bellows tube filled with refrigerant (freon or its analogues). This tube fits tightly to the evaporator or is located in the air flow of the refrigerator compartment. As the temperature rises, the gas inside the tube expands, creating pressure on the walls of the bellows, which, in turn, acts on the lever system, closing the contacts. Cooling causes the gas to compress and open the circuit.
Electronic analogues work differently: there are no moving mechanical parts in the sensor itself. The temperature-sensitive element changes its electrical resistance as the temperature changes. The microprocessor reads these changes and issues a command to start or stop the compressor. This approach allows you to achieve high accuracy of temperature control, down to 0.1 degrees, which cannot be achieved in simple mechanical systems.
Design of a mechanical thermostat
A mechanical thermostat is a sealed housing, inside of which there are electrical contacts and a switching mechanism. A long capillary tube (bellows) is attached to the body, which is a sensitive element. Inside this tube and expansion chamber there is saturated freon vapor, which reacts to temperature changes by transferring pressure to the membrane.
The user adjusts the temperature through a special handle connected to a screw that changes the spring tension. By turning the handle, you change the force required to open the contacts. The tighter the spring is, the lower the temperature must drop for the trip to operate. This is the same temperature rangethat you set manually.
Inside the case there is also a differential - the temperature difference between the moment the compressor is turned on and off. This parameter is usually fixed and is about 2–3 degrees Celsius. If there were no differential, the contacts would “rattle”, turning the motor on and off every second when the limit temperature was reached, which would quickly disable the equipment.
What is inside the bellows tube?
Inside the tube there is the same gas as in the cooling system (usually R134a or R600a), but in small quantities and under a certain pressure. It is not directly involved in the cooling cycle, but serves only to transfer thermal expansion to the mechanics of the switch.
Electronic thermostats and sensors (NTC)
Modern refrigerators, especially models with system No Frost and inverter compressors, are increasingly equipped with electronic control units. In such systems, the role of a mechanical thermostat is performed by temperature sensors (thermoresistors). They are semiconductor elements whose resistance varies nonlinearly depending on the ambient temperature.
An electronic thermostat has no moving parts, which makes it more reliable in vibration conditions, but it is sensitive to power surges. The signal from the sensor is sent to the main control board, which processes the data and makes decisions about the operation of the compressor, fans and defrost system. This allows you to implement complex algorithms, for example, “smart” cooling or the “Vacation” mode.
Unlike mechanics, where you can hear a characteristic click when triggered, the electronic sensor operates silently. Diagnosis of such elements requires knowledge of their nominal resistance at a certain temperature (usually 25°C). If the actual resistance is very different from the table one, the control module perceives this as an error and can block the operation of the refrigerator, displaying a fault code on the display.
Table: Comparison of mechanical and electronic thermostats
For a better understanding of the differences and choosing a diagnostic method, consider a comparative table of the main characteristics of both types of devices. This will help determine what exactly you are dealing with during repairs.
| Characteristics | Mechanical thermostat | Electronic sensor (NTC) |
|---|---|---|
| Principle of operation | Gas expansion in bellows | Change in semiconductor resistance |
| Control accuracy | Low (error 2-4°C) | High (error 0.5-1°C) |
| Diagnostics | Testing contacts with a multimeter | Measuring resistance with an ohmmeter |
| Maintainability | Replacing the entire unit | Replacing the sensor or boards |
| Replacement cost | Low | High (especially taking into account the work) |
The table shows that mechanical systems are simpler and cheaper to maintain, but are inferior in accuracy. Electronics provide ideal temperature conditions, extending the shelf life of products, but require a more qualified approach if they break down. The choice of thermostat type depends on the class of the refrigerator and its price category.
Typical malfunctions and symptoms of breakdown
The most common problem with mechanical thermostats is the loss of tightness of the bellows tube. If the gas has evaporated, pressure is not created inside and the contacts simply do not close. In this case, the compressor will not start at all and the refrigerator will stop cooling. It is impossible to determine this visually; only an instrumental check is required.
The second common option is contacts “sticking” in the closed state. In this case, the compressor runs continuously without turning off. This leads to freezing of food in the refrigerator compartment and the formation of an ice coat. If you notice that ice is appearing on the food, and the motor is humming incessantly, the first thing to suspect is the thermostat.
- 🔌 The refrigerator does not turn on: The indicator is on, there is light, but the compressor is silent - probably there is an open circuit in the thermostat.
- 🧊 Ice build-up: "snow" or ice appears in the refrigerator compartment on the back wall - the thermostat does not give the command to stop.
- 📉 Short cycle: the compressor turns on for 1-2 minutes and turns off - the calibration may be broken or there is a gas leak from the bellows.
- 🔊 Clicks: frequent clicks inside the chamber may indicate that the contacts are burning and not working. hold the load.
☑️ Diagnostics before calling a technician
Diagnostics and replacement of the thermostat with your own hands
To check the mechanical The thermostat must be removed. It is usually located under a plastic cover inside the refrigerator compartment or on the top of the cabinet outside. After removing the handle and protective casing, you will see the unit itself with suitable wires. Important: before any work, be sure to disconnect the refrigerator from the power supply!
The removed thermostat is checked with a multimeter in dial mode. The probes are connected to the contacts, and the bellows tube is placed in the cold (for example, in the freezer for 10-15 minutes). As it cools down, a click should be heard and the resistance should change (open or close depending on the type). If the chain does not respond to cold, the part must be replaced.
⚠️ Attention: When installing a new thermostat, it is extremely important to correctly bend the bellows tube so that it follows the path of the old one. Creases, twisting or touching sharp edges of the case are unacceptable - this will lead to rapid failure of the new part.
The replacement process takes from 30 to 60 minutes. After installing the new part and turning on the refrigerator, do not load food immediately. Allow the unit to return to operating mode for 2-3 hours, monitoring the operation of the compressor. If the on/off cycles have become regular and the temperature has stabilized, the repair can be considered successful.
Frequently asked questions (FAQ)
Is it possible to temporarily remove the thermostat from the circuit so that the refrigerator works?
Technically, you can close the contacts directly, and the compressor will work constantly. However, this is an emergency measure that will lead to freezing of the evaporator and spoilage of food. You can use the refrigerator in this mode only for a short time, before purchasing a new spare part.
Why does the new thermostat click, but the refrigerator does not freeze?
If there are clicks, then the electrical part is working. The problem may be that the bellows tube does not fit tightly to the evaporator and does not sense the temperature. There may also be a defect in the compressor itself or a freon leak in the cooling system, which is not related to the thermostat.
How to choose an analogue if the original thermostat is not on sale?
Three parameters are important: the length of the bellows tube, the type of contacts (normally closed or open) and the temperature range. You can use universal models by adjusting the length of the tube and adjusting with a screw, but it is better to look for cross-codes in spare parts catalogues.
Does the location of the refrigerator affect the operation of the thermostat?
Yes, if the refrigerator is too close to the wall or heat source (battery, stove), the thermostat may incorrectly read the temperature of the air around it, which will lead to frequent inclusions. It is necessary to observe the clearances for ventilation specified in the instructions.
Can the electronics itself “heal” after a power surge?
Sometimes a complete power reset helps (unplug for 15-20 minutes). This reboots the controller. If the error in the temperature sensor was caused by a software glitch, this will help. If the sensor itself or the board burns out, the hardware will need to be replaced.