Understanding what the refrigerator thermostat is made of is the foundation for competent diagnosis of faults and high-quality repair of household appliances. This small but critically important unit is responsible for maintaining the set temperature by opening and closing the electrical circuit of the compressor at the right time. The design of the device can differ radically depending on the generation of the refrigerator and the type of automation used.
In modern models, complex electronic modules, controlled by microcircuits, are increasingly found, while classic units are still equipped with reliable mechanical regulators. Knowledge of the materials and principles of operation of each element helps the technician determine the cause of the breakdown without unnecessary replacement of expensive components. In this article, we will analyze in detail the “anatomy” of a thermostat, look inside its body and find out what physical and chemical processes make it work.
Principal diagram of a mechanical thermostat
The classic mechanical thermostat is a sealed system, working on the basis of changes in the volume of the working fluid when the temperature changes. The main elements here are a sensitive flask, a capillary tube and a bellows (corrugated chamber). This entire system is filled with refrigerant, which responds to thermal fluctuations inside the fridge compartment.
When the temperature in the chamber rises, the gas or liquid in the flask expands, transmitting pressure through the capillary to bellows. The bellows, in turn, stretches and mechanically acts on the lever system, opening the contacts and starting the compressor. When the desired cold is achieved, the process proceeds in the reverse order: the pressure drops, the bellows compresses, and the circuit opens.
⚠️ Attention: The capillary tube of a mechanical thermostat is extremely thin and fragile. Any kinking or damage leads to depressurization of the system and complete loss of functionality of the regulator, since the working fluid evaporates.
The materials used in this system are selected taking into account their thermal expansion and resistance to cyclic loads. The body is usually made of durable plastic or metal, which protects the internal mechanisms from moisture and condensation, which inevitably form during the operation of refrigeration equipment.
Materials of the bellows chamber and capillary tube
The heart of any mechanical thermostat is the bellows chamber. It is made from special alloys of non-ferrous metals, most often phosphor bronze or beryllium bronze. These materials have the unique ability to return to their original shape after thousands of compression and stretching cycles, which guarantees the durability of the device.
The capillary tube connecting the bellows to the sensitive bulb is made of copper or stainless steel. Copper is chosen for its excellent thermal conductivity and flexibility, which allows the tube to be neatly laid along the walls of the refrigerator. The length of the capillary varies depending on the model of the refrigerator and can be from 0.5 to 3 meters.
- 🔹 Phosphor bronze is the main material for bellows due to its high elasticity and corrosion resistance.
- 🔹 M1 copper - used for capillary tubes due to its ability to easily transfer heat energy.
- 🔹 Stainless steel - used in tubes for models where increased tensile strength is required.
- 🔹 Indium-gallium alloys - sometimes used as a working fluid in high-precision sensors.
Inside this system there is a working fluid. In older models it could be freon, in modern ones it could be special gas mixtures or liquids with a high expansion coefficient. The tightness of the connection between the tube and the bellows is ensured by soldering with refractory solders.
Design of the contact group and lever mechanism
The electrical part of the thermostat consists of a contact group that directly switches the voltage to the compressor. The contacts are made of silver or special silver-based alloys (for example, silver-cadmium oxide) to minimize resistance and prevent sparking when the circuit opens under load.
The lever mechanism that transmits force from the bellows to the contacts is made of insulating materials such as textolite or special heat-resistant plastics. This is necessary to ensure electrical safety and prevent short circuits. The springs that create the tension are made of hardened steel.
The temperature range is adjusted with a screw that changes the tension of the spring that counteracts the pressure of the bellows. Rotating this screw changes the contact opening point, allowing the user to adjust the cold in the chamber.
The quality of the contact group materials directly affects the life of the entire refrigerator. Cheap analogues may use contacts with a lower silver content, which leads to their rapid burning and sticking.
Design of electronic thermostats
In refrigerators with the No Frost system and digital control, mechanics have given way to electronics. Here, instead of a bellows and gas, solid-state temperature sensors are used, most often thermistors (NTC-type). Their resistance changes depending on the ambient temperature, which is recorded by the control module.
The basis of the electronic thermostat is a printed circuit board with microelements soldered on it. The housing of such devices is usually made of impact-resistant polystyrene or ABS plastic. The sensors are placed in a separate housing and connected to the board with wires, which simplifies their replacement.
| Component | Manufacturing material | Function |
|---|---|---|
| Thermistor | Metal oxides (Ni, Mn, Co) | Change in resistance when heated |
| Control board | Fiberglass | Signal processing and operating logic |
| Relay contacts | Silver / Tungsten | Compressor switching |
| Sensor housing | Stainless steel / Brass | Protection and heat dissipation |
Electronic systems allow the implementation of complex algorithms, for example, rapid cooling or holiday mode. However, they are more sensitive to power surges in the network and require stable power.
Working fluid: gas, liquid or solid
The choice of working fluid in mechanical thermostats depends on the required temperature range. For refrigeration chambers where temperatures are positive or slightly negative, vapor-gas mixtures are often used. They provide smooth operation and precise adjustment.
For freezers where deep cold is required, liquid fillers or adsorption systems are used. The liquid practically does not compress, so expansion occurs only due to thermal expansion, which gives a very clear response to temperature changes.
⚠️ Attention: It is strictly forbidden to open the flask of a mechanical thermostat. The substance inside may be under pressure or be toxic, and a violation of the seal will make the device unsuitable for further use.
In solid-state electronic sensors, the role of the “working fluid” is played by semiconductor materials. Their properties change predictably, allowing the controller to calculate the temperature with an accuracy of a fraction of a degree. This eliminates the inertia inherent in gas systems.
Modern research is aimed at creating environmentally friendly working fluids that do not contain fluorocarbons, which can negatively affect the ozone layer when disposing of equipment.
Comparison of the reliability and durability of materials
Mechanical thermostats are valued for their simplicity and independence from power surges. Metal bellows components and copper tubes can last for decades without loss of properties. The main enemy here is corrosion and physical damage to the capillary.
Electronic analogues benefit in accuracy and functionality, but their service life is limited by the resource of electronic components and the quality of soldering. Plastic cases can turn yellow and crack over time, and relay contacts can oxidize.
Why are mechanics more reliable?
Mechanical systems do not have complex logic that can be “glitched.” It’s just physics: when it heats up, it expands, when it cools down, it contracts. There is nothing to break if the tube is not damaged.
When choosing a spare part for repair, it is important to take into account not only the mounting dimensions, but also the type of working fluid and the characteristics of the contact group. There are no universal solutions here; each manufacturer uses its own standards.
Frequent malfunctions associated with materials
One of the most common problems is leakage of working gas due to microcracks in the weld of the bellows or capillary tube. This leads to the fact that the thermostat stops “feeling” the temperature and either constantly keeps the compressor on or does not start it at all.
Another common reason is burnt contacts. If a low-quality alloy was used in the pair or the contacts were operated under increased load, carbon deposits will form on them. This increases resistance, causes heating and can lead to melting of the plastic case.
- 🔹 Breakage of the capillary - complete loss of tightness and performance.
- 🔹 Corrosion of springs - change in calibration and shift in temperature regime.
- 🔹 Oxidation of contacts - unstable operation, sparking, humming.
- 🔹 Degradation of plastic - the appearance of cracks in the regulator body.
Diagnosis of such malfunctions requires careful inspection. Often, a visually intact device may have a hidden defect in the internal structure of the metal, which will only appear under load.
Diagnostics and replacement: what to look for
When replacing a thermostat, it is important not only to select an analogue by catalog number, but also to pay attention to the length of the capillary tube and the location of the contacts. Incorrect installation of the tube can lead to its kinking and failure of the new unit.
For electronic models, it is critical to check the input voltage and compliance of the sensor resistance with the values declared by the manufacturer at room temperature. Deviations in resistance of even 10% can lead to incorrect operation of the refrigerator.
☑️ Check before installation
Remember that tampering with the electrical part of the refrigerator requires compliance with safety rules. Before starting work, be sure to disconnect the appliance from the power supply.
⚠️ Attention: Technical characteristics and internal structure of thermostats may vary depending on the specific model and year of manufacture of the refrigerator. Always check the manufacturer's official documentation before replacing.
A competent approach to selecting and installing a thermostat will extend the life of your refrigerator and ensure proper storage of food.
Is it possible to repair a mechanical thermostat if the gas mixture has leaked?
Refill the bellows yourself gas at home is impossible. This requires special equipment for vacuuming and filling, as well as precise knowledge of the composition of the mixture. Such thermostats must only be replaced with new ones.
Why is an electronic thermostat more expensive than a mechanical one?
Electronic devices contain complex microcircuits, precise sensors and require more expensive production. In addition, they provide higher accuracy of temperature control, which justifies their cost.
How often should you change the thermostat in a refrigerator?
Thermostat service life is usually 10-15 years or more. Replacement is required only if it is faulty. Planned replacement is not carried out if the device is working correctly.
Does the flask material affect the accuracy of operation?
Yes, the flask material (usually copper or brass) affects the rate of heat transfer. The better the thermal conductivity of the material, the faster the thermostat reacts to changes in temperature in the chamber.
Is it dangerous to use a universal thermostat?
The use of universal thermostats is permissible if their parameters (temperature range, capillary length, type of contacts) coincide with the original. However, they may not be as accurate as the settings.