When the refrigerator stops freezing or, conversely, cannot stop, the technician often mentions a bimetal malfunction. For the average user, this term sounds mysterious and even scary, causing associations with complex electronics or expensive repairs. However, in fact bimetallic plate is an ingeniously simple mechanical device that serves as the “brain” for the thermoregulation system in many models of household appliances.
Understanding how this element works will help you better understand the causes of breakdowns and not be afraid of service diagnoses. This is not a fragile microchip, but a reliable mechanism based on the laws of physics that have been known for over a hundred years. It is responsible for maintaining a stable temperature inside the chambers, ensuring the safety of your products.
In this article we will analyze in detail the design, principle of operation and typical problems associated with this component, so that you can confidently communicate with specialists or independently diagnose simple malfunctions.
The physical principle of operation of the bimetallic element
The operation of this device is based on lies a fundamental property of metals: when heated, they expand. But different metals do this at different speeds and strengths. Engineers took two strips of different alloys (usually Invar and nichrome or steel), connected them to each other along their entire length and created a material that bends when the temperature changes.
When the temperature inside the refrigerator increases, the plate heats up and bends in one direction. When a certain threshold is reached, it closes the electrical contacts, starting the compressor. As soon as the temperature drops to the desired value, the reverse process occurs: the metal cools down, straightens and opens the circuit.
This cycle is repeated constantly, providing cyclic operation the refrigeration unit. The main advantage of this approach is its absolute independence from software. There is nothing to “glitch” or freeze here, since the physical law of expansion of bodies always works the same way.
⚠️ Attention: If you hear a loud click inside the refrigerator every 15-20 minutes, this is normal. This is how the bimetallic thermostat works its cycle, turning the motor on and off.
It is important to understand that sensitivity such an element depends on the quality of the alloys and the accuracy of their connection. Over time, the metal may “get tired”, but this happens extremely rarely compared to electronic sensors.
It is the mechanical nature of the process that makes bimetal the standard of reliability in budget and mid-range models of refrigerators, where accuracy to tenths of a degree is not required.
Design features and installation location
Externally, a bimetallic thermostat looks like a small box, often with a plastic body, from which a long capillary tube extends. Inside the case there is exactly that same curved plate connected to electrical contacts. The tube is filled with gas or liquid, which transfers temperature from the evaporator to the sensing element.
The installation location of this unit is critical for correct operation. Usually it is mounted directly on evaporator or in close proximity to it. In old models of Atlant or Indesit refrigerators, the thermostat can often be found under the plastic panel inside the refrigerator compartment.
In modern No Frost systems the situation is more complicated. There, bimetal can be used not only to control the compressor, but also to control operation Defrost heating element. In this case, it is located in the evaporator area of the freezer, hidden behind the back wall.
| Type of thermostat | Installation location | Main function | Characteristic feature |
|---|---|---|---|
| Mechanical (Capillary) | Inside the refrigeration chamber | Compressor control | Inside adjustment knob |
| Defroster (Bimetal) | On the evaporator (No Frost) | Protection against overheating of the heating element | Open the circuit at +10°C |
| Defrost controller | In the zone evaporator | Activation of defrost | Closes the circuit at -5°C |
| Protective relay | On the compressor | Motor protection | Clicks when starting |
The design may vary depending on the manufacturer. For example, in refrigerators Liebherr or Bosch bimetallic elements are often integrated into complex control systems, but the physical principle remains the same.
Access to these parts in models with a “no-frost” system usually requires complete defrosting of the unit during the day, so as not to damage fragile plastic and tubes during dismantling.
Types of bimetallic devices in refrigeration equipment
Not all bimetals are the same. Depending on the tasks that engineering thought sets for them, they are divided into several types. Understanding the difference between them is necessary for correct diagnosis.
The first type is classic thermostats. They control the main operating cycle of the refrigerator. Their task is to prevent the temperature from rising above a given level. They operate in a wide range, from -20 to +10 degrees Celsius, depending on the user’s settings.
The second type is defroster (thermal switches). These elements are created for No Frost systems. Their task is to break the power supply circuit of the defrost heating element when the ice melts and the temperature on the evaporator reaches, for example, +10°C. This prevents heating of the empty evaporator and spoilage of products.
The third type is defrost thermal sensors. They, on the contrary, close the circuit when enough ice accumulates on the evaporator and the temperature drops to a certain minimum (usually around -5...-7°C). This is a signal for the timer or control board that it is time to start defrosting.
⚠️ Attention: Do not try to replace the defroster with a regular thermostat or vice versa. They have different temperature response thresholds, which will lead to either underheating or constant operation of the heating element.
There are also bimetallic ones protective relays, which are installed directly on the compressor starting device. They protect the engine from overheating by opening the circuit if the motor housing becomes too hot.
Each of these elements has its own markings and color coding, by which technicians determine its purpose and temperature range.
Typical malfunctions and their symptoms
Although bimetal is a reliable part, it does not last forever. Over time, the metal loses its elastic properties, the contacts oxidize or burn, and the capillary tube may lose its seal. How to understand that the problem lies here?
The most common symptom is that the refrigerator stops turning off. The compressor hums non-stop, trying to build up the temperature, but the bimetallic plate simply does not open the contacts. In this case, the food in the refrigerator compartment may freeze, and a “fur coat” will form on the back wall.
The opposite situation is that the unit does not turn on at all. The plate cooled down, but the contacts did not close. As a result, it becomes warm inside, the light is on, but there is no cold. This often happens if the contacts are “stuck” in the open state or oxidized.
In No Frost systems, a common problem is defroster malfunction. If it does not open the circuit when defrosting is completed, the heating element may continue to heat, which is dangerous. If it does not close when defrosting begins, the evaporator becomes overgrown with ice, blocking the air circulation channels, and the refrigerator stops freezing.
- 🔥 Continuous operation: The motor does not turn off, it is too cold inside the chamber, ice is freezing.
- ❄️ Lack of cold: The compressor is silent, although the chamber is warm, there is light.
- 💧 Ice in the freezer: If the compressor is working properly, an ice lump forms in the No Frost chamber (problem with defrosting).
- ⚡ Knocks out plugs: Short circuit inside thermostat (rarely, but it happens).
Diagnostics often requires testing the contacts with a multimeter. In a cold state, some contacts should ring, while in a hot state, they should open. The discrepancy of this algorithm indicates a malfunction.
Why does the refrigerator click?
The click is the sound of the contacts opening or closing inside the bimetallic plate. If the clicks become very frequent (every minute), the tightness of the capillary tube may be broken or the calibration of the plate is lost.
Instructions for checking and replacing the thermostat
If you decide to check the bimetal yourself, make sure that the refrigerator is unplugged. Safety is paramount as we work with electricity and refrigerant.
You must first gain access to the part. In most cases, you need to remove the top cover of the refrigerator (if the regulator is on top) or dismantle the internal plastic panel in the refrigerator compartment. Be careful with plastic latches, they become brittle in the cold.
To check, you will need a multimeter in dial mode. Remove the wires from the thermostat contacts. Connect the multimeter probes to the contacts. At room temperature the circuit should be closed (if it is a normally closed contact). To check the opening, you can gently heat the regulator body with a hairdryer or lower the sensitive tube into warm water.
⚠️ Attention: Do not heat the bimetal with an open fire! High temperatures can melt the plastic case or damage the insulation of the wires, which will make the part unusable.
If the part does not ring or, conversely, always rings (even when heated), it must be replaced. The analogue is selected strictly according to the labeling. Installation of the new element is carried out in the reverse order.
It is important to position the capillary tube correctly. It should fit snugly against the evaporator in the same place where the old one was. Often, a special metal bracket or clamping bar is used for fixation.
☑️ Algorithm for replacing the thermostat
Comparison of bimetal and electronic sensors
With the development of technology in modern refrigerators, mechanical bimetals are gradually giving way to electronic temperature sensors (thermistors). What is the difference and what is better?
Electronic sensors transmit data to the control module, which decides whether to turn on the compressor. This allows you to achieve high accuracy (up to 0.1 degrees) and implement complex algorithms, for example, fast cooling or working via Wi-Fi. Bimetal works “head-on”, reacting only to physical expansion.
However, electronics are more sensitive to power surges and humidity. The bimetallic plate is practically “immortal” in terms of electronics - it doesn’t care whether the voltage jumps or not, it only reacts to heat. This makes mechanics preferable in conditions of unstable power supply.
The cost of replacing bimetal is usually lower, and the procedure itself is simpler, since it does not require reprogramming the control board. An electronic sensor can be more expensive, and replacing it sometimes requires calibration.
However, in the premium segment and in areas where precise humidity control is needed (for example, the “zero zone” for meat and fish), only electronic systems are used. Bimetal there is simply not capable of providing the necessary accuracy.
The choice between them is a choice between reliable simplicity and high technology. For normal food storage, both options do an excellent job if they work.
Frequently asked questions (FAQ)
Is it possible to temporarily short-circuit a bimetal to make the refrigerator earned?
Technically this is possible, but it is highly not recommended. If you short-circuit the thermostat contacts, the compressor will run continuously until you manually turn it off. This will lead to freezing of food, icing of the evaporator and, ultimately, to breakdown of the compressor due to overheating. Use this method only for a short-term check (5-10 minutes).
Why does the new bimetal not fit?
Capillary tubes can have different lengths. If the new tube is longer, it can be carefully twisted into a ring (at least 5 cm in diameter) and placed in free space without bending it. In short, this is a problem, since it cannot be increased, you will have to look for another model. The main thing is that the sensitive element fits tightly to the evaporator.
How often do you need to change a bimetallic thermostat?
The service life of a high-quality bimetal is 10-15 years or more. It does not require scheduled replacement. They change it only when symptoms of a malfunction appear: the refrigerator has stopped holding the temperature or does not turn off. In No Frost systems, defrosters may fail more often, once every 5-7 years.
Does the type of refrigerant affect the operation of the bimetal?
No, the bimetallic plate reacts only to the temperature of the surface to which it is attached, or the temperature of the air in the chamber. The type of freon (R134a, R600a) affects the pressure in the system and the design of the compressor, but not the principle of operation of the mechanical thermostat.