The modern No Frost system, which ensures the absence of ice on the walls of the chamber, is based on a clear alternation of cooling and defrosting cycles. The key element that controls the transition to defrost mode is defrost relay, which is often confused with a timer or sensor. It is this component that makes the decision to turn on the heating element (heating element) at the moment when the compressor stops. Understanding the principle of its operation is necessary for every owner of refrigeration equipment who wants to independently diagnose malfunctions.
During operation, ice inevitably forms on the evaporator, which, without periodic removal, would block heat exchange. The defrost relay, receiving a signal that the compressor has completed operation, closes the heater circuit, starting the ice melting process. If this mechanism fails, the refrigerator stops freezing properly, and a coat of snow grows in the freezer. Let's analyze in detail exactly what processes occur inside this small device.
⚠️ Attention: Before starting any diagnostic work or replacing the relay, be sure to disconnect the refrigerator from the power supply. Working with live electrical components is deadly.
Principle of operation and role in the No Frost system
The operation of the defrost relay is inextricably linked with the operation of the defrost timer (or electronic control module) and thermostat. When the timer switches the refrigerator from cooling mode to defrosting mode, it opens the compressor and fan circuits, but closes the heater circuit. However, in order to prevent the evaporator from overheating or the heating element turning on when there is no need for it, a defrost relay (often bimetallic type) is included in the circuit. It acts as a fuse and temperature controller.
The device responds to the temperature of the evaporator. While the evaporator is cold and covered with ice, the relay contacts are closed, allowing current to flow to the heating element. As soon as the ice melts and the temperature of the evaporator rises to a certain threshold (usually about 10-15 degrees Celsius), bimetallic plate the relay inside bends and opens the circuit. This stops heating even if the timer has not yet switched the system back to cooling. This mechanism saves energy and protects the plastic parts of the chamber from melting.
In some models of refrigerators, for example, in technology Indesit or Stinol, the relay can perform the function of protection against overheating, opening the circuit at critical temperatures. In more complex electronically controlled systems, the role of a mechanical relay is taken over defrost sensor, which sends signals to the control board. However, the principle remains the same: heating must be stopped immediately after the ice melts.
It is important to understand the difference between a timer and a relay. The timer sets the frequency of the cycles, and the relay controls the temperature at the time of defrosting. If the timer “commands” to heat, but the relay “sees” that there is nothing left to heat (or, on the contrary, it is dangerous), it blocks the process. A failure in the operation of any of these elements leads to an imbalance in the temperature regime.
Design and types of defrost relays
Structurally, the defrost relay is a compact device enclosed in a plastic case. Inside there is a sensitive element that responds to temperature changes. In classical mechanical systems the most common bimetallic relays. The principle of their operation is based on the different coefficient of thermal expansion of two metals soldered together. When heated, such a plate bends, opening or closing electrical contacts.
There are several main types of devices used in refrigeration technology:
- 🔹 Bimetallic relay: the most common type, working exclusively on mechanics, without electronics.
- 🔹 Thermal relay: uses the property of materials to expand when heated, often used as protection.
- 🔹 Electronic sensor (thermistor): changes its resistance depending on the temperature and transmits data to the control module.
The device body is usually made of heat-resistant plastic that can withstand temperature changes and humidity. The contacts inside are made of highly conductive materials, such as silver or silver-based alloys, to avoid sparking and sticking. In models Samsung and LG there are often combined devices where both a defrost sensor and an overheating sensor are assembled in one housing.
The relay is attached directly to the evaporator tube or in close proximity to it to ensure accurate temperature reading. A special clamp or clip is used for this. Reliability of contact between the relay body and the evaporator tube is critical for the correct operation of the entire system.
Why does the relay click?
If you hear periodic clicks in a running refrigerator, this may be normal - this is how the thermostat or defrost relay works. However, if the clicks are accompanied by the lights turning off or the compressor stopping, the relay may be stuck or faulty.
Symptoms of a malfunctioning defrost relay
Diagnosing faults begins with observing the behavior of the refrigerator. If defrost relay fails, the system stops correctly switching from cooling mode to defrosting mode or vice versa. The most obvious symptom is the formation of a thick layer of ice and snow on the back wall of the freezer. Ice can block the ventilation ducts, causing cold air to stop flowing into the refrigerator compartment.
If the relay is “stuck” in the closed state, the heater can work constantly, even when it is not necessary. This causes the refrigerator to become too warm, food begins to spoil, and the compressor runs continuously trying to compensate for the heat. Overheating can damage the plastic elements of the internal lining of the chamber.
Main signs of breakdown:
- ❄️ Extensive ice on the evaporator, which is not removed by the standard defrosting system.
- 🔥 Heating of the back wall of the freezer during operation compressor.
- 🔊 Absence of the characteristic “gurgling” or crackling sound, which is usually heard during defrosting.
- 📉 An increase in temperature in the refrigerator compartment with a working compressor.
Sometimes a relay malfunction is accompanied by a short circuit, which can lead to automatic operation switch in the apartment. If the plugs immediately pop out when you turn on the refrigerator, most likely the heating element is broken or the relay contacts are closed. In such cases, immediate diagnosis is required.
⚠️ Attention: If you notice melting of the plastic inside the camera or smell burnt wiring, immediately turn off the device. Operating such a refrigerator can lead to a fire.
Instructions for checking the relay with a multimeter
For accurate diagnostics, you need to ring the relay using a multimeter. First get to the evaporator by removing the back panel in the freezer. The defrost relay is usually mounted on or near one of the evaporator tubes. Carefully disconnect the terminals, having previously photographed their position so as not to confuse them during assembly.
Switch the multimeter to resistance measurement mode (Ohm). At room temperature, the contacts of a serviceable bimetallic relay should be open (resistance infinity), since the temperature is above the operating threshold. If the multimeter shows zero or a value close to it at room temperature, it means that the contacts are stuck - the relay is faulty.
☑️ Checking the defrost relay
For a complete check, the relay can be cooled. To do this, put it in the freezer for 15–20 minutes (pre-pack it in a bag to prevent moisture from getting in). After cooling, the contacts should closed (the multimeter will show 0 Ohm). If after freezing the circuit does not ring, it means that the bimetallic plate has lost its properties and does not bend - the device requires replacement.
The table below shows typical values and states for different temperatures:
| State | Temperature | Expected resistance | Action |
|---|---|---|---|
| Cold | Below -5°C | 0 Ohm (Closed) | Evaporator is heating |
| Warm | Above +10°C | ∞ (Open) | Disables heating |
| Faulty (Stuck) | Any | 0 Ohm | Constant heating |
| Faulty (Open) | Any | ∞ | No defrosting |
When checking, also inspect the wires going to the relay. Often the insulation rubs against the sharp edges of the metal body of the evaporator, which leads to a short circuit. If the wires are intact and the multimeter readings do not correspond to the norm, the relay definitely needs to be replaced.
Replacement and features of installing a new component
Replacing the defrost relay is a procedure that is accessible even to a beginner, if you are careful. The main rule: the new component must have identical response temperature characteristics. There are no universal relays; each manufacturer (for example Danfoss, Klixon) has a series with different on and off thresholds. Using an unsuitable part will lead to incorrect operation of the refrigerator.
When installing a new relay, make sure that it is pressed tightly against the evaporator tube. For better thermal contact, it is sometimes recommended to use special thermal paste, although in many models simply a tight mechanical fit through a metal clip is sufficient. Do not bend the evaporator tubes too much - they are very thin and easily burst, which will lead to freon leakage.
After replacement, reassemble the refrigerator in the reverse order. Don't rush to load products right away. Let the appliance idle for a few hours to ensure that the defrost cycle is running properly and that ice does not form again. Check that the compressor turns off when the set temperature is reached.
Prevention and service life of the defrost system
The service life of the defrost relay depends on the quality of electricity and the operating conditions of the refrigerator. Frequent power surges can lead to burnt contacts. To extend the life of equipment, it is recommended to use surge protectors or voltage stabilizers, especially in old houses or rural areas.
Regular defrosting of the refrigerator (even a No Frost system) at least once a year helps keep the system clean. The accumulation of dust on the condenser and contamination of the drainage holes create additional stress on the entire thermoregulation system, forcing the relays and heating elements to work more intensively.
If you notice that the refrigerator has started to operate louder or the cycles have become shorter/longer than usual, do not ignore these signals. Timely replacement of an inexpensive relay can save the compressor from overload and costly repairs. Monitor the condition of the door seal - if it allows warm air to pass through, the ice on the evaporator will grow faster, and the relay will operate more often, exhausting its resource.
Is it possible to temporarily remove the relay and connect the wires directly?
Doing this is strictly not recommended. Direct connection of the heating element without temperature control will result in the heater operating constantly. This can cause plastic parts inside the chamber to melt, food to spoil due to heat, and even cause a fire. The relay is an important safety element.
Why does the refrigerator not turn on for a long time after replacing the relay?
This may be due to the fact that the thermostat has not yet reached the temperature to turn on, or the new relay has different characteristics. Also check if the wires are connected correctly. If the refrigerator does not turn on for more than 2-3 hours, the compressor or start relay may be damaged.
How often do you need to change the defrost relay?
The service life of a bimetallic relay is on average 5-7 years of active operation. However, with frequent power surges or poor-quality assembly, it may fail earlier. Scheduled replacement is not required, change only when symptoms of a malfunction appear.
Does room humidity affect the operation of the relay?
The relay itself is sealed, but high humidity can lead to oxidation of the contacts at the junction of the wires or to the formation of an ice crust on the sensor itself, which will distort the temperature readings. It is important to ensure ventilation around the refrigerator.