A modern refrigerator is a complex mechanism where each element performs a critical function to maintain food freshness. One of the key components of the No Frost system is defrost relay, which often becomes the cause of breakdown when it fails. Understanding the principles of its operation helps not only to diagnose a malfunction, but also to avoid expensive repairs in a service center.
During operation, an ice crust inevitably forms on the evaporator, which interferes with heat transfer. For the system to work effectively, this ice must be melted regularly. It is for turning on and off the heating element at the right time that a small but powerful device is responsible - the defrost relay, which controls defrosting cycles.
If you notice that your refrigerator Indesit or LG has stopped freezing, and a “snowdrift” has formed on the back wall of the chamber, the problem may lie precisely in this component. In this article we will analyze in detail the design of the relay, its role in the electrical circuit and methods for testing functionality.
Main function and principle of operation
The main task of the defrost relay is to automatically control the heating element (heating element) of the evaporator. It closes the electrical circuit only when the refrigerator compressor is turned off, and opens it when a certain temperature is reached or after a specified time has elapsed. This prevents overheating of the system and saves energy.
The principle of operation is based on thermal or mechanical expansion of materials inside the device body. Classic models use a bimetallic plate that bends when cooled, closing the contacts. When the heating element heats the evaporator, the plate changes shape again and opens the circuit. The critical contact opening temperature is usually from +10°C to +15°C, which guarantees complete melting of the ice without the risk of damaging plastic parts.
There are also electronic timer relays that control the process based on sensor readings and a built-in algorithm. Such systems are more accurate, but more difficult to diagnose. In both cases, the goal is the same - to ensure regular and safe defrosting of the hidden evaporator.
⚠️ Attention: Replacing the defrost relay with a universal analogue without taking into account the temperature threshold of operation can lead to the fact that the refrigerator will not defrost completely or, conversely, the heating element will heat up for too long.
The malfunction of this unit is often confused with a breakdown of the heating element or thermostat itself. However, if the heating element is intact, and no current is supplied to it at the time of defrosting, the relay is most often the culprit. Its resource is limited by the number of operation cycles, so over time the contacts may oxidize or “burn out.”
Types of defrost relays in modern refrigerators
There are several types of devices on the household appliances market that provide defrosting. The choice of a specific type depends on the model of the refrigerator and the year of its manufacture. Understanding the differences helps you choose the right spare part for repair.
- 🔹 Thermal relay (bimetallic): The most common type, working solely on the physics of changing the shape of metal when heated and cooled.
- 🔹 Thermal relay: Uses the properties of an expanding gas or liquid in a capillary tube to open contacts.
- 🔹 Electronic timer relay: Controlled by a microprocessor, receiving signals from evaporator temperature sensors.
The thermal relay is often called the “evaporator defrost relay”. It is compact and mounts directly on or near the evaporator tubes to accurately read the temperature. Electronic analogues can be located on a common control board, which complicates their visual search during diagnostics.
When selecting a spare part, it is important to pay attention not only to the appearance, but also to the markings. Even if two relay housings look identical, their temperature characteristics may differ. Using an unsuitable component will lead to incorrect operation of the system No Frost.
Symptoms of defrost relay malfunction
Diagnostics begins with observing the behavior of the refrigerator. If the relay ceases to perform its functions, this inevitably affects the climate inside the chambers and the operation of the compressor. Ignoring the first signs can lead to more serious damage.
The most striking symptom is icing of the evaporator. Ice blocks the circulation of cold air, and the refrigerator compartment becomes warm, although the freezer may work at high speed. The compressor works almost non-stop, trying to gain temperature.
Pay attention to the following signs indicating problems with the defrost system:
- ❄️ Freezing of “coat” or ice on the back wall of the refrigerator compartment.
- 🔊 Unusual crackling or clicking in the area freezer during operation.
- 🌡️ An increase in temperature in the main chamber above the set values.
- 💧 The appearance of a puddle of water under the refrigerator due to the drainage tray overflowing with melt water.
Sometimes the relay can “stick” in the closed state. In this case, the heating element will heat up constantly, even when it is not necessary. This is dangerous due to overheating of plastic structural elements and melting of the wiring. If you smell a burning smell or see blackened plastic around the evaporator, immediately unplug the appliance.
Instructions for testing and diagnostics
To check the defrost relay, you will need a multimeter capable of measuring resistance (Ohm) and ringing the circuit. Before starting any work, be sure to disconnect the refrigerator from the power supply. Safety is the number one priority when repairing appliances.
Get to the evaporator by removing the back panel in the freezer. The defrost relay is usually mounted on the evaporator tube or located in close proximity to it. Carefully disconnect the wires and remove the device. Visually inspect the housing: it should not have cracks, signs of burning or melting.
☑️ Diagnostics of the defrost relay
In a cold state (at temperatures below -5°C), the thermal relay contacts must be closed. The multimeter in dial mode should make a sound and the resistance arrow should approach zero. If the device shows “infinity” (open), then the relay is faulty and requires replacement. If you check the relay at room temperature, it may be open - in this case, it needs to be cooled, for example, by putting it in the freezer for 15-20 minutes.
It is also worth checking the integrity of the heating element itself, since often these elements fail together. The resistance of a working heating element is usually from 200 to 400 Ohms, depending on the power and model of the refrigerator.
⚠️ Attention: Never check the resistance of the relay or heating element while they are connected to the network or until the capacitors on the control board are discharged. This can lead to failure of the multimeter and electric shock.
If the relay is electronic, a simple test may not give a result. In such cases, the presence of a control signal from the board to the relay is checked at the start of the defrost cycle. This requires more in-depth knowledge of electronics and the diagram of a specific refrigerator.
Table of typical faults
Systematization of data helps to quickly identify the problem. Below is a table comparing symptoms with possible causes and solution methods.
| Symptom | Probable cause | Solution method | Difficulty |
|---|---|---|---|
| Ice on evaporator, compressor hums constantly | Thermal relay is faulty (break) | Replacing the defrost relay | Medium |
| The refrigerator does not freeze, the heating element is cold | The heating element has burned out or the relay does not supply current | Checking the circuit with a multimeter | Medium |
| Burning smell, melted plastic | The relay is “stuck” in the closed state | Urgent replacement of the relay and heating element | High |
| Defrost cycles are too frequent | Incorrect temperature threshold of the relay | Installation of original spare parts | Low |
As can be seen from the table, most problems are solved by replacing the component. However, it is important to ensure that the new relay meets the manufacturer's specifications. Some models, for example Bosch or Siemens, use unique connectors or fasteners.
The process of replacing the defrost relay with your own by hand
If the diagnostics confirm the malfunction, proceed with replacement. This process requires precision, but does not always require special tools. The main thing is to act consistently and not use force where it is not necessary.
First remove the old relay. It can be held on by plastic latches or a metal clamp. Disconnect the terminals, remembering or photographing the order in which the wires are connected. An error in the connection may result in a short circuit.
Install the new relay in the same position as the old one. It is especially important for the thermostat to fit tightly to the evaporator tube for correct temperature reading. Connect the wires, observing the color coding or the marks made earlier.
What happens if the wires are mixed up?
In most cases, the thermal relay does not have polarity, since it is a simple switch. However, if you mix up the ground and phase wires in an electronically controlled system, you can burn the refrigerator control board.
After assembly, do not rush to load food. Turn on the refrigerator and let it run idle for a few hours. Make sure it gets up to temperature and turns off periodically. After a few days, check the evaporator for ice formation.
⚠️ Attention: Relay specifications (on/off temperature) may vary slightly between different manufacturers. Always check the markings on the body of the new part with the documentation for your refrigerator or the data of the old spare part.
Prevention and service life of the system
The defrost relay is a consumable item, but its service life can be extended. The main enemy of electronics and mechanics in a refrigerator are voltage surges and excessive moisture. A stable power supply significantly increases the service life of all components.
Regular defrosting (even for the No Frost system, at least once a year) helps clean the drainage system and reduce the load on the sensors. If the drainage is clogged, water can flow inside the components, causing corrosion of the relay contacts.
- 🧹 Keep the condenser at the back of the refrigerator clean - overheating of the system harms all components.
- 🔌 Use a surge protector to protect against power surges.
- 🚪 Do not leave the door open for a long time - this causes excessive icing.
The average service life of a high-quality relay is 7-10 years. If your refrigerator is more than 15 years old, it makes sense to preventively replace the relay whenever the freezer is opened in order to avoid repeated repairs in six months.
Is it possible to temporarily do without a defrost relay?
Technically, you can close the circuit directly, but this will lead to constant operation of the heating element, which will cause overheating, increased energy consumption and the risk of fire. It is strictly not recommended to operate a refrigerator without a working defrost relay.
Why does the defrost relay click?
The click is the normal sound of the bimetallic strip opening or closing inside the housing. If the clicks become too frequent or are accompanied by a humming noise, the relay may be faulty or incorrectly selected.
Where to buy an original relay?
It is better to buy original spare parts at authorized brand service centers or specialized stores of spare parts for household appliances, indicating the exact model of the refrigerator.
Does the room temperature affect the operation of the relay?
Yes, if the refrigerator is in an unheated room at a temperature below +10°C, the thermorelay may incorrectly read the evaporator temperature, which will lead to failures in defrosting cycles.