The situation when refrigerator stops cooling, and the compressor only hums for a few seconds and clicks, trying to start, is familiar to many owners of household appliances. In most cases, the cause of this behavior is not a breakdown of the motor itself, but a failure of the starting mechanism, which is responsible for starting the engine. It is starting relay that supplies the initial impulse to the winding, allowing the rotor to gain the necessary speed.
The most common type of such devices in modern models is positor relay, the operation of which is based on the properties thermistor. Understanding the principle of its operation and the ability to carry out basic diagnostics will help you save a significant amount on calling a technician, since even an inexperienced user can often replace this part. However, before starting any work, it is necessary to clearly understand the device of the unit and follow safety rules.
In this article we will look in detail at how it works posistor, what tools are needed for testing and how to distinguish a burnt element from a working one. You will receive comprehensive information about why clicks occur, how to correctly remove the part and what to look for when visually inspecting the internal filling of the housing.
The principle of operation and the design of the starting relay
Start relay is a small device, usually enclosed in a black plastic case, which is attached directly to the side of the compressor. Inside it there is a key element - posistor (the thermistor with a positive temperature coefficient of resistance). In a cold state, the resistance of this element is minimal, which allows current to freely pass through the starting winding of the engine.
As soon as voltage is applied to the compressor, current passes through posistor and heats it. When a certain temperature is reached (usually about 100-120 degrees Celsius), the resistance of the thermistor increases sharply, essentially breaking the starting winding circuit. This process takes a split second and is necessary for the motor to switch to operating mode using only the working winding.
⚠️ Attention: A posistor relay is a disposable element. If it fails, it cannot be repaired or repaired; only a complete replacement with a new analogue with identical characteristics is possible.
The device body also serves the function of protecting contacts and attaching to the terminals of the compressor. There may be additional elements inside, such as a capacitor (in some circuits) or thermal protection, although in purely starting models the main task is precisely the switching of the windings at the time of start. Understanding this logic of operation is important for correct diagnosis.
Why a posistor?
Previously, electromagnetic relays were used, which worked on the principle of closing contacts when the current drops. The posistor circuit is more reliable, since it does not have mechanically moving contacts that could stick or spark. However, the thermistor itself degrades over time and loses its properties.
Main signs of malfunction and diagnostics
It is possible to determine what starting relay is faulty by characteristic external signs that appear immediately after the refrigerator is plugged into the network. The first and most obvious symptom is the failure of the compressor to start with a characteristic hum, which ends abruptly with a click. This cycle can be repeated at certain intervals until the thermal relay turns off the power completely.
When visually inspecting the removed part, obvious signs of overheating can be detected. If the plastic case is blackened, melted or has cracks, this is a sure sign that a breakdown or short circuit has occurred inside. The smell of burning, which often accompanies such breakdowns, should also alert you.
- 🔊 The compressor hums for 3-5 seconds, then a click is heard and silence sets in.
- 🔥 The relay housing has traces of soot, melting or deformation.
- ❄️ The refrigerator does not gain temperature, although the light inside is on.
- ⚡ The circuit breaker in the electrical panel trips when you try to start.
Sometimes the cause may not be the relay itself, but a jammed compressor, which creates an excessive load, leading to the combustion of the starting element. Therefore, before installing a new part, it is recommended to ensure that the motor shaft rotates freely. If the motor is working properly, then the problem lies precisely in the starting circuit.
Necessary tools and measures security
Before you start checking starting relay, you need to prepare a minimum set of tools. The main diagnostic device will be multimeter (tester), which allows you to measure circuit resistance and contact integrity. Without this device, a high-quality check is impossible, since a visual inspection does not always reveal internal defects of the posistor.
You will also need pliers or a screwdriver to remove the protective cover of the compressor and fix the wires. Gloves to protect your hands from possible dirt and oil, as well as a flashlight for better illumination of the work area, since access to the compressor is often limited, will not be superfluous.
⚠️ Attention: All work on dismantling and checking the relay should only be carried out with the refrigerator completely disconnected from the power supply. Unplug the plug from the socket and make sure there is no voltage.
Allow the equipment to cool for 15-20 minutes before starting work. This will protect you from burns and allow the posistor (if it is still functioning) to return to a cold state for correct testing.
☑️ Preparation for diagnostics
Step-by-step instructions for removing the relay
The dismantling process start relay is quite simple, but requires care so as not to damage the suitable wires or terminals of the compressor. First you need to remove the back panel of the refrigerator that covers the engine compartment. It is usually attached to latches or several screws, which can be easily unscrewed with a Phillips screwdriver.
Find the compressor - this is a black metal cylinder at the bottom of the unit. A plastic box is attached to its side, in which our desired device is located. In some models, the relay is simply put on the three pins of the compressor, in others it is fixed with a metal bracket or a plastic clip.
If the relay is held by a bracket, gently squeeze it with pliers and remove it. Then, using light rocking movements, pull the relay housing down. Do not pull sharply so as not to break the wires going to the thermostat or thermal protection, if they are integrated into the structure.
| Mounting type | Tool | Action | Risk of damage |
|---|---|---|---|
| Tension (on pins) | Without tools | Remove by rocking down | Low |
| Metal bracket | Pliers | Squeeze the clamp antennae | Medium |
| Plastic clamp | Screwdriver | Squeeze latch | High (fragile plastic) |
After removal, carefully inspect the connectors. They must be clean, without oxides and traces of soot. If the contacts are blackened, they can be carefully cleaned with fine sandpaper, but most often such symptoms require replacing the entire assembly.
Methods of checking with a multimeter
The most reliable diagnostic method is checking posistor using a multimeter switched to resistance measurement mode (usually the range is 200 Ohm or 2 kOhm). First, measure the resistance between the contacts to which the starting winding is connected. In good cold condition, it should be from 10 to 50 Ohms, depending on the model.
If the multimeter shows one (infinity) or zero, this indicates an open or short circuit inside the element, respectively. It is also important to check that there is no short to body. Touch one probe to any contact, and the other to the metal part or relay body - the device should show infinity.
The difficulty of checking a posistor is that it changes its properties when heated. To conduct a full test, some technicians use a hair dryer to heat the part while measuring resistance. When heated, the resistance of a working posistor should increase sharply to several megaohms.
Pay attention to the condition of the internal ceramic tablet. If, when opening the case (which is possible, but it violates the integrity), you see cracks or blackening on the thermistor itself, its functionality is excluded. However, it is often impractical to open the sealed case.
Replacement of the part and selection of an analogue
If diagnostics confirm a malfunction, you need to purchase a new one starting element. It is critical to select a device with identical characteristics, since different compressor models require different starting current. The part number is usually stamped on the body or indicated in the technical documentation of the refrigerator.
Installation is carried out in the reverse order of removal. Insert the new relay onto the compressor terminals as far as it will go, making sure that the contacts are tightly connected. Attach the retaining bracket or clamp. It is important to correctly orient the part: there is often an arrow on the body indicating the “UP” (up) direction, which should look strictly vertical.
⚠️ Attention: Incorrect orientation of the relay (arrow down or sideways) can lead to incorrect operation or rapid failure of the new part due to a violation of the heat sink or the position of the internal contacts.
After assembly, do not rush to close the back panel. Plug in the refrigerator and listen. The compressor should start with a characteristic sound, then the hum will become smooth and quiet. If the start-up was successful, let the equipment run for 10-15 minutes, checking the heating of the relay housing - it may warm up slightly, but should not smoke or crackle.
Frequently asked questions (FAQ)
Is it possible to start a refrigerator without a starting relay?
Theoretically, you can start an asynchronous motor using an external starting circuit, but it is absolutely impossible to operate the refrigerator in this mode. Without a relay, the compressor will not be able to switch to operating mode, will overheat, consume huge current and burn out within a few minutes or hours. This is an emergency mode, not a solution to the problem.
Why does a new relay burn out after a short time?
If a new one starting relay burns out soon after replacement, this often indicates a more serious problem. Possible reasons: jamming of the compressor piston group, an interturn short circuit in the motor windings, or a malfunction in the electrical network (voltage surges). Also, the cause may be improper installation or an incorrectly selected type of relay.
Where can I find the part number for replacement?
The part number (part number) is usually printed directly on the body of the burnt relay. If the marking is erased or unreadable, the number can be found in the service documentation of the refrigerator, on a sticker inside the chamber or according to the compressor model (for example, Danfoss, Secop, LG), choosing an analogue in a specialized store.
Does the room temperature affect the operation of the posistor?
Yes, it does. The posistor is sensitive to ambient temperature. If the refrigerator is in a very cold room (below +10°C), the posistor cooling time increases, which can lead to less frequent but longer startup attempts. In a hot room, the relay may cool down faster, but the risk of overheating with frequent starts is higher.