Modern refrigerators with a system No Frost they often encounter the problem of the formation of an ice “coat” on the evaporator, which leads to disruption of air circulation and an increase in temperature in the chambers. In most cases, the culprit of this phenomenon is a failed starting element of the defrost system, which in technical documentation is often called positor or a thermistor. This small part is responsible for the timely activation of the heating element to defrost the ice, and its failure can turn an effective unit into a useless box.
If you notice that the refrigerator is humming, but not freezing, or a thick layer of ice is visible on the back wall of the freezer, do not rush to call a technician. Even a beginner can diagnose this malfunction if he has a basic set of tools. In this article, we will look in detail at how to identify a breakdown, what tools will be needed, and whether it is possible to do without expensive service.
It is important to understand that a posistor is not an ordinary resistor, but an element with a positive temperature coefficient of resistance. Its physicochemical properties change depending on the heating, which allows it to act as an automatic switch in the defrost circuit. Incorrect operation of this component can lead to combustion of the compressor or heating element, so timely checking is critical for the longevity of your equipment.
The design and principle of operation of a posistor in the No Frost system
To effectively diagnose the problem, you need to have a clear understanding of exactly how this unit functions. Posistor in the system The refrigerator defrost system works in tandem with a thermal relay, forming a single unit, often called a start-up relay or simply a “pill”. In a cold state, the resistance of the posistor is minimal, which allows current to pass freely through the starting winding of the compressor, starting the engine.
As soon as the motor starts working, a current passes through the posistor, heating its crystalline structure. The resistance increases sharply, practically breaking the starting winding circuit. The compressor continues to operate on the main winding. If we are talking about a defrosting system, then a posistor (thermistor) controls the temperature of the evaporator, closing the heating element circuit only when the sensor detects the accumulation of ice.
⚠️ Attention: In some models of refrigerators Indesit i Atlant the posistor is built directly into the body of the thermal relay. An attempt to disassemble this assembly to replace only the ceramic element often leads to a violation of the seal and improper operation of the protection, so change the assembly as a whole.
The operating principle is based on the physics of semiconductors. When a certain temperature (Curie point) is reached, the posistor material abruptly changes its properties. This provides reliable overload protection and correct control of defrosting cycles. Violation of this balance leads to the fact that the refrigerator either stops turning on or works non-stop without reaching the set mode.
The service life of this element is usually 5-7 years, after which the material degrades. It is impossible to visually determine wear; an instrumental check is required. Users often confuse a posistor with a conventional temperature sensor, but their functions and electrical characteristics differ significantly, which is important to consider when selecting spare parts.
Typical symptoms of a posistor malfunction
It is possible to determine that the problem lies in the starting element or defrost sensor by a number of indirect signs. First of all, pay attention to the nature of the compressor operation. If you hear a click when you turn it on, the motor hums for 2-3 seconds, and then a second click is heard and the hum stops - this is a classic symptom of a faulty starting unit.
In the case of the system No Frost the symptoms may be less obvious. The refrigerator may freeze, but only intermittently, or snow will begin to accumulate in the freezer. This indicates that the defrost circuit is not closing and the ice is not being removed. Over time, the ice plug blocks the cold air supply channels, and the refrigerator compartment becomes warm, although the compressor runs continuously.
- 🔊 The compressor makes a loud hum and clicks, but does not start.
- 🌡️ The temperature in the chambers is higher than normal, despite the motor running.
- ❄️ A thick ice crust is visible on the evaporator, blocking the fan.
- 🔥 The housing of the starting relay or posistor has traces of melting or blackening.
Another sign may be a burning smell in the compressor compartment area. This indicates that the posistor overheated and began to melt when trying to start the engine, or a short circuit occurred. In such a situation, operating the device is dangerous and requires immediate disconnection from the network.
Sometimes the refrigerator can behave “capriciously”: it turns on the fifth time or only after a long period of inactivity. This indicates that the characteristics of the posistor have “floated away”, and it does not provide the necessary current for a confident start of the motor rotor. It is impossible to leave the equipment in this state, as this leads to an inter-turn short circuit of the compressor windings.
Necessary tools for diagnostics
To carry out a quality test, you will need a minimum set of tools that most home craftsmen can find. The main device will be multimeter (tester), capable of measuring resistance in a wide range. Without this device, diagnostics will only be of a guessing nature and will not give an accurate result.
You will also need screwdrivers to remove the protective panels and compressor casing. In some models, for example LG or Samsung, access to the units may be hidden by decorative overlays that require careful dismantling. Do not forget to prepare pliers or wire cutters if you need to bite off the wires to strip the contacts, although it is better to use soldering.
For safety and ease of work, prepare:
- 🔌 Dielectric gloves to protect against accidental electric shock.
- 🔦 A flashlight, since the compressor compartment is often poorly lit.
- 🧹 A brush to clean the contacts from dust and oxides before checking.
- 📝 A marker to mark the wires before disconnecting them to avoid confusion.
It is important that the multimeter probes are in good working order and the battery in the device is not discharged, otherwise the resistance readings will be distorted. Before starting work, make sure that the refrigerator is completely disconnected from the power supply. Working with live electrical circuits without proper qualifications is prohibited.
☑️ Preparation for diagnostics
Step-by-step instructions: how to check a posistor with a multimeter
The testing process begins with gaining access to the part. Turn the refrigerator over or reach the compressor from the back. Remove the protective cover; it is usually secured with latches or bolts. Locate the start relay, which fits directly onto the compressor rod. Carefully remove it, having first photographed the wiring diagram.
Open the relay housing. Inside you will see a coil, a bimetallic plate and itself posistor (often this is a white or yellowish cylinder with two contacts). To check, switch the multimeter to resistance measurement mode (Ohms) to the 200 Ohm or 2 kOhm limit. Touch the posistor contacts with the probes.
In a cold state, a working posistor should show low resistance, usually in the range from 3 to 50 Ohms, depending on the model. If the device shows one (infinity) or zero, then the element is faulty. You can also carry out a "hot" test, but this requires caution and experience.
Normal resistance: 3...50 OhmFault (open): 1 or OL
Fault (short circuit): 0 Ohm
To check the thermistor of the defrost system (if it is placed separately on the evaporator), it also needs to be “ringed”. The resistance of a working sensor at room temperature is usually about 6-10 kOhm. When heated in the hands, the resistance should fall (for NTC) or increase (for PTC), which is easy to track on the multimeter screen.
⚠️ Attention: When checking, do not hold the probes with your fingers, as skin resistance can introduce an error in the measurements, especially at high limits. Hold the probes by the insulated handles.
Table of normal resistance values
For ease of diagnosis, we present a summary table of values. Please note that the exact parameters may vary depending on the manufacturer of the refrigerator and the type of compressor installed. Always check the technical documentation for your specific model, if available.
| Element type | Condition | Normal resistance | Fault symptom |
|---|---|---|---|
| Starting posistor | Cold | 3 - 50 Ohm | Infinity or 0 Ohm |
| Thermal defrost | Closed (cold). kOhm | 0 Ohm (Ringing) | Resistance > 0 Ohm |
| Defrost sensor (NTC) | At +20°C | 6 - 10 kOhm | Sharp change during heating |
| Defrost heating element | Good | 200 - 400 Ohm | Open (infinity) |
As can be seen from the table, the spread of values can be significant. It is not only the presence of resistance that is critical, but also its stability. If the numbers on the multimeter screen “jump” without changing the temperature, it means that there is poor contact or degradation of the material inside the element.
Particular attention should be paid to the defrost heating element. If the posistor and relay are working properly, but the defrost does not work, the heating element is often to blame. Its resistance must be stable. Also check that there is no breakdown on the housing: one probe on the contact of the heating element, the other on the metal body - the device should show infinity.
Is it possible to temporarily remove the posistor?
Theoretically, you can start the compressor without a starting element by applying a pulse manually, but this is dangerous. Without a posistor, the starting winding will burn out in a few seconds. Operating the refrigerator without a working start relay is strictly prohibited.
Replacing the posistor and assembling the system
If the diagnostics showed a malfunction, the element must be replaced. You can buy a new posistor or relay assembly in spare parts stores for household appliances. When purchasing, pay attention not only to the appearance, but also to the labeling. For different compressor powers, posistors with different characteristics are required.
The replacement process is simple: the old element is soldered off or disconnected (if there are terminals), and a new one is installed in its place. If the posistor is built into the relay, the entire unit changes. Make sure the contacts are tight and there is no tension on the wires. After installation, reassemble everything in the reverse order.
- 🛠️ Disconnect the refrigerator from the network before starting work.
- 🧩 Select an analogue with identical resistance parameters.
- 🔌 Securely fix the wires so that vibration does not break the contact.
- ✅ After assembly, check the operation of the refrigerator in idle mode.
After replacement, turn on the refrigerator and listen to it start. The motor should start working on the first try, without prolonged buzzing or clicking. During the first hour of operation, monitor the temperature in the chambers and the absence of extraneous sounds. If everything went well, the posistor successfully copes with its task.
Common mistakes in diagnostics and repair
One of the most common mistakes is trying to clean posistor contacts with sandpaper. This breaks the coating and leads to rapid failure. If an element is called incorrectly, it only needs to be changed. Users also often confuse the check sequence, starting with the heating element, although the problem may be in the sensor.
Another mistake is ignoring the wiring condition. Even if the posistor is working properly, oxidized contacts in the connector can give the same resistance as a breakdown. Always check the integrity of the entire circuit, including the wires going to the compressor and evaporator.
Do not forget about the thermal relay. If the posistor starts the motor, but after 10-15 minutes the refrigerator turns off with a click, perhaps the problem is not in the posistor, but in the thermal release, which reacts to overheating. A comprehensive check of all elements of the starting system is mandatory.
Is it possible to check a posistor without a multimeter?
It is impossible to accurately determine the resistance without a device. However, if when power is applied (for a short time!) the posistor does not heat up or, on the contrary, instantly heats up and smokes, this is an indirect sign of a malfunction. But this method is dangerous and is not recommended.
Why does a new posistor burn out?
A common cause is a jammed compressor. If the motor cannot turn over, the starting winding draws enormous current and the posistor burns out trying to start it. In this case, replacing the posistor will not help - the compressor needs to be repaired.
How often should the posistor be changed?
This element does not require scheduled replacement. It changes only in the event of a malfunction. The average service life is 5-10 years, but with frequent power surges in the network, it may fail earlier.
Does the temperature in the room affect the operation of the posistor?
Yes, in extremely cold rooms (below +10°C) the viscosity of the oil in the compressor increases and the starting currents increase. The posistor may not be able to start up, which will lead to overheating. In such conditions, refrigerators do not operate correctly.
Where is the posistor located in the No Frost refrigerator?
The starting posistor is located in the black casing at the back of the refrigerator, on the compressor. The defrost sensor (thermistor) is located on the evaporator, hidden behind the back wall of the freezer.