Sudden failure of the refrigerator is always an unpleasant situation, especially when there is food left in the chamber. One of the most common reasons why the compressor does not start or makes only a quiet click is the failure of the start-up relay, or rather, its key element posistor. This small ceramic component takes on the role of the starting winding, providing initial torque to the motor.
Fortunately, diagnosing this part does not require sophisticated laboratory equipment. You will only need a basic set of tools and multimeter. Understanding the principle of operation and the sequence of actions will allow you to independently determine whether you need to buy a new part, or whether the problem lies deeper in the power supply system.
Before you begin disassembling, you must clearly understand that working with electrical appliances requires compliance with safety rules. Unplugging is not just a recommendation, but a requirement. In this article we will look at how to visually assess the condition of an element, how to measure resistance and interpret the obtained values.
The operating principle and design of a PTC resistor
A posistor, often called PTC thermistor, is a semiconductor element whose electrical resistance directly depends on temperature. At room temperature, it has low resistance, which allows current to flow freely through the starting winding of the compressor, creating the necessary magnetic phase shift to start the engine.
When the refrigerator is turned on, a current passes through the posistor, which causes it to heat up. As soon as the temperature reaches a certain point (usually about 100-120 degrees Celsius), the resistance of the element sharply increases hundreds of times. This actually breaks the starting winding circuit, and the engine continues to operate on the main winding.
Key feature The fact is that after cooling the posistor becomes conductive again. However, it takes time to cool down before restarting, usually 3 to 5 minutes. This is what explains the pause that you hear between relay clicks when trying to start a faulty refrigerator.
⚠️ Attention: The posistor operates at high temperatures. If you have just tried to start the refrigerator, do not touch the relay parts with bare hands to avoid burns. Allow the assembly to cool for at least 10 minutes before diagnostics.
Structurally, the element is enclosed in a plastic case and connected to a common relay board. There is a ceramic tablet inside which may degrade over time. Mechanical shocks during transportation or power surges in the network often become the reason why starting device ceases to perform its functions correctly.
Visual diagnostics and initial inspection
The first stage of testing should always be a thorough visual inspection. Even without the use of measuring instruments, a number of critical defects can be identified. To do this, you need to remove the back cover of the refrigerator, unscrew the fixing clips from the start relay and remove the assembly along with the compressor.
Carefully open the plastic housing of the relay. Inside you will see the posistor itself - usually it is a white or grayish cylindrical element with two contacts. Pay attention to its appearance: there should be no traces of burning, cracks, blackening or melting of the plastic around the contacts.
If you find that the ceramic part has cracked or has obvious signs of thermal destruction, further measurements may not be required - the element definitely requires replacement. Also check the condition of the contacts: oxidation or carbon deposits can prevent the passage of current even with good internal resistance.
It often happens that on the outside the posistor looks absolutely intact, but inside there is a break in the conductive layer. In this case, you cannot rely on your eyes, and you must proceed to an instrumental check using tester.
Necessary tools for testing
For high-quality diagnostics, you will need a digital multimeter. Analog dial gauges are also applicable, but a digital display is more convenient for reading accurate resistance values. Make sure that the device is working properly and that the battery in it is not discharged, as this may distort the readings.
Set the multimeter to resistance measurement mode (indicated by the symbol Ω or Ohm). Select a measurement limit of 200 Ohm or 2 kOhm, since the resistance of a serviceable cold posistor is usually in the range from 10 to 50 Ohm.
In addition to the multimeter, prepare:
- 🔧 A flat-blade screwdriver to carefully open the relay housing.
- 🧤 Dielectric gloves to protect hands when working with electrical components.
- 📱 Smartphone with a camera to record the disassembly process and connection diagram.
- 🧹 Dry rags for cleaning contacts from dust and possible carbon deposits.
Any attempts to check the resistance “hot” or without removing the part from the circuit will lead to incorrect data and can damage the multimeter itself.
Step-by-step instructions: how to test a posistor with a multimeter
The checking process is quite simple, but requires care. First you need to disconnect the posistor from the relay contact group. Usually it is simply inserted into a connector or soldered; Most modern models use a detachable connection, which simplifies the task.
Attach the multimeter probes to the posistor contacts. Polarity does not matter in this case. The display should show the resistance value. For a working cold element, the normal range is considered to be from 15 to 50 Ohm, depending on the model and manufacturer.
If the device shows one (infinity) or zero, this indicates a malfunction. One means an open circuit inside the ceramic, and zero means a short circuit. In both cases, the element will not be able to create the necessary impulse to start the compressor.
☑️ Algorithm for checking the posistor
There is a method for testing “for heating”, but it requires caution and an additional 12-24V power source. By applying voltage to a cold posistor, you should see the resistance begin to rise and the current to fall. However, at home, it is easier and safer to limit yourself to measuring cold resistance and external inspection.
Table of typical values and faults
For ease of interpretation of measurement results, we have prepared a summary table. It will help you quickly compare the device readings with the probable condition of the part. Remember that the values may vary slightly depending on the ambient temperature.
| Multimeter reading | Element condition | Probable cause | Actions |
|---|---|---|---|
| 15 - 50 Ohm | Good | Norm for cold state | Check the compressor and relay |
| Infinity (1) | Faulty (Open) | Internal destruction of ceramics | Replacement of the posistor |
| 0 Ohm | Faulty (short circuit) | Short circuit of internal contacts | Replacing a posistor |
| Unstable numbers | Defective contacts | Oxidation or poor contact of probes | Clean contacts and repeat |
If your measurements showed a value outside the norm, or the behavior of the device was unstable, it is better not to take risks. The cost of a posistor is low, and replacing it takes a minimum of time. Installing a questionable element can lead to repeated failure or, in the worst case, damage to the compressor windings.
Why does the resistance change?
The resistance of a posistor nonlinearly depends on temperature. At 25°C it is minimal, but when heated to 100°C it can increase to 10-20 kOhm. It is this jump that turns off the starting winding.
Fault symptoms and indirect signs
Understanding how a refrigerator behaves when a posistor breaks down helps to narrow down the search before starting diagnostics. The most characteristic symptom is the so-called “cyclic start”. The compressor tries to turn on, the relay clicks, the hum lasts 2-3 seconds, then a pause follows, and everything repeats.
In some cases, the compressor may not respond to turning on at all if the starting winding circuit is completely broken. However, a quiet hum is often heard, indicating that voltage is supplied to the main winding, but there is not enough time to start.
Pay attention to the following indirect signs:
- 🔊 A characteristic metal click every 3-5 minutes.
- 🌡️ Strong heating of the compressor housing when the refrigerator is not working.
- 💡 Blinking light in the room when trying to start (voltage drop).
- 🔥 Smell of burning or melted plastic in the area of the motor compartment.
⚠️ Attention: Long-term operation of the refrigerator with a faulty starting device can lead to overheating and burning of the compressor windings. If the described symptoms appear, immediately disconnect the device from the network.
Sometimes the problem lies not only in the posistor, but also in the starting relay itself or in the motor windings. If replacing the posistor does not help, it is necessary to test the compressor windings for an interturn short circuit or break.
Replacement of the element and assembly of the unit
If the diagnostics confirmed a malfunction, the posistor must be replaced. When purchasing a new part, pay attention to the markings - it must match the original in terms of resistance and geometric dimensions. There are no universal solutions here, although many models are interchangeable within the same series.
Installation is carried out in the reverse order of disassembly. Insert the new posistor into the connectors, making sure that the contacts fit tightly. Assemble the relay housing by snapping the clips into place. When installing the relay on the compressor, make sure that it fits tightly and the contacts do not loosen.
After assembly, turn on the refrigerator. The first startup may take several minutes, since the system needs time to stabilize the pressure and cool the posistor (if there have been startup attempts). You should hear a confident start of the compressor and an even hum.
In some rare cases, it may be necessary to select resistance by experiment if the original cannot be found, but this is a task for experienced craftsmen. For standard home repairs, it is enough to find an analogue with an identical rating.
Is it possible to temporarily close the posistor contacts?
Theoretically, you can start the compressor by applying voltage directly to the starting winding, but doing this is strictly not recommended without special knowledge. This can lead to the winding burning out, since it is not designed for long-term operation. The posistor automatically turns it off, but if it is shorted, this will not happen.
Why does a new posistor burn out immediately after installation?
If the new element fails instantly, the reason is most likely not in it. Check the compressor for interturn short circuit or jamming of the piston group. The problem may also be an unstable mains voltage or a malfunction of the main starting relay.
Where can I find the posistor markings?
The marking is usually applied to the side surface of the ceramic cylinder in black or blue. The resistance (for example, 15R, 22R) and the permissible current are indicated there. If the marking is not readable, look for the relay model number and consult the diagram or spare parts catalogs.
Does the temperature in the room affect the operation of the posistor?
Yes, it does. In a cold room (for example, on a balcony in winter), the posistor resistance may be lower than the nominal value, which will change the start-up time. In hot weather, the cooling process will take longer, increasing the pause between attempts to start the compressor.