When a modern refrigerator suddenly stops cooling, and the compressor is either silent or makes a short hum and clicks, most owners begin to panic. Often the cause of a breakdown is not an expensive motor-compressor, but a tiny part hidden inside a black or gray plastic case on the back wall of the unit. This part is commonly called a “tablet”, although technically it is posistor or a thermistor with a positive temperature coefficient of resistance.
It is this small element that is responsible for starting the engine, providing voltage to the starting winding in the first seconds of operation. If the tablet fails, the refrigerator stops turning on or operates intermittently, requiring immediate attention. Understanding the operating principle of this unit will help you diagnose the problem yourself and, perhaps, avoid calling a technician.
In this article we will analyze in detail the device of the start relay, explain the physical processes occurring inside the tablet when heated, and provide step-by-step instructions for replacing it. You will learn why this part burns out, how to distinguish a faulty posistor from a breakdown of the compressor, and what precautions must be taken during repairs.
The principle of operation of the starting relay and the role of the posistor
In order to start the asynchronous electric motor with which the refrigerator compressor is equipped, it is necessary to create a rotating magnetic field. In operating mode, this field is created by one winding, but to start, a force is required, which is provided by the second - the starting winding. Start relay this is what briefly connects this second winding to the power supply. Inside the relay body, in addition to the electromagnetic or induction mechanism, there is often the same ceramic element - a tablet.
A posistor is a semiconductor element, the resistance of which increases sharply as the temperature rises. In a cold state, when the refrigerator is turned off, the resistance of the tablet is minimal, and it freely passes electric current to the starting winding. At the moment voltage is applied, current begins to flow through the tablet and the winding, causing the posistor to heat up.
Literally after 1-3 seconds, the temperature of the tablet reaches a critical point (Curie point), and its resistance increases thousands of times, actually breaking the starting winding circuit. By this time, the engine has already gained the required speed and continues to operate on the main winding. This cycle is repeated every time the compressor starts.
Why does the relay click?
The click you hear is the sound of the thermal relay contacts opening or the sound of the posistor cooling. If the refrigerator clicks and does not start, it means that the tablet has either already burned out (broken), or the compressor cannot start due to other reasons, and the thermal relay emergency turns off the power.
Main reasons for the tablet to fail
Despite the simplicity of the design, the tablet in the relay is a consumable item. Over time, the ceramic element degrades, loses its properties or is physically destroyed. The main reason for failure is thermal fatigue material. Each cycle of turning the refrigerator on and off is heating up to hundreds of degrees and subsequent cooling, which leads to microcracks in the structure of the posistor.
Another common reason is a power surge. A sudden surge of current can instantly melt or crack a ceramic tablet. It is also worth considering the condition of the compressor itself: if the engine is “heavy” (has mechanical wear or jamming), it consumes a starting current higher than normal, which leads to overheating and combustion of the starting element.
Often users are faced with a situation where an apparently intact tablet ceases to perform its function. Irreversible chemical changes could have occurred inside it, changing the temperature coefficient of resistance. In this case, the starting winding circuit does not break, which leads to overheating of the engine and tripping of the protection.
Symptoms of a faulty starting device
You can determine that the problem lies in the tablet or relay by characteristic signs. The most obvious symptom is that the refrigerator does not turn on, although the indicators are on and there is voltage in the network. In this case, you can hear a characteristic hum of the motor, lasting several seconds, followed by a click and silence.
Another sign is overheating of the rear wall of the refrigerator in the lower part, where the compressor is located. If the starting winding does not turn off due to stuck contacts or a malfunction of the tablet, the motor operates in abnormal mode, consuming excess current. This can lead to a fire hazard.
⚠️ Attention: If you smell burning or see melted plastic on the relay body, immediately unplug the refrigerator. Operation of such a device is prohibited until complete repair and replacement of burnt out elements.
It is also worth paying attention to the frequency of switching on. If the refrigerator begins to turn on and off too often (the so-called “cold start”), this may indicate that the tablet does not have time to cool down and restore conductivity, or, conversely, does not hold the circuit.
Diagnostics and checking the tablet with a multimeter
Before buying a new part, you need to make sure it is faulty. To do this, you will need easy multimeter access to the back of the refrigerator. First, disconnect the unit from the power supply and remove the protective casing from the compressor. Carefully disconnect the wires from the start relay and remove the unit itself.
Disassemble the relay housing to get to the tablet. Typically it is a white or grayish disk mounted in a terminal block. Visually inspect the element: the presence of cracks, blackening, chips or melting clearly indicates the need for replacement. However, even visually the entire element may not work.
To check, set the multimeter to resistance measurement mode (Ohm). Touch the contacts of the tablet with the probes. In a cold state, a working posistor should show a resistance in the range from 10 to 50 Ohms (the exact value depends on the model). If the device shows one (infinity) or zero, the part is faulty.
☑️ Algorithm for checking the relay
Table of compatibility and characteristics of posistors
When selecting a replacement, it is important to take into account not only the geometric dimensions, but also the electrical parameters. Tablets may differ in diameter, thickness, initial resistance and actuation current. Using the wrong element may result in the compressor either not starting or burning out.
| Parameter | Description | Impact on operation |
|---|---|---|
| Initial resistance | Cold resistance (usually 10-60 Ohm) | Determines the strength of the inrush current. If high - weak start, too low - overheating. |
| Operation current | Current at which switching occurs | Must correspond to the power of the compressor. Incorrect selection will lead to untimely shutdown. |
| Geometric dimensions | Diameter and height of the tablet | Affects the possibility of installation in the relay housing and the tightness of contact with the terminals. |
| Time response | Heating time to the Curie point | Critical for synchronization with the moment the engine picks up speed. |
The most common tablets with a diameter of about 13 mm, but there are other options. It is critically important when buying a new part to pay attention not only to diameter, but also on the resistance marking indicated on the body of the old element or in the technical documentation of the refrigerator.
If it is not possible to find an exact analogue, it is permissible to install a posistor with similar characteristics, but the difference in resistance should not exceed 10-15%. In extreme cases, you can use universal kits that come complete with a new relay housing and are suitable for most compressor models.
Instructions for replacing a tablet in a relay
The replacement process does not require complex equipment, but demands accuracy. After you have purchased a suitable posistor, make sure that the refrigerator is completely de-energized. If you haven't already done so, remove the start relay from the compressor and disassemble its housing. Usually it consists of two halves, fastened with latches or screws.
The old tablet must be dismantled. It can be soldered to the contact plates or simply inserted into the terminal block. If the element is soldered, use a soldering iron with a power of at least 60 W, as the massive contacts quickly dissipate heat. Be careful not to overheat the adjacent plastic elements.
Install the new tablet in place of the old one. If it is inserted into the grooves, make sure it fits snugly. If soldering is required, use quality solder and flux. Assemble the relay housing, check that the wires are securely fastened and install the unit on the compressor fitting. The compressor pins must fit tightly into the relay contact groups.
⚠️ Attention: When installing the relay on the compressor, make sure that it is located strictly in accordance with the “UP” marking or the arrow pointing up. Incorrect orientation will disrupt the operation of the thermal release.
After assembly, connect the refrigerator to the network. It is considered normal to start the compressor 3-5 minutes after plugging it in (the waiting time is necessary to equalize the pressure in the system and cool down the posistor if it was heated during testing).
Prevention and service life of the device
The service life of the start relay and tablet directly depends on the operating conditions of the refrigerator. Frequent power surges in the network are the main enemy of electronics and starting equipment. For protection, it is recommended to use a voltage stabilizer or surge protector, especially in the private sector or old houses.
It is also important to ensure good ventilation of the compressor compartment. If the refrigerator is located close to the wall or in a niche, the heat is not removed, and the relay operates under elevated temperature conditions, which reduces the life of the tablet. Regularly, at least once a year, clean the rear grille of dust and cobwebs.
If your refrigerator is a model with an inverter compressor, it is worth noting that the starting principle may be different in them, and the classic posistor tablet may not be there, giving way to electronic control modules. In such cases, diagnosis requires a different approach.
Is it possible to repair a tablet?
It is impossible to restore a physically destroyed or burnt tablet. This is a one-time item. Attempts to glue it or clean the contacts will not give a long-term effect and can lead to a fire.
Why does the refrigerator hum for 5 seconds and turn off?
This is a classic symptom of a faulty starter or jammed compressor. A humming sound means that current is being supplied to the windings and the motor is trying to start. If after a few seconds a click is heard and there is silence, it means that the thermal protection relay has tripped. Most often, the tablet is to blame because it does not open the circuit or does not close it, or the compressor itself is “jammed” mechanically.
Is it possible to start a refrigerator without a tablet?
Theoretically, you can apply voltage to the starting winding manually or through a button, but it is absolutely impossible to operate the refrigerator in this mode. Without automatic shutdown of the starting winding, the engine will burn out in a matter of minutes. A tablet is a necessary element of safety and functionality.
Where to buy a tablet for a refrigerator relay?
These parts are sold in spare parts stores for household appliances, on radio markets or in online stores. You need to search by the query “positor for refrigerator” or by spare part code. They are often sold complete with a new relay housing, which simplifies replacement.
Is it dangerous to change a relay yourself?
Replacing a relay is safe if you follow the main rule: work only with the device completely disconnected from the network. There is no high voltage inside the refrigerator after unplugging it from the outlet. However, if you are not confident in your abilities or are afraid of damaging the compressor tubes, it is better to entrust the work to a professional.