Many household owners Technicians are faced with a situation where the refrigerator compressor hums, but no cold is generated, or the unit refuses to start at all. Often the cause of such failures is the failure of a small but critical element of the engine starting circuit - starting relay, inside which there is a thermistor element called a posistor. Understanding exactly how a posistor works in a refrigerator will help you carry out initial diagnostics yourself and understand whether it’s worth calling a technician or you can do it on your own.
This part performs the function of an automatic switch, closing the circuit of the compressor starting winding only for a split second at the moment of start. If this mechanism is broken, the motor does not receive the necessary impulse to rotate the rotor, which leads either to a complete stop of the unit, or to characteristic clicks and humming without subsequent starting. In this article, we will analyze in detail the physical principle of operation of a posistor, its role in the system No Frost and in classical models, and also consider the algorithm for checking the serviceability of the element.
Before starting to disassemble the equipment, you must clearly understand that a posistor is not an ordinary relay with mechanical contacts, but a semiconductor device that changes its resistance depending on temperature. It is the thermal effect of the electric current that causes it to “turn off” the starting winding, preventing overheating of the motor. The critical parameter for replacement is the exact match of the nominal resistance and the diameter of the hole for the terminals, since even a minimal deviation can lead to combustion of the compressor.
Physical principle of operation thermistor
To understand why the posistor is so important, you need to turn to the physics of semiconductors. A posistor (or PTC thermistor) is a ceramic element whose resistance increases sharply when heated. In a cold state, that is, immediately after turning on the refrigerator, the resistance of this part is minimal and amounts to only a few tens of ohms. This allows current to flow freely through the starting winding of the electric motor, creating the necessary torque.
The starting process occurs as follows: at the moment the voltage is applied, the current passes through the posistor and the starting winding, causing the compressor rotor to gain speed. At the same time, the posistor itself begins to heat up under the influence of passing current. When the temperature reaches a certain threshold (usually about 100-120 degrees Celsius), its resistance increases abruptly to tens of kilo-ohms, essentially breaking the starting winding circuit. Further operation of the motor is carried out only due to the working winding.
It is important to note that after cooling the posistor does not return to its original state instantly. It takes time, usually 2 to 5 minutes, to cool down and become a conductor again. This is what explains the pause between repeated attempts to start the compressor if the first start was unsuccessful. If you hear a click, then a hum, and then click again after a few minutes, this is a classic symptom of a protection system in which the posistor plays a key role.
Start relay design and the role of the posistor
The starting relay in which the posistor is installed is usually a plastic case mounted directly on the terminals of the compressor. Inside this compact block there is the thermistor itself, a contact group and sometimes thermal protection. The design may vary depending on the manufacturer of the refrigerator, be it Indesit, Atlant or Bosch, but the general principle remains the same. The posistor in this assembly acts as a contactless switch, which makes it more reliable compared to mechanical analogs that are prone to sticking.
Inside the relay body, the posistor is fixed on spring contacts or soldered to the board in the case of modern inverter systems, although in the latter thermistors are used less frequently. The main purpose of the design is to ensure tight contact with the compressor terminals to transmit current and at the same time effectively remove the heat necessary for switching modes. Violation of the tightness of the relay housing can lead to moisture ingress, which causes oxidation of the contacts and incorrect operation of the posistor.
It is also worth mentioning the thermal protection relay, which is often built into the same housing or located nearby. If the posistor for some reason does not turn off the starting winding or the current in the circuit is too high, the thermal protection will break the power circuit, preventing the motor from burning out. This is a double safety system, where the posistor is responsible for normal starting, and the thermal relay is responsible for emergency shutdown.
Below is a table illustrating the dependence of the posistor state on temperature and its effect on the operation of the compressor:
| Thermal posistor state | Element temperature | Resistance | Compressor operating mode |
|---|---|---|---|
| Cold (rest) | 20–25 °C | Low (15–50 Ohm) | Ready to start, starting circuit closed windings |
| Heating | 60–100 °C | Increasing | Engine RPM |
| Hot (working) | > 120 °C | High (> 10 kOhm) | Starting winding is disabled, work on the working winding |
| Cooling | Decreasing to 60 °C | Decreasing | Pause before the next starting cycle |
Symptoms of a faulty starting device
You can determine that the problem lies in the posistor or relay by a number of characteristic signs. Most often, users notice that the refrigerator has stopped freezing, the light inside is on, but the compressor is silent or makes strange sounds. One of the most obvious symptoms is a series of clicking sounds repeated at intervals of several minutes. This indicates that the relay is trying to start the motor, the posistor heats up, opens the circuit, but the engine does not have time to reach operating mode.
Another sign is that the compressor starts, runs for a few seconds, then a click is heard and it stops. This may indicate that the posistor is “broken” and does not open the starting winding circuit, which is why the current in the motor increases sharply and the thermal protection is triggered. In some cases, a faulty posistor may not conduct current at all when cold, then the compressor will not start at all and there will be no clicks.
Visual inspection can also give clues. If you remove the relay and shake it, sometimes you can hear a ringing sound, which indicates the destruction of the ceramic element inside. In addition, blackening of the relay housing or the presence of a burning smell clearly indicates overheating and the need for replacement. However, you should not rely only on external signs, since the posistor may look intact, but not perform its function.
⚠️ Attention: If the compressor runs continuously without stopping and is very hot, but there is no cold, the problem may not be in the posistor, but in a freon leak or a malfunction of the motor itself. Do not change the relay without thorough diagnostics.
Diagnostics and testing with a multimeter
To accurately check a posistor, you will need a multimeter (tester) and a minimum set of tools. Before starting work, be sure to disconnect the refrigerator from the power supply! Remove the rear cover at the bottom of the unit where the compressor is located and carefully remove the start relay. Usually it is simply removed from the pins of the compressor; sometimes it is necessary to loosen the clamp or disconnect the wires.
The check is carried out in two stages. First you need to measure the resistance between the contacts of the posistor in a cold state. Switch the multimeter to resistance measurement mode (200 Ohm or 2 kOhm range). Touch the probes to the thermistor terminals. A working posistor should show a resistance in the range from 15 to 50 Ohms (the exact value depends on the model, but it should not be zero or infinite). If the device shows “1” (infinity) or “0” (short circuit), the part is faulty.
The second stage of testing is the heating test. This is more difficult to do at home without special equipment, but you can try to gently warm up the posistor with a hairdryer (not with an open fire!) and measure the resistance again. For a working element, the resistance should increase sharply when heated. If there are no changes, the posistor has lost its properties. Also check the integrity of the supply wires and the condition of the contacts for oxidation.
- 🔌 Turn off the power: unplug the refrigerator cord from the outlet before any manipulations.
- 🔍 Visual inspection: look for cracks, melting, traces of soot on the relay body.
- 📏 Resistance measurement: use multimeter for checking conductivity in a cold state.
- 🔥 Heating test: make sure that the resistance increases as the temperature of the element increases.
☑️ Checklist before buying a new relay
The process of replacing a posistor or relay
Replacing a starting relay with a posistor is a procedure accessible to a person with basic technical skills. The main rule: buy exactly the same relay model that was installed previously. Markings are usually applied to the plastic case (for example, R6, RT, PTC and a set of numbers). If it is impossible to find the original, you can choose an analogue, focusing on the diameter of the hole for the compressor pins and the nominal resistance.
The replacement process itself is simple: remove the old relay by disconnecting the wires (if they are not on the connectors, but soldered, they will have to be carefully unsoldered and cleaned). Install the new relay in place, pushing it onto the compressor contacts until it stops. Connect the wires, observing polarity if indicated, although this is often not critical for the starting winding. Make sure that the relay sits tightly and does not loosen.
After installation, turn on the refrigerator and listen. The compressor should start with a characteristic hum, after a few seconds the clicking should stop (or become barely audible), and the unit should begin to gain cold. If the clicks are repeated, perhaps a relay with unsuitable characteristics was selected or the problem is deeper. In some cases, it is necessary to replace the entire starting device assembly, since posistors are rarely sold separately.
⚠️ Attention: When installing a new relay, make sure that it is oriented correctly (usually the arrow on the body points up or is marked “TOP”). Incorrect installation can lead to overheating and rapid failure of the new part.
Is it possible to temporarily start a refrigerator without a posistor?
Theoretically, if you close the contacts of the starting winding manually when turning it on, the compressor will start. However, it is absolutely impossible to leave it running in this mode - the engine will burn out in a few minutes due to overheating of the starting winding. This method is suitable only for a short-term check of the performance of the motor itself.
The influence of external factors on the service life
The service life of the posistor and starting relay directly depends on the operating conditions of the refrigerator. Frequent power surges in the network are one of the main enemies of electronics. Sudden changes can lead to breakdown of the ceramic element or welding of contacts. To protect expensive equipment in conditions of unstable power supply, it is recommended to use voltage stabilizers or surge protectors of good quality.
Humidity and ambient temperature also play a role. If the refrigerator is installed in a damp room (for example, in a basement or in an unheated cottage), moisture may condense inside the relay. This leads to corrosion of the contacts and changes in the properties of the posistor. In addition, poor ventilation in the installation niche of the refrigerator causes overheating of the compressor and relay, which accelerates the degradation of materials.
The frequency of switching on and off also affects the resource. In older models or if the thermostat is set incorrectly, the compressor may turn on too often, not allowing the posistor to cool down. This leads to operation in borderline modes and a reduction in the service life of the part. Modern inverter compressors do not have this drawback, since they operate smoothly, but their starting elements (if any) must be in good working order.
- ⚡ Voltage surges: the main cause of electrical breakdown of the posistor.
- 💧 Humidity: causes oxidation of contacts and a change in resistance.
- 🌡️ Overheating: insufficient clearance between the back wall and the furniture impairs cooling.
- 🔄 Frequent cycles: wear and tear due to constant heating and cooling without a full cycle.
Frequently asked questions (FAQ)
Is it possible to replace a posistor with a universal analogue?
Yes, in many cases you can use a universal starting relay if it matches the diameter of the hole for the compressor bayonets and the power range. However, it is best to look for an original spare part using the catalog number indicated on the body of the old part to guarantee correct operation.
Why, after replacing the relay, does the refrigerator click but does not start?
This may mean that the problem is not in the relay, but in the compressor itself (interturn short circuit, piston group wedge) or in the wiring. It is also possible that a relay with the wrong resistance has been selected. Professional diagnostics of the motor is required.
How much does it cost to replace a posistor in a service center?
The cost consists of the price of the part itself (which is usually inexpensive) and the work of a technician to visit and diagnose. On average, replacing a start relay is cheaper than repairing a compressor, but prices may vary depending on the region and model of the refrigerator.
Is it dangerous to operate a refrigerator with a faulty posistor?
Yes, it is dangerous. Constant attempts to start “sit down” the compressor, overheat the windings and can lead to complete combustion of the engine, the replacement of which is not economically feasible. In addition, there is a risk of the wiring catching fire if there is a short circuit.
Can a posistor fail due to old age?
Yes, even without voltage surges, the ceramic element degrades over time. Materials lose their properties after thousands of heating and cooling cycles, so on old refrigerators (10+ years) the breakdown of the posistor is a natural wear and tear process.