Modern systems No Frost ensure comfortable operation of refrigeration equipment, relieving owners of the need for regular manual defrosting. However, behind this automation lies a complex chain of electronic components, each of which plays a critical role. One of the key elements that ensures the uninterrupted operation of the defrost system is posistor (or a thermistor with a positive temperature coefficient).
Many users are faced with a situation where a coat of ice builds up in the freezer, and the fan stops moving air, although the compressor itself continues to work. In 80% of cases, the cause of this behavior is the failure of this particular small ceramic element. Understanding the principle of its operation allows you not only to diagnose a breakdown, but also to save on the services of a service center by performing the replacement yourself.
In this article we will analyze in detail the device of a posistor, its interaction with heating elements and a defrost timer, and also consider a troubleshooting algorithm. You will learn why this component is called a “smart switch” and how its correct operation affects the life of the compressor and the energy consumption of your refrigerator.
⚠️ Attention: Before starting any work on diagnosing or replacing the internal components of the refrigerator, be sure to disconnect the device from the electrical network. Working with live electrical wiring is deadly.
The operating principle and design of a thermistor
A posistor is a semiconductor resistor, the resistance of which increases sharply when a certain temperature is reached. Unlike conventional resistors, whose resistance varies linearly, this element works as a threshold switch. In a cold state (at temperatures below +60...+130°C, depending on the model), its resistance is only a few tens of Ohms, which allows current to flow freely through the circuit.
When electric current passes through the posistor, it heats up. As soon as the temperature of its crystal reaches the Curie point (critical temperature), the resistance increases abruptly to several megaohms. In effect, the circuit opens and the current stops flowing. This property makes it an ideal component for controlling defrosting heating elements in refrigerators.
In a defrosting system, a posistor often acts as a starting relay or a protective element. When the defrost mode is turned on, it passes current to the heating element. When the heating elements heat up to the desired temperature, the posistor “cuts off” the energy supply, preventing overheating of the evaporator and plastic parts of the chamber. Such a mechanism does not require complex electronics and is highly reliable.
Technical nuances
Why ceramics?: The posistor is based on barium titanate. This material has unique ferroelectric properties that allow the resistance to change by several orders of magnitude with a slight change in temperature. It is the ceramic nature of the element that ensures its durability and resistance to vibration, which is critical for working in tandem with a compressor.
The role of the posistor in the No Frost system
The system No Frost is based on the circulation of cold air, which is driven through the evaporator by a fan. During operation, condensation inevitably forms on the evaporator, which freezes in the form of frost. If this frost is not removed, it will turn into an ice monolith, blocking airflow and leading to equipment failure. This is where the defrost circuit comes into play, the central element of which is often a posistor.
In a classic circuit with a mechanical timer or electronic module, the posistor is connected in series with the evaporator heaters. When the timer switches the refrigerator to defrost mode, voltage is applied to the circuit. The posistor at this moment is cold and has low resistance, so the heating elements begin to actively heat up, melting the ice.
The key function of the component is to automatically complete the defrost cycle. As soon as the ice melts and the temperature of the sensor (or the posistor itself in some circuits) rises, the resistance of the element will increase and the heating will stop. This prevents a situation where heating elements continue to heat “dry”, which could melt plastic air ducts or damage seals.
- 🔹 Overheat protection: prevents a critical increase in temperature in the evaporator zone.
- 🔹 Energy efficiency: turns off powerful heating elements immediately after completion defrosting, saving electricity.
- 🔹 Automation: allows the system to operate without user intervention and complex humidity sensors.
Symptoms of a thermistor malfunction
Diagnosis of refrigerator malfunctions often begins with an analysis of the symptoms. If posistor fails, it immediately affects the climate inside the chambers. The most striking sign is the formation of icing on the back wall of the freezer or in the evaporator niche, even if visually it seems that the No Frost system is working.
If the posistor breaks down (when it is constantly closed), heating elements can heat up continuously or turn on at the wrong time, which leads to defrosting of food in the freezer and compressor overheating. In the event of a break (when the resistance is infinitely high even in a cold state), the defrost simply will not turn on. Ice will build up layer by layer until it blocks the fan.
Fan blockage is a secondary but very common symptom. When the ice coat becomes too thick, the fan blades begin to touch the ice. You will hear a characteristic crackling, humming or grinding sound. Soon the fan may stop completely, and the cold will stop flowing into the refrigerator compartment, although the temperature in the freezer will remain the same for some time.
⚠️ Attention: Do not ignore extraneous sounds (crackling, humming) from the freezer compartment. Prolonged operation of the fan with blocked blades can lead to combustion of its motor or destruction of the plastic impeller.
Diagnostics and testing with a multimeter
To accurately determine the malfunction, a visual inspection is not enough. You will need a digital multimeter set to resistance (ohms) mode. Before starting the test, make sure that the refrigerator is disconnected from the network, and the posistor itself has cooled down to room temperature, since when heated it will show high resistance, which can be mistaken for serviceability or a break.
Remove the back panel in the freezer to get to the evaporator. Find the defrost circuit: usually these are heating elements surrounding the evaporator, and a small ceramic element connected to them in a plastic case or filled with a compound. Disconnect the connectors leading to the posistor to exclude the influence of other circuit components on the measurements.
Touch the multimeter probes to the posistor contacts. When cold, a healthy element should show low resistance, usually in the range of 10 to 100 ohms (the exact value depends on the model, for example, for Indesit or Stinol it is often around 30-50 ohms). If the device shows “1” (infinity) or “0” (short circuit), the part requires replacement.
☑️ Multimeter testing algorithm
Table of typical parameters and compatibility
When selecting a replacement, it is important to take into account not only the geometric dimensions, but also the electrical characteristics. PTC resistors can differ in nominal resistance, operating current and switching temperature. Using an unsuitable analogue may lead to incorrect operation of the defrost system.
Below is a table with approximate parameters of common types of posistors used in household refrigeration equipment. Please note that data may vary depending on the component manufacturer.
| Type / Model | Resistance at 25°C (Ohm) | Trigger current (A) | Switching temperature (°C) | Application |
|---|---|---|---|---|
| PTC-1 (Analog) | 15 - 25 | 2.0 - 3.0 | 60 - 80 | Small heating elements, starting relays |
| PTC-2 (Standard) | 30 - 50 | 1.5 - 2.5 | 90 - 110 | Refrigerators Indesit, Ariston |
| PTC-3 (Powerful) | 50 - 80 | 3.0 - 5.0 | 120 - 140 | Industrial chambers, large heating elements |
| PTC-4 (Specific) | 100 - 150 | 0.5 - 1.0 | 70 - 90 | Motor protection systems |
Step-by-step instructions for replacing a component
Replacing a posistor is a procedure that is accessible even to a beginner, if you are careful. The main rule: do not damage the insulation of the wires and restore connections correctly. You will need a screwdriver, side cutters, electrical tape or heat shrink, and, preferably, a soldering iron for more reliable contact, although you can get by with twisting with high-quality insulation.
First, dismantle the old element. If it is secured to the heating element with clamps or clips, carefully remove them. Cut the wires as close to the posistor body as possible, leaving enough margin for connection to the new element. Strip the ends of the wires by 5-7 mm.
Install a new posistor in place of the old one, securing it in the same way. Connect the wires: twist the wires, preferably solder the connection with tin to prevent oxidation, and carefully insulate. Use heat-shrink tubing, warming it with a hairdryer or lighter (carefully!) to get a sealed connection that is resistant to moisture.
After installation, reassemble the structure in the reverse order: install the evaporator panel, tighten the screws. Plug in the refrigerator. In the first 10-15 minutes, listen to see if the defrosting process is in progress (if the cycle coincides), or just wait until the temperature rises. After an hour, check to see if there is any extraneous noise or if cold is starting to form.
⚠️ Attention: The quality of the insulation of the connections is critical. There is always moisture from condensation in the evaporator area. Poor insulation will lead to a short circuit and failure of the control module or burnout of the wiring.
Frequently asked questions and answers (FAQ)
Is it possible to temporarily remove the posistor from the circuit to make the refrigerator work?
Theoretically, if you short-circuit the circuit, the heating elements will heat up constantly, which will lead to defrosting of the food and overheating. If the circuit is broken, the defrost will not turn on at all, and the refrigerator will quickly become clogged with ice. You cannot remove the posistor is a key element of safety and automation.
Why does the new posistor burn out immediately after replacement?
The reason may be be an incorrect selection of the part (wrong current or voltage), a short circuit in the heating elements themselves (breakdown to the housing) or a malfunction of the defrost timer, which supplies voltage at the wrong time. Also check the condition of the wiring for chafing.
How often should you change a posistor in a refrigerator?
The service life of a posistor is usually 5-10 years or more. It is not a consumable item that is changed on a schedule. Replacement is made only in the event of a malfunction, when the system stops defrosting correctly.
Does the brand of the refrigerator affect the choice of posistor?
Yes, indirectly. Refrigerators LG, Samsung or Bosch can use different defrost control schemes. However, the physical principle of operation of a posistor is the same. The main thing is the (coincidence) of electrical parameters (resistance, current) and geometric dimensions for installation in a regular place.