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Types and Principles of Air Conditioning Temperature Sensors

——The air conditioner temperature sensor is a negative temperature coefficient thermistor, referred to as NTC, also known as a temperature probe. The resistance value decreases with the increase of temperature, and increases with the decrease of temperature. The resistance value of the sensor is different, and the resistance value at 25℃ is the nominal value.

Plastic-encapsulated sensors are generally black, and are mostly used to detect ambient temperature, while metal-encapsulated sensors are generally stainless steel silver and metallic copper, which are mostly used to detect pipe temperature.

The sensor is generally two black leads side by side, and the resistor is connected to the socket of the control board through the lead plug. There are generally two sensors in the air conditioner room. Some air conditioners have two separate two-wire plugs, and some air conditioners use one plug and four leads. In order to distinguish the two sensors, most air conditioner sensors, plugs and sockets are made to be recognizable.


——The sensors commonly used in air conditioners are:

Indoor ambient temperature NTC

Indoor tube temperature NTC

Outdoor pipe temperature NTC, etc.

Higher-end air conditioners also use outdoor ambient temperature NTC, compressor suction and exhaust NTC, and air conditioners with indoor unit blowing air temperature NTC.


——The common role of temperature sensors

1. Indoor ambient temperature detection NTC (negative temperature coefficient thermistor)

According to the set working state, the CPU detects the temperature of the indoor environment through the indoor ambient temperature (referred to as the inner ring temperature) NTC, and controls the compressor to be powered on or powered off to stop.

The variable frequency air conditioner performs variable frequency speed regulation according to the difference between the set working temperature and the indoor temperature. When running at high frequency after starting up, the larger the difference, the higher the compressor operating frequency.

2. Indoor tube temperature detection NTC

(1) In the cooling state, the indoor tube temperature NTC detects whether the indoor coil temperature is too cold, and whether the indoor coil temperature drops to a certain temperature within a certain period of time.

If it is too cold, in order to prevent the indoor unit coil from frosting and affecting the exchange of indoor heat, the CPU compressor will be shut down for protection, which is called supercooling protection.

If the indoor coil temperature does not drop to a certain temperature within a certain period of time, the CPU will detect and judge the refrigeration system problem or lack of refrigerant, and the compressor will be shut down for protection.

(2) Anti-cold air blowing detection, overheating unloading, overheating protection, heating effect detection, etc. in the heating state. When the air conditioner starts heating, the operation of the indoor fan is controlled by the temperature of the inner tube. When the temperature of the inner tube reaches 28 to 32 °C, the fan will run to prevent the heating from starting to blow out cold air, causing physical discomfort.

During the heating process, if the indoor pipe temperature reaches 56°C, it means that the pipe temperature is too high and the high pressure is too high. At this time, the CPU controls the outdoor fan to stop to reduce the absorption of outdoor heat, and the compressor does not stop, which is called heating unloading.

If the temperature of the inner tube continues to rise after the outdoor fan is stopped, and reaches 60°C, the CPU will control the compressor to stop the protection, which is the overheat protection of the air conditioner.

In the heating state of the air conditioner, within a certain period of time, if the tube temperature of the indoor unit does not rise to a certain temperature, the CPU will detect the problem of the refrigeration system or the lack of refrigerant, and the compressor will be shut down for protection.

It can be seen from this that when the air conditioner is heating, both the indoor fan and the outdoor fan are controlled by the indoor pipe temperature sensor. Therefore, when repairing the operation failure of the heating-related fan, pay attention to the indoor pipe temperature sensor.

3. Outdoor pipe temperature detection NTC

The main function of the outdoor tube temperature sensor is to detect the heating and defrosting temperature. Generally, after the air conditioner is heated for 50 minutes, the outdoor unit enters the first defrosting, and the subsequent defrosting is controlled by the outdoor tube temperature sensor, and the tube temperature drops to -9 ℃, start defrosting, and stop defrosting when the tube temperature rises to 11-13 ℃.

4. Compressor exhaust gas detection NTC

Avoid overheating of the compressor, detect lack of fluorine, reduce the frequency of the inverter compressor, control the flow of refrigerant, etc.

There are two main reasons for the high discharge temperature of the compressor. One is that the compressor is in an overcurrent working state, mostly due to poor heat dissipation, high pressure and high pressure, and the other is the lack of refrigerant or no refrigerant in the refrigeration system. The electric heat and frictional heat of the compressor itself cannot be well discharged with the refrigerant.

5. Compressor suction detection NTC

In the refrigeration system of the air conditioner with the electromagnetic throttle valve, the CPU controls the refrigerant flow by detecting the temperature of the return air of the compressor, and the stepper motor controls the throttle valve.
The compressor suction temperature sensor also plays the role of detecting the cooling effect. There is too much refrigerant, the suction temperature is low, the refrigerant is too little or the refrigeration system is blocked, the suction temperature is high, the suction temperature without refrigerant is close to the ambient temperature, and the CPU Detect the suction temperature of the compressor to determine whether the air conditioner is working normally.

Post time: Nov-07-2022