How does the air conditioning temperature sensor work?

1. The commonly used NTCs for air conditioners include indoor ring temperature NTC, indoor coil NTC, and outdoor coil NTC. The higher-grade air conditioners also use external ring temperature NTC, compressor suction, and exhaust NTC. In the circuit of NTC, the temperature change causes the NTC resistance value to change, and the voltage of the CPU terminal also changes. The CPU determines the operating state of the air conditioner according to the change of the voltage.

2. After the air conditioner temperature sensor is connected in series with a resistor, the voltage of 5V (+3.3V used in some air conditioners) is divided, and the voltage after the voltage divider is sent to the CPU. Since the air conditioner temperature sensor adopts a negative temperature coefficient thermistor, the resistance value decreases when the temperature rises, and the resistance value increases when the temperature decreases.

Therefore, the input voltage law of the CPU is: when the temperature rises, the input voltage of the CPU rises, and when the temperature decreases, the input voltage of the CPU decreases. This changed voltage enters the internal analysis processing of the CPU to determine the current tube temperature or room temperature, and controls the operating state of the air conditioner through internal procedures and artificial settings.

3. Since the sampling voltage sent to the CPU varies with temperature and varies widely, the manufacturer usually designs it at 25 degrees, and the sampling voltage is designed to be half of the power supply voltage, so as to give temperature changes. There is plenty of room for voltage changes to hatch. If the sampling voltage is designed too high or too low, it will not reflect the current temperature change.

Since the resistance values of the series resistors of R1, R2 and R3 are constant, if the internal resistance of the CPU interface is not considered (in fact, the internal resistance of the interface is relatively large, it may not be considered), then it is necessary to ensure that its A, B, C three CPU input point voltage is about 2.5V (at 25 degrees). RT1, RT2, RT3 theses three sensors can only use three series resistance (R1, R2, R3) with the same value of the sensor. Otherwise, the voltage drop at this point deviates more.