A temperature sensor is a functional component that senses temperature and converts the sensed temperature change into an electrical signal. Before we practice the temperature sensor detection and replacement, we must first have a certain understanding of the device orientation, structural characteristics and operating principle of the temperature sensor.
1. The device orientation and structure of the temperature sensor
In the indoor unit of an air conditioner, there are generally two temperature sensors, namely the room temperature sensor and the pipeline temperature sensor. The temperature sensor head of the indoor temperature sensor is generally installed on the surface of the evaporator, that is, the front side of the air inlet, which is mainly used to detect the temperature in the room; The temperature sensing head of the pipeline temperature sensor is generally mounted on the pipeline of the evaporator, and is fixed in the copper pipe by a clip, which is mainly used to detect the temperature of the evaporator pipeline. Let's take a closer look at the location of the temperature sensor in the indoor unit of the air conditioner.
The primary function of these two temperature sensors is to sense the operating temperature at that time, and transmit the temperature directly to the system control integrated circuit to maintain the normal operation of the air conditioner.
The indoor temperature sensor and the pipeline temperature sensor are related to the main control circuit through signal lines and plug-ins, and the sensed indoor temperature signal and the temperature signal of the evaporator are sent into the microprocessor, and the current operation status of the air conditioner is determined by microprocessing calculation.
The temperature sensor is essentially a thermistor, which is to measure the temperature and temperature-related parameters by using the characteristics of the resistance value of the thermistor to change with the change of temperature.
2. The operation process of the temperature sensor
1) Indoor temperature sensor input signal
When TH1 detects a change in temperature, the change in resistance causes a voltage change in the voltage divider circuit, and the room temperature signal is sent to the 38 pins of the microprocessor. A capacitor is connected in parallel at both ends of the room temperature sensor TH1, and the voltage at the input end of this temperature sensor is about 2V at normal temperatures.
2) Return the line temperature sensor input signal
The transmission signal of the pipeline temperature sensor TH2 is divided by the resistor and then input by the 37 pins of the microprocessor. This voltage signal reflects the temperature of the indoor unit coil. Under normal conditions, the input voltage of this temperature sensor is about 3V.
In addition, temperature sensors can be divided into two categories: PTC sensors and NTC sensors according to their different sensing characteristics. Among them, the NTC sensor is a negative temperature coefficient sensor, that is, the resistance value of the sensor decreases with the increase of temperature; PTC sensors are positive temperature coefficient sensors, that is, the sensor resistance increases with the increase of temperature.
3. Detection method of temperature sensor
In the air conditioner, the temperature sensor is an indispensable control device, if the temperature sensor is damaged or abnormal, it will generally cause the air conditioner not to operate, the air conditioner outdoor unit does not run and other problems, so it is very necessary to grasp the maintenance method of the temperature sensor.
There are generally two ways to detect temperature sensors: one is to detect the power supply signal and output voltage (the voltage fed into the microprocessor) of the temperature sensor on the road; One is to detect the resistance in different temperature environments under the open circuit condition.
When detecting the relevant voltage value of the temperature sensor on the road, take out the circuit board in the indoor unit from its circuit board bracket, then connect various components, turn on the power supply, and detect the temperature sensor in the air conditioner under the road condition.
Before testing, the connection between the temperature sensor and other components should be clarified, the relevant voltage value under normal conditions should be analyzed or pinpointed, and then the test should be carried out to determine the good or bad according to the test results. The following figure shows the detection of the temperature sensor of an air conditioner.
It can be seen that under normal conditions, one pin of the indoor temperature sensor and the pipeline temperature sensor is connected to the 5V power supply voltage after the inductor, so under normal conditions, the power supply terminal voltage of the two temperature sensors should be 5V, otherwise it should be judged whether the sensor is an open circuit fault.
The other pin is connected to the voltage divider point of the resistor voltage divider circuit, and the voltage is sent into the microprocessor, under normal conditions, the voltage sent by the indoor ambient temperature sensor to the microprocessor should be about 2V, and the voltage value of the pipeline temperature sensor sent to the microprocessor should be about 3V, and the temperature changes, and the voltage also changes, and the range is 0.55-4.5V. Otherwise, the temperature sensor is abnormal.
The indoor temperature sensor and pipeline temperature sensor of the air conditioner are related to the 5V power supply circuit through the inductor, and the voltage at the end is detected by the DC voltage block of the multimeter under normal conditions. If the voltage is normal, it means that the power supply of the temperature sensor is normal; If there is no voltage, it detects whether the sensor is open or whether the power supply part is abnormal.
When the temperature sensor is operating, the temperature change signal is converted into an electrical signal, and then sent to the relevant pins of the microprocessor through the socket and resistor, and the voltage value of the microprocessor pin can be detected by the DC voltage block of the multimeter, and the voltage value of 2 to 3V should be measured under normal conditions.
If the power supply voltage of the temperature sensor is normal, and the voltage of the pressure point of the socket is 0V, the external sensor is mostly damaged and should be replaced. In general, if the voltage at the input pin of the sensor signal of the microprocessor is higher than 4.5V or lower than 0.5V, it can be determined that the temperature sensor is damaged. In addition, the open circuit of the external voltage divider resistor of the temperature sensor will also cause the air conditioner to not operate and the temperature sensor will be alarmed when it is turned on.
The open-circuit detection temperature sensor refers to the separation of the sensor from the circuit, and the quality of the temperature sensor is judged by detecting the change of the resistance value of the temperature sensor under different temperature conditions (normal temperature and high temperature) without power.
At room temperature, the pipeline temperature sensor is detected, that is, the pipeline temperature sensor is placed in the indoor environment, and the resistance value is detected by the resistance of the multimeter, under normal conditions, the resistance value of the evaporator pipeline temperature sensor is about 6.45K, and the resistance value of the indoor ambient temperature sensor is about 6.18K.
When detecting a temperature sensor at high temperatures, it is possible to artificially raise the ambient temperature of the temperature sensor, such as filling a cup with hot water and placing the sensing head of the temperature sensor in the water cup. After that, use a multimeter for testing.
The temperature sensor of the air conditioner is a negative temperature sensor. Therefore, under high temperature conditions, the resistance strain of the detection room temperature sensor and pipeline temperature sensor is small, as in the above test. At high temperatures, the resistance of the indoor ambient temperature sensor is about 1.87K, and the resistance of the pipeline temperature sensor is about 1.022K.
If the resistance value of the temperature sensor in room temperature, hot water and cold water does not change or does not change significantly, it means that the temperature sensor has been abnormal and should be replaced in time. If the resistance of the temperature sensor is always very large (tending to infinity), it is clear that the temperature sensor is showing a problem, if the temperature sensor is normal during open circuit detection, and the voltage value of its pins is too high or too low during road detection, it is necessary to do further testing of the circuit part to eliminate the problem.
When the resistance value of the temperature sensor becomes smaller, it is equivalent to detecting an increase in temperature, and the microprocessor will think that the indoor temperature or the temperature of the evaporator pipeline is higher than a certain value after receiving the signal sent by the sensor. So as to control the air conditioner, indoor electromechanical fan, and motor to run all the time; If the resistance of the temperature sensor becomes larger, it is equivalent to detecting a temperature drop, and the microprocessor will also refer to this signal (not a normal signal) to control the air conditioner accordingly, causing abnormal operation of the air conditioner.
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