How the thermistor works.Smart Human Body Infrared Sensor

　　Thermistor is a kind of sensor resistance, the resistance value of thermistor changes with the change of temperature, which is different from the general fixed resistance. The resistance value of metal increases with the increase of planting degree, but the opposite is true for semiconductor, its resistance value decreases sharply with the increase of temperature, and shows a nonlinearity, as shown in the figure below.

　　It can be seen from the figure that when the temperature changes are the same, the resistance value of the thermistor changes about 10 times that of the lead thermal resistance, so it can be said that the thermistor is particularly sensitive to temperature changes. This temperature characteristic of semiconductor is because the conduction mode of semiconductor is carrier (electron, hole) conduction. Since the number of carriers in a semiconductor is much less than the number of free electrons in a metal, it has a high resistivity. As the temperature increases, the number of carriers participating in conduction in the semiconductor will increase, so the conductivity of the semiconductor will increase and its resistivity will decrease.

　　The role of thermistor:

　　1. Temperature measurement

　　As a thermistor sensor for measuring temperature, the structure is generally simpler and the price is lower. Thermistors without an outer protective layer can only be used in dry places; sealed thermistors are not afraid of moisture erosion and can be used in harsh environments. Due to the large resistance of the thermistor sensor, the resistance and contact resistance of its connecting wires can be ignored, so the thermistor sensor can be used in long-distance temperature measurement for several kilometers, and the measurement circuit mostly adopts bridge circuits.

　　2. Temperature compensation

　　Thermistor sensors can compensate for the humidity of certain components within a certain temperature range. For example, the moving coil in the head of the moving coil instrument is made of copper wire, and the temperature rises and the resistance increases, causing temperature errors. Therefore, the negative temperature coefficient thermistor can be connected in parallel with the manganese copper wire resistance in the loop of the moving coil, and then connected in series with the compensated component, thereby offsetting the error caused by the temperature change.

　　3. Overheat protection

　　Overheating protection is divided into direct protection and indirect protection. For small current applications, the thermistor sensor can be directly connected to the load to prevent overheating damage to protect the device. For high current applications, it can be used to protect relays, transistor circuits, etc. For example, a sudden change thermistor sensor is embedded in the stator winding of the motor and connected in series with the relay. When the motor is overloaded, the stator current increases, causing heat generation. When the temperature is greater than the sudden change point, the current in the circuit can suddenly change from a few tenths of a milliamp to tens of milliamps, so the relay operates to achieve overheating protection.

　　4. Liquid level measurement

　　When a certain heating current is applied to the NTC thermistor sensor, its surface temperature will be higher than the surrounding air temperature, and its resistance value will be smaller at this time. When the liquid is higher than its installation height, the liquid will take away its heat, causing its temperature to drop and its resistance to rise.

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