Temperature sensor: application and principle of temperature sensor

　Temperature sensor:The application range of temperature measurement is very wide, not only the production process needs temperature control, some electronic products also need to measure their own temperature, such as the computer to monitor the temperature of the CPU, the motor controller to know the temperature of the power drive IC, etc., the following is some introduction. Commonly used temperature sensor.
Temperature is a parameter that often needs to be tested in practical applications. From steel manufacturing to semiconductor production, many processes must be achieved by temperature. The temperature sensor is the bridge between the application system and the real world. This article provides a brief overview of the different temperature sensors and describes the interface to the circuit system.
Thermistor
There are many types of sensors that can be used to measure temperature, and thermistors are one of them. Many thermistors have a negative temperature coefficient (NTC), which means that their resistance will increase as the temperature drops. Among all passive temperature sensors, thermistors have high sensitivity (I. e., the change in resistance when the temperature changes by 1 degree), but the resistance/temperature curve of the thermistor is non-linear.
Thermistors typically have an error range to specify consistency between samples. Depending on the material used, the error value is usually between 1% and 10%. Some thermistors are designed to be interchangeable and used in situations where field adjustment is not possible. For example, for an instrument, the user or field engineer can only replace the thermistor, but cannot perform calibration. The thermistor is more accurate than ordinary thermistors, and its price is higher and the price is higher.
self-heating problem
Since the thermistor is a resistor, a certain amount of heat will be generated when the current flows through, so the circuit designer should ensure that the pull-up resistor is large enough to prevent the thermistor from overheating, otherwise the system will measure the thermistor. heat, not the temperature of the surrounding environment.
The effect of the energy consumed by the thermistor on temperature is expressed by the dissipation constant, which refers to the number of milliwatts required to make the temperature of the thermistor 1°C higher than the ambient temperature. The dissipation constant varies with the thermistor's package, pin specification, package material, and other factors.
The allowable self-heating and current limiting resistance of the system depends on the measurement accuracy. Compared with a measurement system with an accuracy of ± 1°C, a measurement system with a measurement accuracy of ± 5°C can withstand greater thermistor self-heating.
It should be noted that the resistance value of the pull-up resistor must be calculated to limit the self-heating power consumption in the entire measurement temperature range. After the resistance value is given, due to the change of the resistance value of the thermistor, the power consumption at different temperatures is also different.
It is sometimes necessary to calibrate the thermistor input to obtain proper temperature resolution.
Thermocouple
Thermocouples consist of two different metals. A tiny voltage is generated when heated. The voltage depends on the two metallic materials that make up the thermocouple, iron-constant (type J) and copper-constant (type T), while chromium-aluminum (type K) thermocouples are the three commonly used. The voltage generated by the thermocouple is very small, usually only a few millivolts. When the temperature of the K-type thermocouple changes by 1°C, the voltage change is only 40 μV, so the measurement system must be able to measure a voltage change of 4 μV before it can achieve a measurement accuracy of 0.1°C.