When two different conductors and semiconductors A and B form a loop, and the two ends of the loop are connected with each other, as long as the temperatures at the two nodes are different, one end is called working end or hot end, and the other end is called free end or cold end, then a current is generated in the loop, that is, the electromotive force existing in the loop is called thermoelectromotive force. This phenomenon of electromotive force caused by different temperatures is called Seebeck effect. There are two effects related to Zeebek: First, when current flows through the junction of two different conductors, heat is absorbed or released here (depending on the direction of current), which is called Peltier effect; Secondly, when current flows through a conductor with a temperature gradient, the conductor absorbs or releases heat (depending on the direction of current relative to the temperature gradient), which is called Thomson effect. The combination of two different conductors or semiconductors is called a thermocouple.
2. Working principle of resistance sensor
The resistance value of the conductor changes with the change of temperature. By measuring its resistance value, the temperature of the measured object can be calculated. The sensor based on this principle is the resistance temperature sensor, which is mainly used for temperature measurement in the temperature range of -200-500℃. Pure metal is the main manufacturing material of thermal resistance, and the material of thermal resistance should have the following characteristics:
(1), the temperature coefficient of resistance should be large and stable, and there should be a good linear relationship between resistance and temperature.
(2) High resistivity, small heat capacity and fast reaction speed.
(3) The material has good reproducibility and manufacturability, and the price is low.
(4) In the temperature measurement range, the chemical and physical properties are stable.
At present, platinum and copper, which are the most widely used in industry, have been made into standard temperature resistance.
3. Infrared temperature sensor
In nature, when the temperature of an object is higher than absolute zero, due to its internal thermal motion, it will continuously radiate electromagnetic waves to the surrounding area, including infrared rays with the wave band of 0.75 ~100μ m. The infrared temperature sensor is made by using this principle.
4. Digital temperature sensor
It adopts digital temperature sensor produced by silicon technology and PTAT structure, which has good output characteristics of accuracy and temperature dependence. The output of PTAT is modulated into a digital signal by a duty ratio comparator, and the relationship between duty ratio and temperature is as follows: DC=0.32 0.0047*t, where t is Celsius. The output digital signal is compatible with the microprocessor MCU, and the duty cycle of the output voltage square wave signal can be calculated through the high-frequency sampling of the processor, thus the temperature can be obtained. Because of its special technology, the resolution of this temperature sensor is better than 0.005K K.
5. Logic output temperature sensor
Set the temperature range, once the temperature exceeds the specified range, send an alarm signal to start or close the fan, air conditioner, heater or other control equipment. At this point, you can choose the logic output temperature sensor. LM56, MAX650 1-MAX6504, MAX6509/65 10 are its typical representatives.
6. Analog temperature sensor
Common analog temperature sensors are LM39 1 1, LM335, LM45, AD22 103 voltage output type and AD590 current output type.
AD590 is the current output temperature sensor of analog devices. The power supply voltage is 3~30V, the output current is 223μA(-50℃)~423μA( 150℃), and the sensitivity is 1μA/℃. When the sampling resistor R is connected in series in the circuit, the voltage across R can be used as the output voltage. Please note that the resistance of r should not be too large to ensure that the voltage across the AD590 is not less than 3V. The transmission distance of the output current signal of AD590 can reach more than 1km. As a high-resistance current source, it can reach up to 20MΩ, so there is no need to consider the error caused by the extra resistance introduced by the selector switch or CMOS multiplexer. It is suitable for multipoint temperature measurement and remote temperature measurement control. Geshenfuwo has a large number of different related technical materials to consult.
Wireless temperature sensor