Working Principle of Temperature Sensor

Temperature transducer, using the law of the change of various physical properties of materials with temperature to convert temperature into a usable output signal. Temperature sensor is the core part of temperature measuring instrument, and there are many varieties. According to the measurement method, it can be divided into two categories: contact type and non-contact type. According to the characteristics of sensor materials and electronic components, it can be divided into thermal resistors and thermocouples. Modern temperature sensors are very small in size, which makes them more widely used in various fields of production practice, and also provides countless conveniences and functions for our lives.

There are four main types of temperature sensors: thermocouples, thermistors, resistance temperature detectors (RTDs), and IC temperature sensors. IC temperature sensors include analog output and digital output types.

1. Working principle of thermocouple  
When two different conductors and semiconductors A and B form a loop and their two ends are connected to each other, as long as the temperature at the two junctions is different, one end has a temperature of T, called the working end or hot end, and the other end has a temperature of TO, called the free end (also called the reference end) or cold end, then current will be generated in the loop, as shown in Figure 2-1 (a), that is, the electromotive force in the loop is called thermoelectric electromotive force. This phenomenon of electromotive force generated due to different temperatures is called the Seebeck effect. There are two effects related to Seebeck: First, when current flows through the connection between two different conductors, heat is absorbed or released here (depending on the direction of the current), which is called the Peltier effect; second, when current flows through a conductor with a temperature gradient, the conductor absorbs or releases heat (depending on the direction of the current relative to the temperature gradient), which is called the Thomson effect. The combination of two different conductors or semiconductors is called a thermocouple. The thermoelectric potential EAB (T, T0) of a thermocouple is synthesized by the contact potential and the temperature difference potential. Contact potential refers to the potential generated by two different conductors or semiconductors at the contact point. This potential is related to the properties of the two conductors or semiconductors and the temperature at the contact point. Thermoelectric potential refers to the potential generated by the same conductor or semiconductor at two different temperatures. This potential is only related to the properties of the conductor or semiconductor and the temperature at the two ends, and has nothing to do with the length, cross-sectional size, and temperature distribution along the length of the conductor. Both contact potential and thermoelectric potential are potentials generated by the different number of electrons concentrated at the end points of the contact. The thermoelectric potential measured by thermocouples is the synthesis of the two. When the circuit is disconnected, there is an electromotive force difference △V between the disconnection points a and b, and its polarity and magnitude are consistent with the thermoelectric potential in the circuit, as shown in Figure 2-1 (b). It is also stipulated that at the cold end, when the current flows from A to B, A is called the positive electrode and B is called the negative electrode. Experiments show that when △V is very small, △V is proportional to △T. The differential thermoelectric potential of △V to △T is defined as the thermoelectric potential rate, also known as the Seebeck coefficient. The sign and size of the Seebeck coefficient depend on the thermoelectric properties of the two conductors that make up the thermocouple and the temperature difference of the junction.

Currently, the International Electrotechnical Commission (IEC) recommends 8 types of thermocouples as standardized thermocouples, namely T-type, E-type, J-type, K-type, N-type, B-type, R-type and S-type.

2. Working principle of  

thermal resistors : The resistance value of a conductor changes with temperature. The temperature of the object being measured can be inferred by measuring its resistance value. The sensor constructed using this principle is a resistance temperature sensor, which is mainly used for temperature measurement in the temperature range of -200-500°C. Pure metal is the main manufacturing material of thermal resistors. The material of thermal resistors should have the following characteristics: ① The resistance temperature coefficient should be large and stable, and there should be a good linear relationship between the resistance value and the temperature.  
② High resistivity, small heat capacity, and fast reaction speed.  
③ The material has good reproducibility and processability, and the price is low.  
④ The chemical and physical properties are stable within the temperature measurement range.  
At present, platinum and copper are the most widely used in industry, and have been made into standard temperature measuring thermal resistors.

3. Infrared temperature sensor

In nature, when the temperature of an object is higher than absolute zero, due to the existence of internal thermal motion, it will continuously radiate electromagnetic waves to the surroundings, including infrared rays with a wavelength of 0.75 to 100 μm. Infrared temperature sensors are made using this principle.
SMTIR9901/02 is an infrared sensor produced by Smartec Company of the Netherlands, which is widely used in the market. It is a silicon-based infrared sensor based on thermopile. A large number of thermocouples are stacked on the bottom silicon base. The high-temperature and low-temperature contacts on the bottom are isolated from their heat by a very thin film. The black absorption layer on the high-temperature contact converts the incident radiation into heat energy. From the thermoelectric effect, it can be seen that the output voltage is proportional to the radiation. Usually, the thermopile uses BiSb and NiCr as thermocouples. In addition, SMT9902sil is embedded with a Ni1000 temperature sensor and a small-angle silicon filter to make the temperature measurement more accurate. Because the infrared radiation characteristics are related to temperature, different filters can be used to measure different temperature ranges. Mature semiconductor technology enables product miniaturization and low cost. To meet certain applications, the infrared sensor opening angle can be designed to be as small as 7°.

4. Analog temperature sensor

Common analog temperature sensors include LM3911, LM335, LM45, AD22103 voltage output type, and AD590 current output type.
AD590 is a current output temperature sensor from Analog Devices, Inc., USA. The supply voltage range 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. Note that the resistance value of R cannot be too large to ensure that the voltage across 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 it does not need to consider the error caused by the additional resistance introduced by the selection switch or CMOS multiplexer. It is suitable for multi-point temperature measurement and long-distance temperature measurement control.

5. The logic output temperature sensor

sets a temperature range. Once the temperature exceeds the specified range, an alarm signal is issued to start or shut down the fan, air conditioner, heater or other control equipment. At this time, a logic output temperature sensor can be selected. LM56, MAX6501-MAX6504, MAX6509/6510 are typical representatives.
LM56 is a high-precision low-voltage temperature switch produced by NS, with a built-in 1.25V reference voltage output terminal. It can only carry a maximum load of 50μA. The power supply voltage is from 2.7~10V, the maximum operating current is 230μA, the sensitivity of the built-in sensor is 6.2mV/℃, and the sensor output voltage is 6.2mV/℃×T+395mV.

6. Digital temperature sensor

It is a digital temperature sensor produced by silicon technology. It adopts PTAT structure. This semiconductor structure has accurate and good output characteristics related to temperature. The output of PTAT is modulated into a digital signal by a duty cycle comparator. The relationship between duty cycle and temperature is as follows: DC=0.32+0.0047*t, t is degrees Celsius. The output digital signal is compatible with the microprocessor MCU. The duty cycle of the output voltage square wave signal can be calculated through the high-frequency sampling of the processor to obtain the temperature. Due to its special process, the resolution of this temperature sensor is better than 0.005K. The measurement temperature range is -45 to 130℃, so it is widely used in high-precision occasions.