Thermistor is a kind of semiconductor resistor, and its resistance value is very sensitive to temperature changes. It has many unique advantages and uses and is widely used in automatic control, wireless electronic technology, remote control technology and temperature measurement technology. In this experiment, the resistance-temperature characteristics of thermistor are studied by bridge method, which deepens the understanding of resistance-temperature characteristics of thermistor. Keywords: thermistor, unbalanced DC bridge, resistance-temperature characteristic 1, brief introduction Thermistor is a device made according to the strong dependence of the conductivity of semiconductor materials on temperature, and its resistance-temperature coefficient is generally (-0.003~+0.6)℃- 1. Therefore, thermistors can be generally divided into: Class I.. Resistor negative temperature coefficient (NTC) is often made of semiconductor metal oxides formed by some transition metal oxides (mainly oxides of copper, nickel, cobalt and cadmium) under certain sintering conditions, and in recent years, it is also made of single crystal semiconductors and other materials. At home, it mainly refers to MF9 1~MF96 semiconductor thermistor. Because the transition metal oxides that make up this kind of thermistor are basically ionized at room temperature, that is, the carrier concentration is basically independent of temperature, the relationship between mobility and temperature is mainly considered when the resistivity of this kind of thermistor changes with temperature. With the increase of temperature, the mobility increases and the resistivity decreases. Most of them are used in temperature measurement and temperature control technology, and can also be made into flowmeter and power meter. ⅱ. Thermistor elements with positive temperature coefficient of resistance (PTC for short) are usually made of barium titanate materials containing a small amount of rare earth elements such as titanium and barium by ceramic technology and high temperature firing. The change of resistivity of this kind of thermistor with temperature mainly depends on the carrier concentration, while the change of mobility with temperature is relatively negligible. The number of carriers increases exponentially with the increase of temperature, and the more carriers, the smaller the resistivity. It is widely used, except for temperature measurement, temperature control and temperature compensation in electronic circuits, and can also be made into various heaters, such as hair dryers. 2. Experimental device and principle experimental device: FQJ-II unbalanced DC bridge for teaching, FQJ unbalanced bridge heating experimental device (MF5 1 semiconductor thermistor (2.7kΩ) and temperature sensor for temperature control are built in the heating furnace), and several connecting wires. Experimental principle According to the semiconductor theory, the relationship between the resistivity of general semiconductor materials and the absolute temperature is (1-1), where A and B are constants of the same semiconductor material, and their values are related to the physical properties of the material. Therefore, according to the law of resistance, the resistance value of thermistor can be written as (1-2), where the distance between two electrodes is the cross section of thermistor. For a specific resistance, and b are constants, which can be determined by experimental methods. For the convenience of data processing, if both sides of the above formula are logarithmic, the above formula indicates that there is a linear relationship with (1-3). In the experiment, as long as the values of various temperatures and corresponding resistances are measured and plotted as abscissa and ordinate, the obtained graph should be straight, and the values of parameters A and B can be obtained by graphic method, calculation method or least square method. The temperature coefficient of resistance of thermistor is given by the following formula (1-4). Substituting the B value obtained by the above method and the room temperature into the formula (1-4), the temperature coefficient of resistance at room temperature can be calculated. The resistance of thermistor at different temperatures can be measured by unbalanced DC bridge. The diagram on the right shows the schematic diagram of unbalanced DC bridge. There is a load resistance between B and D. You can get the value by measuring it. When the load resistance→, that is, the bridge output is in an open circuit state, =0, at this time, the bridge output =0 means that only the voltage is output, that is, the bridge is in a balanced state. For the accuracy of measurement, the bridge must be pre-balanced before measurement, so that the output voltage is only related to the resistance change of a certain arm. If R 1, R2, R3 are fixed, R4 is the resistance to be measured, and R4 = RX, then when R4→R4+△R, the output voltage due to bridge imbalance is: (1-5) When measuring MF5 1 thermistor, the unbalanced DC bridge is a vertical bridge, then, 3. Study on Resistance-temperature Characteristics of Thermistor According to the resistance-temperature characteristics of MF5 1 semiconductor thermistor (2.7kΩ) in Table1,the bridge circuit is studied, and the value of R sum of each arm resistance is designed to ensure that the voltage output will not overflow (this experiment =1000.0Ω, = 4323.0Ω). According to the type of bridge, preset the balance, turn the "function switch" to the "voltage" position, press the G and B switches, turn on the experimental heating device to raise the temperature, measure the value of 1 every 2℃, and list the measurement data (Table 2). Table 1 MF 5 1 resistance temperature characteristic temperature of semiconductor thermistor (2.7kΩ)℃ 25 30 35 40 45 50 55 60 5 resistance Ω 2700 225187013411. 868 748 Table II Unbalanced Bridge Voltage Output Form (Vertical) MF5 1 Thermistor i 1 2 3 4 5 6 7 8 9 10 Temperature T℃10.412.414.4/Kloc. 6.4 65438+22.4 24.4 26.4 28.4 Thermodynamics TK2836-107.8-126.4-144.4 0-259.2-529.9-789-100. .9-1630.1-1The linear equation calculated by the least square method is the mathematical expression of the resistance-temperature characteristic of MF5 1 semiconductor thermistor (2.7kΩ). 4. Error of experimental results The mathematical expression of resistance-temperature characteristics of MF5 1 semiconductor thermistor obtained through experiments is as follows. According to the obtained expression, the measured value of resistance-temperature characteristics of thermistor is calculated, which is in good agreement with the reference value given in table 1 As shown in the following table: Table 3 Comparison temperature℃ 25 30 35 45 45 50 55 60 65 Reference value RT ω 2700 2225187015731341160. ω 2720 223819001587140812321074 939 823 Relative error% 0.740.581.600 0.894. We can clearly find that with the increase of temperature, 5. The influence of internal thermal effect During the experiment, because there is always a certain working current when measuring thermistor with unbalanced bridge, the thermistor has large resistance, small volume and small heat capacity, so Joule heat will quickly make the thermistor produce a stable additional internal thermal temperature rise higher than the external temperature, which is called internal thermal effect. When accurately measuring the temperature characteristics of thermistor, the influence of internal thermal effect must be considered. This experiment will not be further studied and discussed. 6. Experimental summary Through experiments, we can clearly find that the resistance of thermistor is very sensitive to the change of temperature, and its resistance decreases exponentially with the increase of temperature. Therefore, using the resistance-temperature characteristics, various sensors can be made, which can transform the tiny temperature change into resistance change and form a large signal output, especially suitable for high-precision measurement. Due to the small size of the device and the wide selection range of shapes and packaging materials, it is especially suitable for temperature and humidity sensors in high temperature, high humidity, vibration and thermal shock environments, and can be applied to various production operations, with great development potential. References: [1] Zhu Jiangfeng, Lu Lijuan, Lu Xiaodong. College Physics Experiment [M] [2] Yang, Yang Jiexin, General Physics Experiment (Part II: Electromagnetism) [M] Beijing: Higher Education Press [3] College Physics Experiment Writing Group. College Physics Experiment [M] Xiamen: Xiamen University Press [4] Lu Shenlong, Experimental Teaching of Resistance-temperature Characteristics of Thermistors [J]