thermistor

Thermistors are sensitive components made by utilizing the temperature dependent electrical resistivity of semiconductor materials.

1. Composition and Structure of Thermistors Thermistors are mainly composed of thermistor components, leads, and a housing. Their structure and symbols are shown in Figure 6-9. According to different usage requirements, thermistors can be made into different shape structures, and their typical structures are shown in Figure 6-10.diode

2. Types of Thermistors: Semiconductor thermistors can be divided into three types based on their resistance temperature characteristics: positive temperature coefficient thermistors (PTC), negative temperature coefficient thermistors (NTC), and critical temperature LR4091 coefficient thermistors (CTC), as shown in Figure 6-11. As can be seen from the figure, the characteristics of PTC and CTC thermistors with temperature changes are dramatic, so they cannot be used for wide range temperature detection and control. They are suitable for making temperature switches or detection components in a narrow temperature range. The characteristic of NTC thermistor changing with temperature is a gradual variation, suitable for use as a temperature measurement component in a wide temperature range, and is currently the most commonly used thermistor.diode

3. Characteristics of NTC Thermistor Bl-lRr=Roe7% (6-24), where D is the measured temperature (K); T ο is the reference temperature (K); B is the material constant (K) of the thermistor, which can be obtained experimentally, usually B=2000~6000K; Rr is the resistance value (Ω) of the thermistor at temperature Ding (K); R ο is the resistance value (Ω) of the thermistor at temperature T ο (K).

If we define the temperature coefficient of a thermistor as the relative change in resistance value when the temperature changes by 1 ℃, we can obtain the equation (6-24): soil. dRr: B (6-25) RrdTT2. For example, when B: 4000K and D: 293.16K (t=20C), we can obtain a=-4.75%/℃, which is more than 10 times that of a platinum resistor. Due to the large change in resistance caused by temperature changes, the impact of lead resistance is small during measurement, and the volume is small, making it very suitable for measuring weak temperature changes. According to equation (6-25), the mouth increases rapidly with decreasing temperature. Due to the severe nonlinearity of thermistors, they need to be linearized in practical use. diode

A simple method to linearize a thermistor is to connect a fixed resistor with a small temperature coefficient in parallel, so that the relationship between equivalent resistance and temperature is linear within a certain temperature range. The required resistance value R of the fixed resistor can be calculated according to the following formula:

Rr2 (RTl+RT3) -2RT, r3R=- (6-26) RT, in the formula+RT3-2RT2, RT, The resistance value of the thermistor at the lowest temperature within the measurement range; R is the resistance value of the thermistor at the highest temperature T3; The resistance value of the thermistor at the midpoint is T=(T3)/2.