1. Characteristics and functions of thermistors. Thermistors are a type of sensitive element that can be divided into positive temperature coefficient thermistors and negative temperature coefficient thermistors based on different temperature coefficients. The type characteristic of thermistors is their sensitivity to temperature, which displays different resistance values at different temperatures. A nonlinear resistor typically made of semiconductor ceramic materials fired from oxides of manganese, cobalt, and nickel that are extremely sensitive to temperature and have little thermal inertia. Its resistance value changes with temperature. Thermistors are divided into three categories based on temperature coefficient: negative temperature coefficient (NTC), positive temperature coefficient (PTC), and critical temperature coefficient. The resistance value of a positive temperature coefficient resistor increases with increasing temperature, while the resistance value of a negative temperature coefficient resistor decreases with increasing temperature. The resistance value of a critical temperature coefficient resistor is basically zero near the critical temperature. Most thermistors are directly heated, meaning that the heat source is obtained by heating the resistor itself when passing through an electric current. In addition, there is a side heating type that requires an external heating source. Common thermistors include circular, gasket shaped, tube shaped, etc. How should thermistors be detected? At present, the most widely used is the negative temperature coefficient thermistor (NTC), which can be divided into temperature measurement type, voltage stabilization type, and ordinary type. There are many types and shapes of them, with common ones being tubular, circular, and so on. Domestic MTC products include MF51-MF57 (for temperature detection), MF11-MF17 (for temperature compensation and control), MF21-MF22 (for circuit voltage stabilization), MF31 (for microwave power measurement) and other series. The application range of positive temperature coefficient sensitive resistors (PTC) is becoming wider and wider. In addition to temperature control and measurement circuits, they are also widely used in the demagnetization circuit of color televisions and household electrical circuits such as refrigerators, electric mosquito repellents, and electric irons. Domestic PTC products include MZ41-MZ42 (for hair dryers, mosquito repellents, hair curlers, etc.), MZ01-MZ04 (for compressor starting circuit of refrigerators), MZ71-MZ75 (for demagnetization circuit of color televisions), MZ61-MZ63 (for motor overheating protection), MZ2A-MZ2D (for current limiting circuit), and other series. 2. Testing method for thermistors: The nominal resistance value of the thermistor is measured using a specialized instrument at a temperature of 25 C. Under amateur conditions, a multimeter resistance range can also be used for testing, but during multimeter testing, thermal effects may occur due to the high working current, which often results in measured values not matching the nominal resistance value. If only a rough measurement of the resistance value of the thermistor is required to determine its type and whether it can work properly, a multimeter can be used for testing as follows: (1) Room temperature testing. Place the multimeter in the resistance position, with the two probes in contact with the two pins of the thermistor. The reading on the multimeter is the resistance value of the tested thermistor at room temperature. If the reading is zero or infinite with the correct selection of resistance gear, it indicates that the thermistor is damaged. (2) High temperature detection. Using an electric soldering iron as a heat source close to the thermistor, if the resistance value displayed on the multimeter shows a significant change compared to the room temperature resistance value, removing the soldering iron will restore the resistance value to the room temperature resistance value, indicating that the thermistor is good. (3) Low temperature testing. Clamp the two pins of the thermistor with a multimeter and place the thermistor in the refrigerator. Under normal circumstances, the resistance displayed on the multimeter of a thermistor with a negative temperature coefficient significantly increases compared to the resistance value at room temperature; The thermistor value with a positive temperature coefficient is significantly lower than the resistance value displayed at room temperature on the multimeter