1. The characteristics and functions of thermistor

Thermistor is usually a non-linear resistor made of a semiconductor ceramic material fired with oxides of manganese, cobalt, and nickel, which is extremely temperature-sensitive and thermally inert, and its resistance changes with temperature. Thermistors are divided into three categories: negative temperature coefficient (NTC), positive temperature coefficient (PTC) and critical temperature coefficient according to their temperature coefficient. The resistance value of the positive temperature coefficient resistor increases with the increase of temperature, the resistance value of the negative temperature coefficient resistor decreases with the increase of temperature, and the resistance value of the critical temperature coefficient resistor is basically zero when it is near the critical temperature.

Most thermistors are of direct heating type, that is, the heat source is obtained by heating when the resistor itself passes current. In addition, there is an indirect heating type, which requires an external heating source. Common thermistors are round, washer-shaped, tube-shaped, etc.

At present, the negative temperature coefficient thermistor (NTC) is widely used, which can be divided into temperature measurement type, voltage stabilization type, and ordinary type. There are many types and different shapes, the common ones are tubular, disc shape and so on. Domestic MTC products include MF51~MF57 (used for temperature detection), MF11~MF17 (used for temperature compensation and temperature control), MF21~ MF22 (used for circuit voltage stabilization), MF31 (used for microwave power measurement) and other series.

The application range of positive temperature coefficient resistors (PTC) is becoming wider and wider. In addition to temperature control and temperature measurement circuits, they are also widely used in degaussing circuits of color TV sets, refrigerators, electric mosquito repellents, electric irons, etc. In the circuit of household appliances. Domestic PTC products include MZ41~MZ42 (used in hair dryers, mosquito repellents, curling irons, etc.), MZ01~MZ04 (used in the compressor starting circuit of refrigerators), MZ71~MZ75 (used in the degaussing circuit of color TVs), MZ61~MZ63 (for motor overheating protection), MZ2A~MZ2D (for current limiting circuit) and other series.

2. Detection method of thermistor

The nominal resistance of the thermistor is measured with a special instrument at a temperature of 25 C. Under amateur conditions, the multimeter can also be used for detection. However, due to the large working current during the multimeter detection, a thermal effect is formed, which often makes the measured value inconsistent with the nominal resistance value. If only rough measurement of the thermistor's resistance is required to determine its type and whether it can work normally, a multimeter can be used to detect it according to the following methods:

(1) Detection at room temperature. Set the multimeter to the electric block, and the two test leads touch the two pins of the thermistor. The reading of the multimeter is the resistance value of the thermistor under test at room temperature. Under the premise of correct selection of electrical barriers, if the reading is zero or infinity, it indicates that the thermistor has been damaged.

(2) High temperature detection. Use the electric soldering iron as a heat source close to the thermistor. If the resistance displayed by the multimeter changes significantly compared to the normal temperature resistance value, remove the electric soldering iron and the resistance value will return to the normal temperature resistance value, indicating that the thermistor is good.

(3) Low temperature detection. Clamp the two pins of the thermistor with a multimeter clamp, and put the thermistor into the refrigerator. Normally, the resistance value of the thermistor with negative temperature coefficient, the resistance value displayed by the multimeter is significantly larger than the resistance value at room temperature; the resistance value of the thermistor with positive temperature coefficient, the resistance value displayed by the multimeter is significantly lower than the resistance value at room temperature.

3. Precautions for testing

(1) When the body temperature is higher than the ambient temperature, pinch the thermistor with your hand, and you should be able to observe the change in the resistance reading.

(2) Each test of PTC thermistor should be carried out after its temperature drops to room temperature.