How to test thermistor with multimeter?
Thermistors are frequently used in current electrical appliances. They produce changes in resistance value through changes in ambient temperature, thereby changing the working state of the circuit. They are widely used in temperature sensors and control systems.
Thermistors can be divided into positive temperature coefficient and negative temperature coefficient according to the relationship between their resistance value and temperature change. The so-called positive temperature coefficient means that the resistance value of the thermistor decreases as the ambient temperature increases.
The nominal resistance value of the thermistor refers to the resistance value when the environment is at 25°C. Therefore, when measuring the resistance value of the thermistor, you need to pay attention to the influence of the ambient temperature on its resistance value. When the ambient temperature is 25°C, the resistance value of the thermistor measured by the multimeter is its nominal resistance value. If the ambient temperature is not 25°C, the measured resistance value does not match the nominal resistance value of the thermistor. normal phenomenon.
If you need to test and determine whether the thermistor has a positive or negative temperature coefficient, you can heat the area around the thermistor when testing the thermistor, such as using a soldering iron close to the thermistor. If the measured resistance value increases , which is the positive temperature coefficient thermistor. On the contrary, it is a negative temperature coefficient thermistor.
How to use a multimeter to judge the quality of capacitors?
Depending on the capacity of the electrolytic capacitor, the R×10, R×100, R×1 K range of the multimeter is usually used for testing and judgment. The red and black test leads are connected to the positive and negative poles of the capacitor respectively (the capacitor needs to be discharged before each test), and the quality of the capacitor can be judged by the deflection of the needle. If the hands of the watch quickly swing to the right and then slowly return to the original position to the left, generally speaking, the capacitor is good. If the hands of the watch do not rotate after swinging up, it means that the capacitor has broken down. If the hands of the watch gradually return to a certain position after swinging up, it means that the capacitor has leaked electricity. If the hands of the watch cannot move up, it means that the electrolyte of the capacitor has dried up and lost its capacity.
It is difficult to accurately judge the quality of leakage capacitors using the above method. When the withstand voltage value of the capacitor is greater than the battery voltage value in the multimeter, the leakage current of the electrolytic capacitor is small when charged forward and large when charged reversely. The R×10 K block can be used to reversely charge the capacitor. Observe whether the point where the needle stays is stable (that is, whether the reverse leakage current is constant), and judge the quality of the capacitor with high accuracy. The black test lead is connected to the negative pole of the capacitor, and the red test lead is connected to the positive pole of the capacitor. If the needle of the meter swings up quickly, and then gradually retreats to a certain place and stays still, it means that the capacitor is good. Whenever the needle of the meter stays unstable at a certain position or gradually stops after staying, it means that the capacitor is good. The capacitor moving slowly to the right has leaked electricity and cannot be used anymore. The hands of the watch generally stay and stabilize within the scale range of 50 to 200 K.
