How to test the thermistor with a multimeter?
Thermistors are frequently used in current electrical appliances. It changes the resistance value through the change of ambient temperature, thereby changing the working state of the circuit. It is widely used in temperature sensors and control systems.
According to the relationship between its resistance value and temperature change, thermistors can be divided into positive temperature coefficient and negative temperature coefficient. The so-called positive temperature coefficient means that the resistance value of the thermistor decreases with the increase of the ambient temperature.
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, it is necessary 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 it is necessary to detect and judge whether the thermistor is a positive temperature coefficient or a negative temperature coefficient, you can heat the surrounding of the thermistor when detecting the thermistor, such as using an electric soldering iron close to the thermistor, if the measured resistance value increases at this time , which is a 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 respectively connected to the positive and negative poles of the capacitor (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 swing to the right quickly, and then slowly return to the original position to the left, the capacitor is generally good. If the hands of the watch do not turn back after swinging, 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. If the hands of the watch cannot move up, it means that the electrolyte of the capacitor has dried up and lost its capacity.
Leakage of capacitors, it is not easy to accurately judge good or bad by the above method. When the withstand voltage value of the capacitor is greater than the voltage value of the battery in the multimeter, according to the characteristic that the leakage current of the electrolytic capacitor is small when it is charged forward, and the leakage current is large when it is charged reversely, you can use the R×10 K gear to reversely charge the capacitor. Observe whether the stop position of the watch needle is stable (that is, whether the reverse leakage current is constant), so as to 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. The hands swing up quickly, and then gradually retreat to a certain place to stay still, indicating that the capacitor is good. The capacitor that moves slowly to the right has leaked and cannot be used any longer. The hands of the watch generally stay and stabilize within the scale range of 50-200 K.
