Detection of electrolytic capacitors with a multimeter
A. Since the capacitance of an electrolytic capacitor is much larger than that of a general fixed capacitor, when measuring, an appropriate range should be selected according to the different capacitances. According to experience, generally, for capacitors between 1 and 47μF, the R×1k gear can be used for measurement, and for capacitors larger than 47μF, the R×100 gear can be used for measurement.
B. Connect the red test lead of the multimeter to the negative pole and the black test lead to the positive pole. At the moment of contact, the pointer of the multimeter deflects to the right by a large degree (for the same resistance gear, the larger the capacitance, the greater the swing amplitude), and then gradually turns back to the left until it stops at a certain position. The resistance value at this time is the forward leakage resistance of the electrolytic capacitor, and this value is slightly larger than the reverse leakage resistance. Practical experience shows that the leakage resistance of an electrolytic capacitor should generally be above several hundred kΩ, otherwise, it cannot work properly. During the test, if there is no charging phenomenon in both the forward and reverse directions, that is, the pointer of the meter does not move, it indicates that the capacitance has disappeared or there is an internal open circuit; if the measured resistance value is very small or zero, it indicates that the capacitor has a large leakage current or has been broken down and damaged and cannot be used any more.
C. For electrolytic capacitors with unclear positive and negative pole markings, the method of measuring the leakage resistance mentioned above can be used for discrimination. That is, first measure the leakage resistance arbitrarily, remember its size, and then exchange the test leads and measure another resistance value. In the two measurements, the measurement with the larger resistance value is the forward connection method, that is, the black test lead is connected to the positive pole and the red test lead is connected to the negative pole.
D. Use the resistance gear of the multimeter and adopt the method of charging the electrolytic capacitor in both the forward and reverse directions. According to the size of the pointer's swing to the right, the capacitance of the electrolytic capacitor can be estimated.
Detection of variable capacitors
A. Gently rotate the rotating shaft by hand, and it should feel very smooth. There should be no feeling of looseness and tightness or even a jamming phenomenon. When the rotating shaft is pushed forward, backward, upward, downward, leftward, and rightward, the rotating shaft should not have a loose phenomenon.
B. Rotate the rotating shaft with one hand and gently touch the outer edge of the moving plate group with the other hand. There should be no feeling of any looseness. A variable capacitor with poor contact between the rotating shaft and the moving plate cannot be used any more.
C. Set the multimeter to the R×10k gear. With one hand, connect the two test leads to the leads of the moving plate and the fixed plate of the variable capacitor respectively. With the other hand, slowly rotate the rotating shaft back and forth several times, and the pointer of the multimeter should not move at the infinite position. During the process of rotating the rotating shaft, if the pointer sometimes points to zero, it indicates that there is a short-circuit point between the moving plate and the fixed plate; if at a certain angle, the reading of the multimeter is not infinite but shows a certain resistance value, it indicates that there is a leakage phenomenon between the moving plate and the fixed plate of the variable capacitor.
