Multimeter testing of electrolytic capacitors
The capacity of electrolytic capacitors is much larger than that of general fixed capacitors. When measuring, select an appropriate range for different capacities. Under normal circumstances, capacitance between 1 and 47 pF can be measured with the R×1k block; capacitance greater than 47 ptF can be measured with the R×100 block. The smaller the capacitor capacity, the larger the electrical blocking ratio selection should be. Before measurement, the capacitor should be fully discharged, that is, the two pins of the electrolytic capacitor should be short-circuited to discharge the residual charge in the capacitor. You can use a multimeter probe to short-circuit the two pins of the capacitor. Large-capacity capacitors must be discharged from the metal part using a screwdriver. After the capacitor is fully discharged, connect the red test lead of the analog multimeter to the negative electrode and the black test lead to the positive electrode. At the moment when it is just turned on, the multimeter pointer should deflect to the right at a large angle, and then gradually return to the left until it stops at a certain position. The resistance at this time is the forward insulation resistance of the electrolytic capacitor, which should generally be above several hundred kiloohms. Change the test leads to measure, and the pointer will repeat the previous phenomenon. The last indicated resistance value is the reverse insulation resistance of the capacitor, which should be slightly smaller than the forward insulation resistance.
In the above measurement, if the pointer of the multimeter does not move during the measurement, it means that the capacitor capacity has disappeared or the internal circuit is open; if the forward and reverse insulation resistance of the capacitor is very small or zero, it means that the capacitor has a large leakage current or an internal short circuit and cannot be used anymore. . For electrolytic capacitors with unknown positive and negative poles, the method of measuring insulation resistance can be used to identify them. That is, first touch the two pins of the capacitor with two test leads of a multimeter to measure the insulation resistance of the capacitor. After changing the test leads, measure again. The larger value is the forward insulation resistance. At this time, the black test lead is connected to the positive electrode of the capacitor.
Detection of variable capacitance
The capacity of the variable capacitor is usually small, and it is mainly used to detect whether there is a short circuit between the moving piece and the fixed piece of the capacitor.
① Rotate the shaft slowly by hand. It should feel very smooth and should not be loose or tight or even stuck. When pushing the rotating shaft forward, backward, up, down, left or right, the rotating shaft should not shake.
② A variable capacitor with poor contact between the rotating shaft and the moving piece cannot be used anymore.
③Place the multimeter in the R×10k gear, connect the two test leads to the terminals of the moving piece and the fixed piece of the variable capacitor with one hand, and slowly rotate the shaft back and forth with the other hand. The pointer of the multimeter should be at infinity. Do not move. If the pointer sometimes points to zero, it means there is a short circuit point between the moving piece and the fixed piece of the variable capacitor; if it is rotated to a certain angle, the multimeter reading is not infinite but a finite resistance, it means there is a short circuit between the moving piece and the fixed piece of the variable capacitor. There is a leakage phenomenon between them.
