How to use a multimeter to detect capacitance?
Detection of fixed capacitance
(1) Detection of fixed capacitance with a capacity above 0.01 pF Adjust the pointer multimeter to the R×10k ohm block, and perform ohm zero adjustment, then touch the two pins of the capacitor with the red and black pens of the multimeter respectively, and observe the pointer of the multimeter changes, as shown in Figure 1. If the pointer of the multimeter swings slightly to the right when the test lead is connected, and then returns to infinity, after changing the test lead, measure again, and the pointer also swings to the right and returns to infinity, then it can be judged that the capacitance is normal; , the pointer of the multimeter swings to near "0", it can be judged that the capacitor has been broken down or serious leakage; If the pointer does not swing, it can be judged that the capacitor is open.
(2) Detection of fixed capacitors with a capacity less than 0.01 pF When detecting small capacitors below 10pF, because the capacitance is too small, use a multimeter to measure, and you can only check whether there is leakage, internal short circuit or breakdown phenomenon: use when measuring The multimeter is R×10k, and the two test leads are connected to the two pins of the capacitor arbitrarily, and the resistance value should be infinite. If the measured resistance is zero, it can be determined that the capacitor is damaged by leakage or internal breakdown.
(3) To detect 10pF～0.01; tF fixed capacitance can adopt the following method. Adjust the multimeter to the R×10k gear, select two triodes 3DC6 (or 9013) with a value greater than 100 to form a composite tube, and its circuit schematic diagram is shown in Figure 2. Using the amplification effect of the composite tube, the charging current of the measured capacitor is amplified to increase the swing range of the pointer of the multimeter. Connect the capacitor to be measured between the base b and the collector c of the composite tube, and connect the red and black test leads of the multimeter to the emitter e and collector c of the composite tube respectively. If the pointer of the multimeter swings slightly and returns to infinity, the capacitor is normal; if the pointer does not move or cannot return to infinity, the capacitor is damaged. During the test operation, especially when measuring a small capacity capacitor, it is necessary to repeatedly exchange the two contact points of the measured capacitor pin to clearly see the swing of the pointer of the multimeter.
Detection of electrolytic capacitors
The capacity of electrolytic capacitors is much larger than that of ordinary fixed capacitors. When measuring, choose a suitable range for different capacities. Under normal circumstances, the capacitance between 1 and 47 pF can be measured with R×1k gear; the capacitance greater than 47 ptF can be measured with R×100 gear. The smaller the capacity of the capacitor, the larger the selection of the blocking factor should be. The capacitor should be fully discharged before measurement, that is, the two pins of the electrolytic capacitor should be short-circuited to discharge the residual charge in the capacitor. The two pins of the capacitor can be short-circuited with a multimeter pen, and the schematic diagram of the capacitor discharge method is shown in Figure 3. Bulk capacitors must be discharged from the metal part of a screwdriver. After the capacitor is fully discharged, connect the red test lead of the pointer multimeter to the negative pole, and the black test lead to the positive pole. At the moment when it is first turned on, the pointer of the multimeter should deflect to the right by a large angle, and then gradually return to the left until it stops at a certain position. The resistance value at this time is the forward insulation resistance of the electrolytic capacitor, which should generally be above several hundred thousand ohms. Change the test leads to measure, the pointer repeats the previous phenomenon, and the last indicated resistance value is the reverse insulation resistance of the capacitor, which should be slightly smaller than the forward insulation resistance.
Detection of Variable Capacitors
The capacity of the variable capacitor is usually small, mainly to detect whether there is a short circuit between the capacitive moving piece and the fixed piece.
① Slowly rotate the shaft by hand, it should feel very smooth, and it should not be loose or tight or even stuck. When the rotating shaft is pushed forward, backward, up, down, left and right, the rotating shaft should not shake.
②Variable capacitors with poor contact between the shaft and the moving piece cannot be used any longer.
③ Put the multimeter in the R×10k gear, connect the two test leads to the leading end of the moving piece and the fixed piece of the variable capacitor with one hand, and slowly turn the rotating 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 that there is a short circuit point between the variable capacitance moving piece and the fixed piece; There is a leakage phenomenon.
