Capacitor testing methods and experience

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Capacitor testing methods and experience [Copy link]

1? Detection of fixed capacitors

A? Detection of small capacitors below 10pF

Because the capacity of fixed capacitors below 10pF is too small, using a multimeter to measure can only qualitatively check whether it has leakage, internal short circuit or breakdown. When measuring, you can choose the multimeter R×10k block, and use two test pens to connect the two pins of the capacitor at will, and the resistance should be infinite. If the measured resistance value (the pointer swings to the right) is zero, it means that the capacitor is leaking or damaged or internally broken down. B? Detect whether the 10PF~0?01μF fixed capacitor has charging phenomenon, and then judge whether it is good or bad. The multimeter uses the R×1k block. The β values of the two transistors are both above 100, and the penetration current should be small. 3DG6 and other types of silicon transistors can be used to form a composite tube. The red and black test pens of the multimeter are connected to the emitter e and collector c of the composite tube respectively. Due to the amplification effect of the composite transistor, the charging and discharging process of the measured capacitor is amplified, so that the swing amplitude of the multimeter pointer is increased, which is convenient for observation. It should be noted that: during the test operation, especially when measuring capacitors with smaller capacity, the pins of the capacitor to be tested should be repeatedly switched to contact points A and B, so that the swing of the multimeter pointer can be clearly seen. C? For fixed capacitors above 0?01μF, the R×10k block of the multimeter can be used to directly test whether the capacitor has a charging process and whether there is an internal short circuit or leakage, and the capacity of the capacitor can be estimated according to the amplitude of the pointer swinging to the right.

2? Detection of electrolytic capacitors

A? Because the capacity of electrolytic capacitors is much larger than that of general fixed capacitors, when measuring, the appropriate range should be selected for different capacities. According to experience, under normal circumstances, the capacitance between 1 and 47μF can be measured with the R×1k block, and the capacitance greater than 47μF can be measured with the R×100 block.

B? Connect the red probe of the multimeter to the negative pole and the black probe to the positive pole. At the moment of contact, the multimeter pointer deflects to the right with a larger deviation (for the same resistance block, the larger the capacity, the larger the swing), and then gradually turns back to the left until it stops at a certain position. The resistance at this time is the forward leakage resistance of the electrolytic capacitor, which is slightly larger than the reverse leakage resistance. Practical experience shows that the leakage resistance of the electrolytic capacitor should generally be above several hundred kΩ, otherwise it will not work properly. In the test, if there is no charging phenomenon in both the forward and reverse directions, that is, the needle does not move, it means that the capacity has disappeared or the internal circuit is broken; if the measured resistance is very small or zero, it means that the capacitor has a large leakage or has been broken down and damaged and cannot be used again. C? For electrolytic capacitors with unclear positive and negative pole markings, the above-mentioned method of measuring leakage resistance can be used to distinguish them. That is, first measure the leakage resistance at random, remember its size, and then exchange the test leads to measure another resistance value. The one with the larger resistance value in the two measurements 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? Using the resistance block of the multimeter, the method of charging the electrolytic capacitor in the forward and reverse directions can be used to estimate the capacity of the electrolytic capacitor according to the amplitude of the pointer swinging to the right.

3. Detection of variable capacitors

A. Gently rotate the shaft by hand, it should feel very smooth, and should not feel sometimes loose, sometimes tight, or even stuck. When the carrier shaft is pushed forward, backward, up, down, left, right, etc., the shaft should not be loose. B. Rotate the shaft with one hand, and touch the outer edge of the moving plate group with the other hand. You should not feel any looseness. Variable capacitors with poor contact between the shaft and the moving plate cannot be used anymore. C. Set the multimeter to the R×10k position, connect the two test leads to the moving plate and the fixed plate 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 the infinite position and not move. In the process of rotating the shaft, if the pointer sometimes points to zero, it means that there is a short circuit point between the moving plate and the fixed plate; if it touches a certain angle, the multimeter reading is not infinite but a certain resistance value, indicating that there is leakage between the moving plate and the fixed plate of the variable capacitor.