Capacitor testing methods and experience

1. Detection of fixed capacitors

A. Detect small capacitance below 10pF

Because the capacity of the fixed capacitor below 10pF is too small, use a multimeter to measure, you can only qualitatively check whether there is leakage, internal short circuit or breakdown. When measuring, the multimeter R×10k block can be selected, and the two pins of the capacitor can be arbitrarily connected with two test leads, and the resistance value should be infinite. If the measured resistance value (the pointer swings to the right) is zero, it indicates that the capacitor leakage damage or internal breakdown.

B. Detect whether the 10PF～001μF fixed capacitor is charged, and then judge whether it is good or bad. Multimeter selects R×1k block. Both betas of the two transistors are above 100, and the penetration current is small. 3DG6 and other silicon transistors can be used to form a composite tube. The red and black test leads 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 amplitude of the multimeter pointer swing is increased, which is convenient for observation. It should be noted that: during the test operation, especially when measuring a capacitor with a smaller capacity, the pins of the capacitor under test must be replaced with the two points A and B repeatedly in order to clearly see the swing of the multimeter pointer.

C. For the fixed capacitance above 001μF, the multimeter's R×10k block can be used to directly test whether the capacitor is charging 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 1~47μF can be measured with R×1k block, and the capacitance greater than 47μF can be measured with R×100 block.

B. Connect the red lead of the multimeter to the negative pole and the black lead to the positive pole. At the moment of contact, the pointer of the multimeter deflects to the right by a larger deviation (for the same electrical block, the larger the capacity, the larger the swing amplitude), and then gradually to the left Turn around 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 electrolytic capacitors should generally be more than a few hundred kΩ, otherwise, it will not work properly. In the test, if there is no charging phenomenon in the forward direction and the reverse direction, that is, the hands are not moving, it means that the capacity disappears or the internal circuit is broken. If the measured resistance value is very small or zero, it means that the capacitor has large leakage or breakdown damage. Can no longer be used.

C. For electrolytic capacitors with unclear positive and negative signs, the above method for measuring leakage resistance can be used to judge. That is, measure the leakage resistance arbitrarily, remember its size, and then exchange the test leads to measure a resistance. The one with the largest resistance in the two measurements is the positive connection method, that is, the black test lead is connected to the positive electrode, and the red test lead is connected to the negative electrode.

D. Use a multimeter to block electricity, and use positive and reverse charging methods to the electrolytic capacitor. According to the magnitude of the swing of the pointer to the right, the capacity of the electrolytic capacitor can be estimated.

3. Detection of variable capacitors

A. Rotate the shaft lightly by hand, it should feel very smooth, and sometimes should not feel tight or even stuck. When the carrier shaft is pushed forward, backward, up, down, left, right and other directions, the shaft should not be loose.

B. Rotate the shaft with one hand and touch the outer edge of the moving piece group with the other hand. There should not be any looseness. The variable capacitor with poor contact between the rotating shaft and the moving piece can no longer be used.

C. Put the multimeter in the R×10k block, 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 rotate the shaft a few times back and forth with the other hand. It should not move at the infinite position. 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 piece and the fixed piece; if it encounters an angle, the multimeter reading is not infinite but a certain resistance value appears, indicating that the variable capacitor is moving. There is a leakage phenomenon between the piece and the fixed piece.