1. Detection of fixed capacitors.
A. Detecting small capacitors below 10pF Because the fixed capacitor capacity below 10pF is too small, use a multimeter to measure, only qualitatively check for leakage, internal short circuit or breakdown. When measuring, you can use the multimeter R×10k block, and connect the two pins of the capacitor with two test leads. The resistance should be infinite. If the measured resistance value (the pointer swings to the right) is zero, the capacitor leakage is damaged or internal breakdown.
B. Detect whether the 10PF~001μF fixed capacitor has a charging phenomenon, and then judge whether it is good or bad. The multimeter uses R × 1k block. The beta values of the two triodes are all above 100 and the penetration current is small. A composite tube of 3DG6 and other types of silicon triodes can be selected. The red and black test leads of the multimeter are respectively connected to the emitter e and the collector c of the composite tube. Due to the amplification effect of the composite triode, the charging and discharging process of the measured capacitor is enlarged, so that the multimeter pointer swing amplitude is increased, thereby facilitating observation. It should be noted that during the test operation, especially when measuring the capacitance of a small capacity, it is necessary to repeatedly exchange the measured capacitor pins to contact A and B points to clearly see the swing of the multimeter pointer.
C. For fixed capacitors above 001μF, use the R×10k block of the multimeter to directly test the capacitor for charging process and whether there is internal short circuit or leakage, and estimate the capacity of the capacitor according to the amplitude of the pointer to the right.
2. Detection of electrolytic capacitors.
A. Because the capacity of electrolytic capacitors is much larger than that of general fixed capacitors, the appropriate range should be selected for different capacities when measuring. According to experience, in general, the capacitance between 1 ~ 47μF can be measured by R × 1k block, and the capacitance larger than 47μF can be measured by R × 100 block.
B. Connect the multimeter red pen to the negative pole and the black pen to the positive pole. At the moment of contact, the multimeter pointer deflects to the right with a large degree of skewness (for the same electrical block, the larger the capacity, the larger the swing), and then gradually to the left. Swing 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 several hundred kΩ or more, otherwise, it will not work properly. In the test, if there is no charging in the forward and reverse directions, that is, the hands are not moving, the capacity disappears or the internal circuit is broken; if the measured resistance is small or zero, the leakage of the capacitor is large or the breakdown is broken. Can no longer be used.
C. 电解 For electrolytic capacitors whose positive and negative signs are unknown, the above method for measuring leakage resistance can be used for discrimination. That is, first measure the leakage resistance arbitrarily, remember its size, and then exchange the test leads and then measure a resistance value. The one with a large resistance value in the two measurements is the forward connection method, that is, the black meter is connected to the positive pole and the red meter is connected to the negative pole.
D. Use the multimeter electric blocker, and use the method of positively and reversely charging 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. Gently rotate the shaft by hand, it should feel very smooth, and should not feel sometimes loose or even stuck. When the axial direction of the load is pushed forward, backward, up, down, left, right, etc., the shaft should not be loose.
B. Rotate the shaft with one hand and gently touch the outer edge of the group with the other hand. There should be no looseness. The variable capacitor with poor contact between the shaft and the rotor cannot be used any more.
C. Place the multimeter in the R×10k block. Connect one of the two test leads to the output end of the variable capacitor and the extension end of the fixed piece with one hand. The other hand will slowly rotate the rotating shaft several times, and the multimeter pointers are Should not move at infinity. In the process of rotating the shaft, if the pointer sometimes points to zero, it means there is a short circuit between the moving piece and the fixed piece; if it touches an angle, the multimeter reading is not infinite but a certain resistance value appears, indicating that the variable capacitor is moving. There is leakage between the film and the stator.