1. Detection of fixed capacitors A) Detection of small capacitances below 10pF. Because the capacity of fixed capacitors below 10pF is too small, measuring with a multimeter can only qualitatively check whether there is leakage, internal short circuit or breakdown. When measuring, you can choose a multimeter R×10k block, and use two meter pens to connect the two pins of the capacitor arbitrarily, 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 damaged by leakage or internal breakdown. B) Detect whether the 10PF～001μF fixed capacitor is charged, and then judge whether it is good or bad. The multimeter selects R×1k gear. The β value of the two transistors is 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 leads of the multimeter are respectively connected to the emitter e and collector c of the composite tube. Due to the amplification effect of the composite triode, the charge and discharge process of the capacitor under test is amplified, and the amplitude of the pointer of the multimeter is enlarged, which is convenient for observation. It should be noted that during the test operation, especially when measuring small-capacity capacitors, it is necessary to repeatedly exchange the contact points A and B of the tested capacitor pin to clearly see the swing of the multimeter pointer. C) For a fixed capacitance above 001μF, the R×10k block of a multimeter can be used to directly test whether the capacitor has the charging process and whether there is internal short circuit or leakage, and the capacitance of the capacitor can be estimated according to the amplitude of the pointer swing to the right. 2. Detection of electrolytic capacitor
s A) Because the capacity of electrolytic capacitors is much larger than that of general fixed capacitors, suitable ranges should be selected for different capacities when measuring. According to experience, in general, the capacitance between 1 and 47μF can be measured with the R×1k block, and the capacitance larger than 47μF can be measured with the R×100 block. B) Connect the red test lead of the multimeter to the negative pole and the black test lead to the positive pole. At the moment of contact, the pointer of the multimeter will deflect to the right by a greater degree (for the same electrical barrier, the greater the capacity, the greater the swing), and then gradually turn to the left Turn around until it stops at a certain position. The resistance value 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 and reverse directions, that is, the hand does not move, it means that the capacity has disappeared or the internal circuit is broken; if the measured resistance value is very small or zero, it means that the capacitor has a large leakage or has been broken down. Can no longer be used. C. For electrolytic capacitors with unknown positive and negative signs, the above method of measuring leakage resistance can be used to distinguish. That is to measure the leakage resistance arbitrarily, remember its size, and then exchange the test leads to measure a resistance value. The larger resistance of the two measurements is the positive connection, 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 the method of forward and reverse charging the electrolytic capacitor. According to the magnitude of the pointer swing to the right, the capacity of the electrolytic capacitor can be estimated. 3. Detection of variable capacitors A) Gently rotate the shaft with your hands, it should feel very smooth, and should not feel loose or tight sometimes or even stuck. When pushing the load shaft forward, backward, up, down, left, right, etc., the rotating shaft should not be loose. B) Rotate the shaft with one hand and gently touch the outer edge of the film set with the other hand. You should not feel any looseness. The variable capacitor with poor contact between the rotating shaft and the moving plate can no longer be used. C) Place the multimeter in the R×10k gear, connect the two test leads with one hand to the moving piece of the variable capacitor and the leading end of the fixed piece, and slowly rotate the shaft several times back and forth with the other hand. The pointers of the multimeter are all Should not move at infinity. In the process of rotating the shaft, if the pointer sometimes points to zero, it indicates that there is a short-circuit point between the moving piece and the fixed piece; if it encounters a certain angle, the multimeter reading is not infinity but a certain resistance value, indicating that the variable capacitor is moving. There is a leakage phenomenon between the film and the stator.
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