Detection of capacitors in refrigeration accessories

Detection of capacitors in refrigeration accessories

The common marking method for capacitors in refrigeration accessories is direct marking, and the commonly used units are pF and μF. However, some small-capacity capacitors use the digital labeling method, which usually has three digits. The first and second digits are valid digits, and the third digit is a multiple, which means how many zeros should be followed. For example: 343 means 34000pF. In addition, if the third digit is 9, it means 10-1 instead of 10 to the 9th power. For example: 479 means 4.7pF. When replacing a capacitor, it should be noted that the withstand voltage value of the capacitor is generally not lower than that of the original capacitor. In circuits with stricter requirements, the capacity generally does not exceed ±20% of the original capacity. In circuits with less stringent requirements, such as bypass circuits, it is generally required to be no less than 1/2 of the original capacitance and no more than 2 to 6 times the original capacitance.

　　1, fixed capacitor detection

　　 Detect small capacitances below 10pF. Because the capacity of fixed capacitors below 10pF is too small, when measuring with a multimeter, it can only be qualitatively checked for 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 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 damaged by leakage or internal breakdown.

　　 Check whether the 10PF～0.01μF 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 is more? You can choose 3DG6 and other types of silicon transistors 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.

　　 For a fixed capacitance above 0.01μF, the R×10k block of a multimeter can be used to directly test whether the capacitor has charging process and whether there is internal short circuit or leakage, and the capacity of the capacitor can be estimated according to the magnitude of the pointer swing to the right.

　　 2. Detection of electrolytic capacitors

　　 The capacity of electrolytic capacitors is much larger than that of general fixed capacitors. Therefore, when measuring, you should select a suitable range for different capacities. Generally, the capacitance between 1 and 47μF can be measured with R×1k block, and the capacitance larger than 47μF can be measured with R×100 block. ?? 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 more to the right (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 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 several hundred kΩ, otherwise, it will not work normally. 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 is very small or zero, it means that the capacitor has a large leakage or has been broken down. Can no longer be used. For electrolytic capacitors with unknown positive and negative signs, the above method of measuring leakage resistance can be used to distinguish. That is, first measure the leakage resistance randomly, remember its size, and then exchange the test leads to measure a resistance value. The larger resistance of the two measurements is the forward connection, that is, the black test lead is connected to the positive electrode, and the red test lead is connected to the negative electrode. Use a multimeter to block electricity and charge the electrolytic capacitor forward and backward. 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

　　 Rotate the shaft gently with your hands, it should feel very smooth, and should not feel loose, sometimes tight or even stuck. When pushing the load shaft forward, backward, up, down, left, right, etc., the rotating shaft should not be loose.

　　 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 cannot be used anymore.

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 respectively, and slowly rotate the shaft several times with the other hand. The pointers of the multimeter should be in The infinity position does 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 piece and the fixed piece; if it encounters a certain angle, the multimeter reading is not infinite but a certain resistance value, indicating that the variable capacitor is moving There is a leakage phenomenon between the film and the stator.