Capacitor detection method and replacement

Capacitors The common marking method is direct marking, and the commonly used units are pF and μF, which are easy to recognize. However, some small-capacity capacitors are marked with numbers, usually with three digits. The first and second digits are valid numbers, and the third digit is the multiple, which means how many zeros are followed. For example: 343 means 34000pF. In addition, if the third digit is 9, it means 10-1, not 10 to the 9th power. For example: 479 means 4.7pF.

When replacing a capacitor, you should pay attention to the withstand voltage of the capacitor, which is generally required to be no lower than the withstand voltage requirement of the original capacitor. In circuits with stricter requirements, the capacity is generally not more than ±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. 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 there is leakage, internal short circuit or breakdown. When measuring, you can choose the multimeter R×10k block, and use two test leads to connect the two pins of the capacitor at will. The resistance should be infinite. If the measured resistance (the pointer swings to the right) is zero, it means that the capacitor is leaking or damaged or has internal breakdown.

B. Check whether the 10PF~0.01μF fixed capacitor has charging phenomenon, and then judge whether it is good or bad. The multimeter uses R×1k gear. 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 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 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 a capacitor with a smaller capacity, it is necessary to repeatedly switch the pins of the measured capacitor 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 range of the multimeter can be used to directly test whether the capacitor is in the charging process and whether there is an internal short circuit or leakage, and the capacity of the capacitor can be estimated based on 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 range, and the capacitance greater than 47μF can be measured with the R×100 range.

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 pointer of the multimeter will deflect to the right with a large deviation (for the same resistance range, the larger the capacity, the larger the swing), and then gradually turn to the left 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 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 value 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 anymore.

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. Use the resistance block of the multimeter to charge the electrolytic capacitor in the forward and reverse directions. The capacity of the electrolytic capacitor can be estimated based on the amplitude of the pointer swinging to the right.

3. Detection of variable capacitors

A. Gently rotate the shaft by hand, and it should feel very smooth, and should not feel loose or tight or even stuck. When the carrier shaft is pushed forward, backward, up, down, left, right, etc., the shaft should not be loose.

B. Use one hand to rotate the shaft and the other hand to touch the outer edge of the moving plate group. You should not feel any looseness. The variable capacitor with poor contact between the shaft and the moving plate cannot be used any more.

C. Set the multimeter to the R×10k position, connect the two test leads to the lead-out terminals of 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 remain at the infinite position. During the rotation of 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 at a certain angle, the multimeter reading is not infinite but a certain resistance value, which means that there is leakage between the moving plate and the fixed plate of the variable capacitor.