The common marking method for capacitors is direct marking, with commonly used units being pF and μF, which are easy to recognize. However, some small-capacitance capacitors use a digital marking method, generally with three digits, where the first and second digits are significant figures, and the third digit is the multiplier, indicating how many zeros to add. For example, 343 represents 34000pF. Additionally, if the third digit is 9, it means 10-1, not 10 to the power of 9. For example, 479 represents 4.7pF.Oil-immersed test transformer is suitable for insulation strength tests of various electrical products, electrical equipment, and insulating materials under specified voltages, to assess the insulation level of products, discover insulation defects in test objects, and measure their ability to withstand overvoltages.
When replacing capacitors, the main consideration is that the voltage rating of the new capacitor should generally not be lower than that of the original. In circuits with stricter requirements, the capacitance should generally not exceed ±20% of the original capacitance. In circuits with less strict requirements, such as bypass circuits, it is generally sufficient that the capacitance is not less than 1/2 of the original and not greater than 2 to 6 times the original.
1. Testing of fixed capacitors
(1) Testing small capacitors below 10pF: Since fixed capacitors below 10pF have too small capacitance, using a multimeter for measurement can only qualitatively check for leakage, internal short circuit, or breakdown. During measurement, select the R×10k range on the multimeter, and connect the two test leads to the two pins of the capacitor arbitrarily. The resistance should be infinite. If the measured resistance (pointer swings to the right) is zero, it indicates that the capacitor is leaking, damaged, or internally broken down.
(2) Testing whether 10pF to 0.01μF fixed capacitors have a charging phenomenon to determine their condition. Use the R×1k range on the multimeter. The β values of the two transistors should be above 100, and the penetration current should be strong. Silicon transistors such as 3DG6 can be selected to form a composite transistor. Connect the red and black test leads of the multimeter to the emitter (e) and collector (c) of the composite transistor, respectively. Due to the amplification effect of the composite transistor, the charging and discharging process of the capacitor under test is amplified, causing the multimeter pointer to swing more, making it easier to observe. It should be noted that during testing, especially when measuring smaller capacitance capacitors, the pins of the capacitor under test should be repeatedly swapped between points A and B to clearly see the pointer swing.
(3) For fixed capacitors above 0.01μF, the R×10k range on the multimeter can be used to directly test whether the capacitor has a charging process, internal short circuit, or leakage, and the capacitance can be estimated based on the amplitude of the pointer swing to the right.
2. Testing of electrolytic capacitors
(1) Because the capacitance of electrolytic capacitors is much larger than that of general fixed capacitors, appropriate ranges should be selected based on different capacitances during measurement. According to experience, generally, capacitors between 1 and 47μF can be measured with the R×1k range, and capacitors larger than 47μF can be measured with the R×100 range.
(2) Connect the red test lead of the multimeter to the negative terminal and the black test lead to the positive terminal. At the moment of contact, the multimeter pointer swings to the right with a large deflection (for the same resistance range, the larger the capacitance, the larger the swing), then gradually returns to the left 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 above several hundred kΩ, otherwise, they cannot work properly. During testing, if there is no charging phenomenon in either forward or reverse direction, i.e., the pointer does not move, it indicates that the capacitance is lost or there is an internal open circuit; if the measured resistance is very small or zero, it indicates that the capacitor has large leakage or has broken down and cannot be used anymore.
(3) For electrolytic capacitors with unclear positive and negative terminal markings, the above method of measuring leakage resistance can be used to distinguish them. That is, first measure the leakage resistance arbitrarily and note its value, then swap the test leads and measure another resistance value. The measurement with the larger resistance is the forward connection, meaning the black test lead is connected to the positive terminal and the red test lead to the negative terminal.
(4) Using the resistance range of the multimeter, the method of forward and reverse charging of the electrolytic capacitor can be used to estimate its capacitance based on the amplitude of the pointer swing to the right.
3. Testing of variable capacitors
(1) Gently rotate the shaft by hand; it should feel very smooth, without any feeling of looseness or tightness, or even sticking. When pushing the shaft in all directions (forward, backward, up, down, left, right), the shaft should not feel loose.
(2) Rotate the shaft with one hand and gently touch the outer edge of the moving plate group with the other hand; there should be no feeling of looseness. Variable capacitors with poor contact between the shaft and the moving plate should not be used anymore.
