How to measure the quality of small capacity capacitors?
1. Detect small capacitances below 10pF. Because the capacity of fixed capacitors below 10pF is too small, if you use an analog multimeter to measure, you 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 the metal measuring rods of the two electric meters to connect the two pins of the capacitor at will, and the resistance should be infinite. If the measured resistance value (the indicator swings to the right) is zero, it means that the capacitor is damaged by leakage or internal breakdown.
2. For fixed capacitors above 0.01μF, the R×10k block of an analog 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 amplitude of the pointer swing to the right.
How to detect the quality of electrolytic capacitors?
1. 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, capacitance between 1 and 47μF can be measured with R×1k block, and capacitance larger than 47μF can be measured with R×100 block.
2. Connect the red meter metal measuring rod of the multimeter to the negative electrode, and the black meter metal measuring rod to the positive electrode. 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 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 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 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.
3. For electrolytic capacitors with unknown positive and negative signs, the above-mentioned leakage resistance measurement method can be used to distinguish. That is, first measure the leakage resistance randomly, remember its size, and then exchange the metal measuring rod of the meter to measure a resistance value.
The larger resistance of the two measurements is the forward connection method, that is, the metal probe of the black meter is connected to the positive electrode, and the metal probe of the red meter is connected to the negative electrode.
4. Use a multimeter to block electricity, and use the method of forward and reverse charging the electrolytic capacitor. According to the amplitude of the pointer swing to the right, the capacity of the electrolytic capacitor can be estimated.
5. Measure the speed of capacitor discharge
Use a digital multimeter on both ends of the capacitor to charge first and then open a circuit to measure the speed at which the voltage drops. Because the capacitor is an open circuit, the only thing that consumes electricity is the leakage current. If you measure the voltage vs. time curve, you can deduct the leakage current. Remember to use a better digital multimeter, because the input impedance of the multimeter itself is limited no matter how large it is. If the leakage current of the capacitor with better quality is not large, then the digital multimeter with lower input impedance will not be accurate.
6. Attention! If you need to measure positive and negative back and forth when measuring large capacity capacitors, short-circuit the capacitor and discharge it to avoid damage to the meter.
According to experience, in high-frequency circuits, switching power supply circuits have many small capacitors that ordinary multimeters cannot correctly judge whether they are good or bad, and some capacitances may increase. It is strongly recommended to use a dedicated digital capacitance meter for measurement.
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