Third, the simple judgment method of the capacitor is good or bad
When inspecting a used capacitor, first connect the two poles with a wire (or other metal) to avoid electric shock damage to the tester due to the internal stored charge.
1. Check the quality of the capacitor with a multimeter
When you suspect that a capacitor is damaged or has a quality problem, you can use a pointer multimeter to make a rough decision. Please refer to the figure below.
Set the multimeter to the R × 1k (or R × 100) block of the resistance bar. Use two test leads to contact the two electrodes of the capacitor under test. Watch the reaction of the hands and determine the quality status of the capacitor according to the reaction.
1) The pointer is quickly placed at zero (0Ω) or near zero, then slowly go back (to the side of ∞Ω), go somewhere and stop. It shows that the capacitor is basically intact, and the closer the return position is to the ∞Ω point, the better the quality, and the farther away, indicating more leakage.
This is because the principle of measuring the resistance of the multimeter is actually adding a fixed value of DC voltage to the conductor under test (provided by the battery installed in the table). At this time, there will be a corresponding current, which will be based on Ohm's law. This current is converted to a resistance value scale on the dial. For example, if the current is 0.03A at a voltage of 9V, the resistance of the conductor is 9V/0.03A=300Ω, and the mark at the 0.03A position on the dial is 300Ω.
For a good capacitor, when a DC voltage is applied to both ends, charging starts, the current will reach the maximum value instantaneously, and it is close to 0Ω for the resistance of the multimeter resistance file. As the charging process progresses, the current also flows. It will gradually decrease. Theoretically, the two plates of the capacitor should be completely insulated. Therefore, the final result of the above charging process should be that the current reaches zero, reflected on the resistance, and finally should return to the ∞Ω point. At the location (ie, where the current is equal to zero). But in fact, all the capacitor plates are not completely insulated, so there will be a small current under the applied voltage, which is called the "leakage current" of the capacitor. This means that the pointer cannot return completely to the ∞Ω point. the reason. The amount of leakage of the multimeter's hands indicates the magnitude of the leakage current. If the return is large, the leakage current is small. If the return is small, the leakage current is large. The leakage current should not be too large, otherwise it will cause some abnormal phenomena of the circuit. If it is serious, it will not work normally. When the leakage current is large, the capacitor will be much hotter than normal.
2) The pointer will not move after the zero position (0Ω) or near zero position, indicating that a short circuit fault has occurred between the two plates of the capacitor, and the capacitor can not be reused.
3) When the two electrodes of the test leads and the capacitor start to be turned on, the pointer does not move at all, indicating that the internal connection of the capacitor has been disconnected (generally occurring at the junction between the electrode and the plate), and naturally cannot be used any more.
2. Use the charging and discharging methods to judge whether the capacitor is good or bad.
When there is no multimeter at hand, the method of charging and discharging can be used to roughly check whether the capacitor is good or bad. The power supply used is generally DC (especially a polarized capacitor such as an electrolytic capacitor, and a DC power supply must be used). The voltage should not exceed the withstand voltage of the capacitor to be tested (marked on the capacitor). A dry battery of 3 to 6 V is commonly used. Or 24V, 48V electric bicycles and automotive batteries. For capacitors that are connected to the AC circuit during operation, AC power can also be used, but when the voltage is high, safety should be taken during operation. Wear insulated gloves or use insulated tools.
After the DC power is applied to both ends of the capacitor, wait a little while to disconnect the power. Then, use a length of wire, one end is connected to one pole of the capacitor, and the other end is connected to the other electrode of the capacitor, while watching whether there is a discharge spark between the electrode and the wire.
If there is a large discharge spark and a squeaky discharge sound, the explanation is good, and the larger the spark capacity is also larger (for the same specification of the capacitor, when using the same power source for charging); the discharge spark and the discharge sound are small , indicating that the quality is not very good; there is no discharge spark, the explanation is bad.
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