Capacitor quality judgment

1. Insulation resistance test

(1) Multimeter method

This method is suitable for measuring the insulation resistance of non-polar capacitors. When measuring, set the multimeter's electrical barrier to 10kΩ, and connect the two test leads to the two pins of the capacitor. For capacitors with small capacitance, the resistance value measured at this time should be infinite or close to infinity; for capacitors with larger capacitance, the pointer of the multimeter will first swing in the clockwise direction, and then it will quickly point to infinity. If the measured resistance value is less than 1MΩ, it means that the capacitor has serious leakage or the medium is damaged, and the capacitor cannot be used. Be careful not to hold the two leads of the capacitor with your hands during the test, otherwise the human body resistance will affect the test results.

(2) Megger method

A megohmmeter is also called a shaker. It can be used to measure the insulation resistance of a capacitor and can measure a more accurate insulation resistance value. But it should be noted that the maximum output voltage of the DC generator in the megohmmeter is 500V DC voltage. If the withstand voltage of the capacitor under test is lower than this voltage, the capacitor will be broken down.

(3) Headphone method

This method is only suitable for judging the insulation resistance of capacitors with larger capacitance. A pair of earphones and a 1.5V dry battery can be used to form a simple test circuit. During the test, when the earphone and the capacitor intermittently collide, the battery charges the capacitor and the earphone will make a "click" sound. If you touch it a few more times, the capacitor will not hear a "click" when it is fully charged, indicating that the leakage current of the capacitor is small and the insulation resistance is normal. If the earphones make a very loud "click" every time they touch each other, it indicates that the capacitor has large leakage and the insulation resistance is abnormal. If there is no sound when it first touches, and there is no sound all the time, it means that the capacitor has been disconnected and cannot be used.

2 Electric capacity test

The capacitance measurement is generally carried out with a capacitor bridge, so that accurate capacitance can be obtained. In the absence of special instruments, the high resistance of a multimeter can be used to estimate the capacitance of a capacitor, but it is only suitable for the measurement of large-capacity capacitors.

The following introduces the method of multimeter to detect the capacitance of electrolytic capacitor. First, set the ohm stop of the multimeter to the position of R x 1k, and short-circuit the meter pen to zero. Connect the black test lead of the multimeter to the positive electrode of the electrolytic capacitor and the red test lead to the negative electrode of the electrolytic capacitor. Because the capacitor was originally uncharged, when the two test leads were just connected to the capacitor, the battery in the multimeter charged the capacitor through the test leads, and the current flowing through the circuit caused the needle of the multimeter to deflect. The greater the positive electrode of the capacitor, the greater the deflection angle of the hands. If the capacitance is small, the deflection angle of the hands during the test will be smaller. According to this principle and actual testing experience, the position where the needle deflection corresponding to different capacitances should reach can be given, and the size of the measured capacitor capacitance can be judged.

As the charge of the capacitor increases, the charging current becomes smaller and smaller. If the capacitor does not leak, after a period of time, when the voltage on the capacitor is equal to the battery voltage, the charging current will be reduced to zero, and the needle of the multimeter will slowly return from the initial deflection position to the infinite resistance position (the starting point) position). In fact, electrolytic capacitors always have leakage current, and it is impossible for the needle to return to the starting point. It is generally believed that as long as the needle can cross 200 scales when returning, the capacitor can be used even if the leakage current is small.

After the test is over, the two pins of the capacitor should be short-circuited for discharge treatment, so that it will not be affected during re-testing.