Using a multimeter to determine capacitor quality
Judging the quality of a capacitor with a multimeter depends on the capacity of the electrolytic capacitor. Usually, the R × 10, R × 100, and R × 1K ranges of the multimeter are selected for testing and judgment. Connect the red and black probes to the negative terminal of the capacitor respectively (discharge the capacitor before each test), and judge the quality of the capacitor by the deflection of the probe. If the pointer quickly swings to the right and then slowly returns to its original position to the left, generally speaking, a capacitor is good. If the pointer no longer rotates after swinging, it indicates that the capacitor has broken down. If the pointer gradually returns to a certain position after swinging up, it indicates that the capacitor has leaked electricity. If the pointer cannot be raised, it indicates that the capacitor electrolyte has dried up and lost its capacity.
It is difficult to accurately determine the quality of some capacitors with leakage using the above methods. When the withstand voltage value of the capacitor is greater than the voltage value of the battery in the multimeter, according to the characteristics of small leakage current during forward charging and large leakage current during reverse charging of the electrolytic capacitor, the R × 10K gear can be used to reverse charge the capacitor, observe whether the pointer stays stable (i.e. whether the reverse leakage current is constant), and judge the quality of the capacitor with high accuracy. Connect the black probe to the negative terminal of the capacitor and the red probe to the positive terminal of the capacitor. If the probe quickly swings up and then gradually retreats to a certain position and stops moving, it indicates that the capacitor is good. Any capacitor whose probe stops unstable at a certain position or gradually moves to the right after stopping has leaked electricity and cannot be used anymore. The pointer generally stays and stabilizes within the 50-200K scale range
When measuring current with the current mode of a multimeter, it is connected in series with the circuit being tested. The smaller the internal resistance of the ammeter, the smaller the impact on the circuit, and the smaller the measurement error. In an ideal situation, the internal resistance of the ammeter should be equal to zero. However, in reality, when measuring current without the current mode of a multimeter, it is connected in series with the circuit being tested. The smaller the internal resistance of the ammeter, the smaller the impact on the circuit, and the smaller the measurement error. In an ideal situation, the internal resistance of the ammeter should be equal to zero, but in reality, it is impossible. Because the movable coil of a multimeter is made of bronze wire, there is always a certain resistance inside the meter head, which is called the internal resistance of the current block. Due to the existence of the internal resistance of the meter head, when the multimeter block measures the current again, the total effective resistance of the tested circuit will increase, which changes the original working state of the tested circuit and generates measurement errors. In order to reduce measurement errors, it is required that the internal resistance of the current block itself be as small as possible. The smaller the internal resistance of the current block, the closer the measurement result is to the actual value. The error analysis of measuring current is shown in the figure, which shows the tested circuit before the current block is connected. The internal resistance of the power supply voltage is ignored, and the current is:
Obviously, when the current I is constant, the larger the RC, the greater the power loss P1 of the current. The following conclusion can be drawn from the above:
1. When the full bias current is the same in the current mode, the smaller the internal resistance of the multimeter's current mode, the smaller the full bias voltage drop, and the smaller the measurement error of the current.
2. For the same multimeter, the larger the current range, the smaller its internal resistance and measurement error.
3. When the total resistance of the tested circuit is much greater than the internal resistance of the current range of the multimeter, the internal resistance of the current range of the multimeter can be ignored.
In summary, when measuring current with a multimeter, since the multimeter is connected in series with the circuit being tested, the smaller the internal resistance when selecting the current mode, the more accurate the measurement result.
