Measuring Method of DC Voltage and Current by Digital Multimeter
When reading, you should look at the scale with the symbol "dc" or "v·ma". At this time, the value pointed by the knob is the maximum limit (range) of DC voltage or current. Before measuring, estimate the size of the measured value, and then turn the transfer switch to the appropriate limit position. The size of the measured value can be read out on a scale. When measuring the voltage between two points on the circuit, the red test pen should be connected to the high potential point, and the black test pen should be connected to the low potential point. When measuring DC current, the ammeter should be connected in series with the branch circuit, so the branch circuit under test must be disconnected first.
If the branch circuit is not disconnected and the two test leads are connected to the two ends of the branch circuit, the ammeter is actually used to measure the voltage, and the ammeter is burned. Using a multimeter to judge the quality of the capacitor depends on the capacity of the electrolytic capacitor. Usually, the r×10, r×100, and r×1k gears of the multimeter are used for testing and judgment. The red and black test leads are respectively connected to the negative pole of the capacitor (the capacitor needs to be discharged before each test), and the quality of the capacitor can be judged by the deflection of the needle.
If the hands of the watch swing to the right quickly, and then slowly return to the original position to the left, the capacitor is generally good. If the hands of the watch do not turn back after swinging up, it means that the capacitor has broken down. If the watch hands gradually return to a certain position after swinging up, it means that the capacitor has leaked. If the hands of the watch cannot move, it means that the capacitor electrolyte has dried up and lost capacity.
Some leakage capacitors are not easy to accurately judge whether they are good or bad by the above method. When the withstand voltage value of the capacitor is greater than the voltage value of the battery in the multimeter, according to the characteristics that the leakage current of the electrolytic capacitor is small when it is charged forward, and the leakage current is large when it is charged reversely, the r×10k gear can be used to reverse charge the capacitor and observe Whether the place where the needle stays is stable (that is, whether the reverse leakage current is constant), and thus the quality of the capacitor can be judged with high accuracy. The black test lead is connected to the negative pole of the capacitor, and the red test lead is connected to the positive pole of the capacitor. The hands swing up quickly, and then gradually retreat to a certain place to stay still, indicating that the capacitor is good. The capacitor that is slowly moving to the right has leaked and is no longer usable. The hands of the watch generally stay and stabilize within the scale range of 50-200k.
