Capacitors are one of the most commonly used electronic components. The common word symbol for capacitors is "C". Capacitors are mainly composed of metal electrodes, dielectric layers and electrode leads, and the two electrodes are insulated from each other. Therefore, it has the basic performance of "blocking DC to AC".
Use a digital multimeter to detect capacitors, as follows.
1. Direct detection with capacitive gear
Some digital multimeters have the function of measuring capacitance, and their ranges are divided into five grades: 2000p, 20n, 200n, 2μ and 20μ. When measuring, the two pins of the discharged capacitor can be directly inserted into the Cx jack on the meter board, and the displayed data can be read after selecting the appropriate range. The 2000p level is suitable for measuring the capacitance less than 2000pF; the 20n level is suitable for measuring the capacitance between 2000pF and 20nF; the 200n level is suitable for measuring the capacitance between 20nF and 200nF; the 2μ level is suitable for measuring the capacitance between 200nF and 2μF. 20μ range, suitable for measuring capacitance between 2μF and 20μF. Experience has proved that some types of digital multimeters (such as DT890B+) have large errors when measuring small-capacity capacitors below 50pF, and there is almost no reference value for measuring capacitors below 20pF. At this time, the series method can be used to measure the small value capacitance. The method is: first find a capacitor of about 220pF, use a digital multimeter to measure its actual capacity C1, and then connect the small capacitor to be measured in parallel with it to measure its total capacity C2, then the difference between the two (C1-C2) is The capacity of the small capacitor to be measured. Using this method to measure small-capacity capacitors of 1 to 20pF is very accurate.
2. Detection with resistance gear
Practice has proved that the charging process of the capacitor can also be observed by using a digital multimeter, which is actually a discrete digital quantity to reflect the change of the charging voltage. Assuming that the measurement rate of the digital multimeter is n times per second, while observing the charging process of the capacitor, n independent and sequentially increasing readings can be seen every second. According to this display feature of the digital multimeter, the quality of the capacitor can be detected and the size of the capacitance can be estimated. Introduced below is the method of detecting capacitors by using the resistance gear of a digital multimeter, which is very useful for instruments without capacitance gears. This method is suitable for measuring large-capacity capacitors ranging from 0.1 μF to several thousand microfarads.
1. Measurement operation method
Turn the digital multimeter to the appropriate resistance gear, the red test lead and the black test lead respectively contact the two poles of the capacitor Cx under test, then the displayed value will gradually increase from "000" until the overflow symbol "1" is displayed. If it always displays "000", it means that the capacitor is short-circuited; if it always displays overflow, it may be an open circuit between the internal electrodes of the capacitor, or the selected resistance file may be inappropriate. When checking electrolytic capacitors, it should be noted that the red test lead (with positive charge) is connected to the positive electrode of the capacitor, and the black test lead is connected to the negative electrode of the capacitor.
2. Measurement principle
The measurement principle of measuring capacitors with resistance gear. During the measurement, the positive power supply charges the capacitor Cx under test through the standard resistor R0, and at the moment when the charging starts, "000" is displayed because Vc = 0. As Vc gradually increases, the displayed value increases. When Vc = 2VR, the meter starts to display overflow symbol "1". The charging time t is the time required for the display value to change from "000" to overflow, and this time interval can be measured by a quartz watch.
