Digital multimeter to measure capacitance good or bad
1, with the capacitance file direct detection
Some digital multimeter has the function of measuring capacitance, its range is divided into 2000p, 20n, 200n, 2μ and 20μ five files. Measurement can be discharged capacitance two pins directly into the table on the Cx jack, select the appropriate range can be read after the display data.
000p file, suitable for measuring capacitance less than 2000pF; 20n file, suitable for measuring capacitance between 2000pF and 20nF; 200n file, suitable for measuring capacitance between 20nF and 200nF; 2μ file, suitable for measuring capacitance between 200nF and 2μF; 20μ file, suitable for measuring capacitance between 2μF and 20μF.
Experience has proved that some models of digital multimeter (such as DT890B +) in the measurement of 50pF below the small capacitance capacitor error is large, the measurement of capacitance below 20pF almost no reference value. At this time, you can use the series connection method to measure the small value capacitance.
Method is: first find a 220pF or so of capacitance, with a digital multimeter to measure the actual capacity of C1, and then to be measured in parallel with the small capacitance to be measured out of its total capacity of C2, then the difference between the two (C1 - C2) that is to be measured the capacity of the small capacitor. With this method to measure 1 ~ 20pF of small capacitance is very accurate.
2, with resistance detection
Practice has shown that the use of digital multimeter can also observe the charging process of capacitors, which is actually a discrete digital quantity to reflect the changes in charging voltage. Set the digital multimeter measurement rate of n times / second, then in the observation of the capacitor charging process, every second to see n independent of each other and the sequential increase in the number of readings. According to this display feature of the digital multimeter, you can test the capacitor and estimate the size of the capacitance.
The following is the use of digital multimeter resistance file detection of capacitors, for not set the capacitance file of the instrument is very practical. This method is suitable for measuring 0.1μF ~ thousands of microfarads of large-capacity capacitors.
1. Measurement method
The digital multimeter will be dialed to the appropriate resistance file, the red pen and black pen were in contact with the measured capacitor Cx poles, then the display value will gradually increase from "000" until the display overflow symbol "1". If always show "000", the capacitor internal short circuit; if always show overflow, the capacitor may be internal open circuit between poles, may also be selected when the resistance file is not appropriate. When checking electrolytic capacitors, it should be noted that the red pen (positively charged) is connected to the positive terminal of the capacitor, and the black pen is connected to the negative terminal of the capacitor.
2.Measuring principle
Measurement principle of capacitors with resistance is shown in Figure 5-11 (b). Measurement, the positive power supply through the standard resistor R0 to the measured capacitor Cx charging, just started charging the moment, because Vc = 0, so the display "000". With the gradual increase in Vc, the display value increases. When Vc = 2VR, the meter starts to display the overflow symbol "1". The charging time t is the time it takes for the display to change from "000" to overflow, and the interval can be measured with a quartz meter.
