Methods for Measuring Resistance with a Digital Multimeter and Precautions
During the measurement, the positive power supply charges the capacitor under test Cx through the standard resistance R0. At the moment when the charging starts, since Vc = 0, the display shows "000". As Vc gradually rises, the displayed value increases accordingly. When Vc = 2VR, the instrument starts to display the overflow symbol "1". The charging time t is the time required for the displayed value to change from "000" to the overflow state, and this time interval can be measured with a quartz watch. When using a DT830 digital multimeter to estimate the capacitance of capacitors ranging from 0.1μF to several thousand microfarads, the resistance range can be selected according to Table 5-1. The table gives the measurable capacitance range and the corresponding charging time.
Measurement Method of Resistance with a Digital Multimeter
When measuring resistance, the reading should be taken on the scale marked with "Ω" (Since the current passing through the meter head is not directly proportional to the resistance under test, the resistance scale on the dial is uneven.). The actual value of the resistance under test is equal to the reading on the scale multiplied by the multiple indicated by the knob. Before the measurement, the two test leads should be short-circuited first, and the zero-adjusting potentiometer should be rotated to make the pointer point to the 0Ω position. Then, select an appropriate range to ensure the accuracy of the measurement. Every time the range is changed, the zero adjustment should be carried out again. In addition, the resistance measurement must be carried out in the absence of a power source and without any other parallel branches. After the resistance (or current) measurement is completed, the transfer switch should be turned to the high voltage range. This is a good habit to prevent mistakenly using the ohmmeter (or current range) to measure the voltage.
Precautions for Measuring Resistance with a Digital Multimeter
Set the digital multimeter to an appropriate resistance range, and the red test lead and the black test lead respectively touch the two poles of the capacitor under test Cx. At this time, the displayed value will gradually increase from "000" until the overflow symbol "1" is displayed. If "000" is continuously displayed, it indicates that the capacitor is internally short-circuited; if the overflow is continuously displayed, it may be that the capacitor has an open circuit between its internal poles, or it may be that the selected resistance range is not appropriate. When testing electrolytic capacitors, it should be noted that the red test lead (with positive charge) is connected to the positive pole of the capacitor, and the black test lead is connected to the negative pole of the capacitor.
