The capacitance value measured by the multimeter keeps changing.
Either the circuit is unstable or the multimeter is almost out of power.
The refresh rate displayed by the digital multimeter is approximately 3 times per second, so it is normal for it to change. But if it jumps around and something is wrong, it may be due to poor contact. Generally, it is the test leads and the circuit under test, or the test leads and the multimeter itself. There may also be poor welding of the multimeter jack, contact problems with the multimeter transfer switch, insufficient battery, etc.
The steps to use a digital multimeter to test the quality of the capacitor are as follows:
1. To determine the polarity, first adjust the multimeter to 100 or 1K ohm. Assume that one pole is positive, connect the black test lead to it, and the red test lead to the other pole. Note the resistance value, and then discharge the capacitor, that is, let the two poles Make contact, and then change the test leads to measure the resistance. The black test lead with the larger resistance is connected to the positive electrode of the capacitor.
2. Adjust the multimeter to the appropriate ohm level. The principle of gear selection is: 1μF capacitor uses 20K gear, 1-100μF capacitor uses 2K gear, and larger than 100, μF uses 200 gear.
3. Then use the red pen of the multimeter to connect the positive pole of the capacitor, and the black pen to the negative pole of the capacitor. If the display slowly increases from 0, and finally displays the overflow symbol 1, the capacitor is normal. If it always displays 0, the capacitor is short-circuited internally. If 1 is displayed, the capacitor is internally open.
How to use a multimeter to measure 220V AC voltage
Set the range switch to AC 500V. At this time, the full scale is 500V, and the reading is based on the scale 1:1. Insert the two test leads into the power socket. The scale pointed by the meter needles is the measured voltage value. When measuring AC voltage, the test leads are not positive or negative. The method of measuring voltage with a multimeter is to first align the range switch within the five ranges marked V (when testing AC voltage, align it with the AC voltage range, and when testing DC voltage, align it with the DC voltage range). When measuring voltage, the meter leads should be connected in parallel to the circuit under test. Select an appropriate range position based on the approximate value of the circuit under test. The maximum value of each dry cell battery is 1.5V, so it can be placed in the 5V range. At this time, the full-scale reading of 500 on the panel should be read as 5. That is 100 times smaller. If the meter hand points to the 300 mark, it reads 3V. Note that the index value on the tip of the range switch is the corresponding value of the full-scale reading of the needle on the meter. When reading the meter, you only need to convert it accordingly to read the actual value. Except for the resistance level, the measurement results are read in this way for all range switch levels. In actual measurement, when the approximate value of the measured voltage cannot be determined, you can first turn the switch to the maximum range, and then reduce the range step by step to the appropriate position. When measuring DC voltage, pay attention to the positive and negative polarity. If the test leads are connected reversely, the meter needle will hit backwards. If you don’t know the positive and negative polarity of this circuit, you can put the multimeter range at the maximum range, quickly test it on the circuit under test, and see how the pen needle deflects to determine the positive and negative polarity.
