Using a multimeter to measure the AC voltage range
Because the meter can only flow DC current, while measuring AC voltage, the AC current must be converted into DC current, which is completed by a rectifier circuit made of two diodes, C1. VD1 and VD2 in the circuit are rectifier diodes in the meter. In order to avoid the DC current in the external circuit from altering the measurement results of the AC voltage, the DC blocking capacitor in the meter blocks the flow of DC current through the meter. Us is the external circuit's AC voltage that needs to be measured.
Through C1, the external circuit's AC voltage is added to the rectifier circuit, where it is used to convert the AC current (the AC current caused by the AC voltage) into a DC current. The pointer on this DC current meter, which solely displays AC voltage values, is deflected.
The following considerations should also be clarified in relation to the AC voltage block measurement principle.
1. To make it easier to operate, connect the red and black gauge sticks in parallel to the external circuit's observed voltage source while measuring AC voltage.
2. The rectifier circuit in the meter will flow a DC current through the meter head even if the AC voltage is being measured.
3. When measuring AC voltage, the meter's battery does not provide power; instead, the circuit being tested's AC voltage source produces the current that deflects the pointer. The measurement is made possible by the meter's massive drop-down resistor (not depicted in the figure). The measured voltage source has very little affect.
4. The pointer cannot deflect and the voltage indicator is 0 when there is no voltage in the circuit being tested. There is also no current flowing through the meter head. The larger the external circuit voltage, the bigger the rectified DC current flowing through the meter head, the greater the pointer's deflection angle, and the greater the stated voltage value are all within the same range.
5. Since the battery in the meter is not used to measure the AC voltage, the battery's voltage has no bearing on how much AC voltage is measured.
6. When measuring AC voltage, there must be a power supply in the external circuit, so the external circuit should also be energized when measuring.
7. Since the direction of the AC current is constantly changing, and the AC voltage block of the pointer multimeter is only used to measure the 50Hz AC, the positive and negative half-cycle amplitudes of this AC are symmetrical, so the AC voltage sent into the meter must pass through the rectifier circuit. The direction of the current flowing through the meter head is determined. In this way, when measuring AC voltage, the red and black gauge sticks have no polarity and can be used interchangeably, which is not like measuring DC voltage or DC current.
8. Because the AC voltage block indicator dial of the pointer multimeter is made for 50Hz sine wave AC, it is inaccurate when measuring non-50Hz sine wave voltage or other frequency sinusoidal voltage. Instead, users should measure using a digital multimeter.
9. The sine wave voltage's effective value serves as the foundation for the AC voltage indicator scale.
