Regarding the principle of measuring AC voltage, the following points should also be explained.
1. When measuring AC voltage, the red and black meter bars are connected in parallel to the measured voltage source in the external circuit, making the operation very convenient.
2. Although the measured voltage is AC, the current flowing through the rectifier circuit in the meter is DC current.
3. When measuring AC voltage, the battery inside the meter is not supplying power, and the current that causes the pointer to deflect is provided by the AC voltage source in the tested circuit. Due to the large step-down resistance in the meter (not shown in the figure), the impact of the measurement on the tested voltage source is also very small.
4. When there is no voltage in the tested circuit, there is no current flowing through the meter head, the pointer cannot deflect, and the voltage indication is zero. In the same range, the higher the voltage in the external circuit, the greater the DC current flowing through the meter after rectification, and the greater the deflection angle of the pointer, the greater the indicated voltage value.
5. Since the battery inside the meter is not used for measuring AC voltage, the voltage of the battery inside the meter does not affect the measurement of AC voltage.
6. When measuring AC voltage, there should be a power supply in the external circuit, so the external circuit should also be energized during the measurement.
7. Due to the constantly changing direction of AC current and the fact that the AC voltage range of a pointer type multimeter is only used to measure 50Hz AC power, the positive and negative half cycle amplitudes of this AC power are symmetrical. Therefore, the AC voltage sent into the meter needs to pass through a rectification circuit, so that the direction of the current flowing through the meter head is determined. In this way, when measuring AC voltage, the red and black meter bars have no polarity and can be interchanged, unlike measuring DC voltage or DC current.
8. The AC voltage range indicator dial of the pointer type multimeter is designed for 50Hz sine wave AC power. Therefore, if the measured voltage is not accurate when measuring non 50Hz sine voltage or other frequency ^ sine voltage, they can be measured with a digital multimeter.
9. The AC voltage indicator scale is calculated based on the effective value of the sine wave voltage.
