The VD1 and VD2 in the circuit are the rectifier diodes inside the meter. As the meter head can only flow direct current, when measuring AC voltage, the AC current needs to be converted into direct current. This is accomplished by a rectifier circuit composed of two diodes. C1 is the DC isolation capacitor inside the meter, which prevents the direct current in the external circuit from flowing through the meter head to prevent the direct current in the external circuit from affecting the measurement results of AC voltage, Us is the AC voltage in the external circuit to be measured.

The AC voltage in the external circuit is added to the rectifier circuit through C1, converting the AC current (generated by the AC voltage) into DC current. This DC current meter tip causes the pointer to deflect, only the AC voltage value.

1. When measuring AC voltage, the red and black meter bars are connected in parallel to the measured voltage source of the external circuit, which is very convenient to operate.

2. Although the AC voltage is measured, the DC current flows through the rectifier circuit in the meter head.

3. When measuring AC voltage, the battery in the meter is not powered, and the current that causes the pointer to deflect is provided by the AC voltage source in the circuit under test. Due to the large step-down resistance in the meter (not shown in the figure), the impact of measurement on the voltage source under test 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. At the same range, the higher the voltage in the external circuit, the greater the DC current flowing through the meter after rectification, the greater the deflection angle of the pointer, and the greater the indicated voltage value.

5. Due to the fact that 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 AC voltage range of the pointer 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 multimeter is designed for 50Hz sine wave AC power. Therefore, when measuring non 50Hz sine voltage or other frequency ^ sine voltage, if the measured voltage is inaccurate, they can be measured using a digital multimeter.

9. The AC voltage indication scale is calculated based on the effective value of the sine wave voltage.,