Schematic Diagram of AC Voltage Range Measuring Principle for Multimeter
The AC voltage in the external circuit is applied to the rectifier circuit through C1, converting the AC current (generated by the AC voltage) into DC current. This DC current meter causes the pointer to deflect, only the AC voltage value.
Regarding the principle of measuring AC voltage levels, the following points need to be explained.
1. When measuring AC voltage, connecting the red and black meter rods in parallel to the measured voltage source in the external circuit is very convenient to operate.
2. Although measuring AC voltage, the rectifier circuit inside the meter shows DC current flowing through the meter head.
3. When measuring AC voltage, the battery inside the meter is not powered, and the current that causes the pointer to deflect is provided by the AC voltage source in the tested circuit. Due to the large voltage reducing resistor inside the meter (not shown in the figure), the measurement has little impact on the tested voltage source.
4. When there is no voltage in the tested circuit, no current flows 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 pointer deflection angle, and the higher 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 source in the external circuit, so the external circuit should also be powered during the measurement.
7. Due to the constantly changing direction of AC current, the AC voltage range of a 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 must pass through a rectifier circuit to ensure 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 rods have no polarity and can be used interchangeably, unlike when measuring DC voltage or DC current.
8. The AC voltage level indicator dial of a pointer type multimeter is designed for 50Hz sine wave AC power, so when measuring non 50Hz sine wave voltage or other frequency sine wave voltage, if the measured voltage is inaccurate, they can be measured with a digital multimeter.
9. The AC voltage indication scale is calculated based on the effective value of the sine wave voltage.
