Analysis of voltage measurement error between digital multimeter and pointer multimeter
If the measured voltage is mains power, which is 50Hz AC, and both meters are qualified, it can only indicate that the internal resistance of the measured voltage is too high. The biggest factor that affects the voltage measurement results of a pointer multimeter and a digital multimeter at the same frequency is the difference in internal resistance, which is significant and not on the same order of magnitude. When the internal resistance of the measured voltage is small, the difference is not significant. When the internal resistance of the measured voltage is large, the measurement results will differ significantly.
In this situation, it is possible that the measured voltage is not the actual 220V live wire power supply line, or the voltage measured after the live wire passes through a certain electrical appliance, or the voltage of the electrical appliance leakage housing.
Excluding the above possibilities can only indicate that one of the two meters is inaccurate and requires maintenance and calibration.
There is an error in measuring voltage. First, you need to figure out what is the frequency of the measured AC voltage in Hz? Is this voltage a pure sine wave?
All kinds of multimeters currently available on the market have their frequency response range and AC waveform indicated in their manuals when measuring AC voltage. For various ordinary digital multimeters, their frequency response is generally 40-1000Hz, and it is required to be a sine wave (distortion ≤ 1%). The measured AC voltage beyond the above range does not guarantee measurement accuracy. This is because most AC/DC conversion circuits in digital multimeters are designed using low-power dual operational amplifier TL062, which has limited GBW (gain bandwidth product), so digital multimeters cannot measure high-frequency AC voltage (of course, it also depends on whether the voltage divider of the multimeter is compensated).
As for the typical pointer type multimeter (which was first invented by Americans and has been around for 100 years), its internal structure is quite simple, consisting of a high sensitivity meter head, diode rectification, and voltage divider (a few pointer type multimeters add an operational amplifier between the meter head and voltage divider to improve sensitivity). Therefore, the measurement accuracy of this ancient and inexpensive multimeter cannot be compared to that of a digital multimeter. The voltage divider of this type of meter generally does not have capacitance compensation, so its frequency response is generally 40-400Hz.
Two meters measure the same AC voltage with a difference of several tens of volts. First, you need to check their voltage divider resistance network to see if any of the resistors have changed value? If everything is normal, can you also check if the pointer of the pointer type multimeter can point to the zero position? For a digital multimeter, can you check if the calibration potentiometer of its AC voltage range is loose?
By the way, if you want to accurately measure the AC voltage of any waveform, it is recommended to purchase a true effective value (TRMS) multimeter. This type of multimeter can accurately measure the AC voltage of various waveforms such as sine waves, triangular waves, rectangular waves, etc., and is independent of distortion.
This portable true effective value multimeter typically uses dedicated TRMS integrated circuits AD736 and AD737 (produced by Analog Devices in the United States) for the AC/DC conversion circuit, allowing for the measurement of AC voltage in any waveform.
