In most cases, traditional AC voltmeters—especially rectifier-type analog voltimeter—can’t be used to measure DC voltage. However, some special AC voltmeters, such as electrodynamometer meters or moving-iron meters, can measure DC voltage, and the reading will match the actual DC value.
In most cases, traditional AC voltmeters—especially rectifier-type analog voltimeter—can’t be used to measure DC voltage. However, some special AC voltmeters, such as electrodynamometer meters or moving-iron meters, can measure DC voltage, and the reading will match the actual DC value.
Detailed Explanation
Why an ordinary AC Voltmeter can‘t measure DC
Traditional AC voltmeters (like permanent-magnet meters) operates based on measuring the average value of the voltage.
They typically use a rectifier circuit (such as diodes) to convert the AC signal into a pulsating DC signal, and then measure the average value of this pulsating DC.
Since AC voltage has both positive and negative halves, they cancel each other out during the rectification process. As a result, the average output becomes close to zero. If a DC voltage is applied to the AC range of such a meter, the pointer will often remain stationary or show highly inaccurate readings. That’s because the meter is designed to respond only to the AC part of the signal, while the DC part is effectively removed by the rectifier circuit.
Why some AC Voltmeters can measure DC?
Some AC voltmeters, like electrodynamometer meters or moving-iron meters, work in a different way.
These instruments do not rely on the average value of the voltage. Instead, they measure voltage based on the root mean square (RMS) value or the thermal effect of current. Their scales are designed according to the square root of the mean of the current squared (√i²).
For DC voltage, the RMS value is actually the same as the DC value itself. So if you connect a DC voltage to this kind of AC meter, it can still give an accurate reading, and the value shown will match the DC voltage.
Although these instruments are generally more expensive than permanent-magnet types, they are widely used in industry for both AC and DC measurements because they have simple structures and relatively small measurement errors, and they can measure both AC and DC.
Conclusion
Ordinary AC analog voltmeters cannot measure DC voltage.Electrodynamometer or moving-iron AC meters can measure DC, and the reading will be equal to the DC voltage applied.
The main reason is how the meter works inside. Some meters use a rectifier circuit and measure the average value, which doesn’t work for DC. Others rely on the heating effect or RMS value, so they can measure both AC and DC.
