If I have a DC voltage source, I can connect a voltmeter across it and get the voltage value. But how do I find out the AC voltage of a voltage source? What will happen if I connect a voltmeter across an AC voltage source? Will the Voltmeter show 0 volts?
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2 Answers
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If you connect a DC voltmeter across the AC mains you will read zero because most DC voltmeter measure the average value of the voltage measured and the average value of a sinusoidal wave is zero. So you will need to use an AC voltmeter to measure the AC mains voltage.
The typical analog AC voltmeter uses a full wave bridge rectifier and measures the average value of the full wave rectified voltage. The meter scale is usually calibrated to give an RMS value. For more details see: https://www.eeeguide.com/ac-voltmeter-using-rectifiers/
Hope this helps.
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Some voltmeters move the needle by using the magnetic field around a current-carrying conductor. For example, two small metal plates or a spiral of fine wire is pushed apart, with the pushing force proportional to the voltage.
With DC the "push" is pretty constant, so an indicator moved by the push stays still and is readable.
With AC the push is constantly changing. Imagine AC with a very low frequency (say, 1/10 Hertz): the indicator would move to match the voltage in effect at each moment (inertia and subtler issues may be small enough to ignore). As the frequency increases, the indicator tries to move much more (and faster) and inertia, air resistance, and many other things become important. At some point the needle can't keep up. Even if it could, your eyes couldn't. So the needle bounces around the average value, which for normal AC is 0V. The higher the frequency the smaller the bounce, because there's not enough time to move very far before being pushed back the other way.
Even a frequency like 60 Hertz is going to be hard to measure with a magnetically-driven needle, or to see. So it will look more and more like the needle is just sitting at zero -- much like can't see the vibrations on a typical tuning fork.
Other voltmeters use different tech, but the issue of keeping up with a rapidly-changing instantaneous voltage is still there. You can see all this more clearly on an oscilloscope, if you adjust its sweep rate to a multiple of the AC frequency.
I'll bet a lot of cheap voltmeters merely put in a diode, which clips or inverts the negative half-cycles. In the latter case, a magnetically-driven needle should show the RMS value (though the numbers written on the dial might be inflated to correct for that).
