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One of my books says that if an AC voltage is applied across a DC voltmeter, its reading will be zero. I think that since average value of AC voltage(in a complete cycle) is zero, DC voltmeter measures it as zero. But I couldn't find a reliable explanation on the internet. Is my logic correct ?

Thank you !

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    $\begingroup$ If the period of the alternating current is much shorter than the response time of a dc voltmeter, the dc voltmeter records the average voltage of zero whereas the ac voltmeter will record the root mean square voltage. $\endgroup$
    – Farcher
    Dec 4, 2017 at 7:44

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Think of it his way.

Assume you have a really, really fast DC voltmeter (and really fast eyes as well). In that case you will see the voltmeter go up and down, positive and negative, as the input AC voltage fluctuates.

In reality, a DC voltmeter is not that fast, so it will start to move up a very tiny bit, and then it is being forced to move down again. The end result is that the voltmeter will simply sit at zero.

If the AC voltage is not symmetrical around zero, then the DC voltmeter will still try to follow it, but instead of sitting at zero, it will show the average of the up and down swings of the input. For a symmetrical AC input, the average is zero.

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  • $\begingroup$ If you think it answers your question. please accept my answer. If not, please let me know what else you require. $\endgroup$
    – hdhondt
    Dec 4, 2017 at 22:47

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