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The DC voltmeter measures the average value of the waveform right?

I have read that ac voltmeters (high end) measure the RMS value based on heating effect while DC voltmeters measure the average value of the waveform.

My question is :

When i calculate the average value of the waveform i get 3.33V (10/3) assuming its a nonperiodic input signal,but the actual answer is 10V WHY???

enter image description here

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  • \$\begingroup\$ You mean: "assuming it IS PERIODIC". Why do you think the answer is 10 V ? Because the average of the waveform is 3.33 V. The peak value is 10 V but that's not what you're asking. \$\endgroup\$ Oct 1 '15 at 12:55
  • \$\begingroup\$ when i measure it i get 10v dont know how?! and does the dc voltmeter measure the rms/average/peak , i think it should be average ,correct me if i am wrong. \$\endgroup\$ Oct 1 '15 at 13:06
  • \$\begingroup\$ Tell me how the dc voltmeter measures for both PERIODIC AS WELL AS NON PERIODIC CASE pls \$\endgroup\$ Oct 1 '15 at 13:09
  • \$\begingroup\$ Only one way to find out if your meter shows the average: look in the datasheet. You're using a signal with (for a DC meter) high frequency components. It depends on the DC meter how it handles that. \$\endgroup\$ Oct 1 '15 at 13:29
  • \$\begingroup\$ A periodic signal is a REPEATING signal, so you show me one period, I know what the other periods are (the same). A non periodic signal can be anything so I cannot say anything about it, will it be 0 V, -10 V or + 12345 V ? I do not know, it can be anything, no sense to talk about it then. Oh but maybe you mean non-periodic in the sense that it is a RANDOM signal with a 10 us sample rate but that the average is always 3.33 V ? \$\endgroup\$ Oct 1 '15 at 13:29
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A DC voltmeter measures the average voltage at its terminal hence it doesn't read anything when a true AC signal is applied. In your case, with the signal shown a DC voltmeter will measure the average and that is 2.5 volts because, as Brian Drummond points out, the duty cycle is 25%. This means the correct answer is "B".

Under no circumstances is the voltage measured by a DC voltmeter 10V. An RC low pass network is very similar to how the DC voltmeter gradually acquires the average voltage of the input: -

enter image description here

The example shows an input square wave with peak of 1V and 50:50 duty cycle. The voltmeter (aka capacitor) gradually attains an average voltage that is 0.5 volts. Note that there is also a slight ripple but, with a conventional meter and a square wave of duty cycle 40 us, the ripple will be negligible.

So, you have either specified the question incorrectly or the answer you were given is incorrect (or I am wrong).

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  • \$\begingroup\$ The DC voltmeter could measure 10 V if it would have a peak detector. Or by coincidence (very slim chance though) it could sample at the time that the signal is 10 V. This is more a theoretical possibility, in practice this should (almost) never happen. My bet is on the DC meter having a peak detector. \$\endgroup\$ Oct 1 '15 at 13:30
  • \$\begingroup\$ Then it's not a DC voltmeter any more - it's a peak detector. \$\endgroup\$
    – Andy aka
    Oct 1 '15 at 13:38
  • \$\begingroup\$ I never said it was a proper DC Voltmeter, maybe it's one from China, then you never know what you get ;-) \$\endgroup\$ Oct 1 '15 at 13:43
  • \$\begingroup\$ NB the actual duty cycle shown is 25%... \$\endgroup\$ Oct 1 '15 at 13:43
  • \$\begingroup\$ @BrianDrummond well spotted I shall amend my answer. Thanks. \$\endgroup\$
    – Andy aka
    Oct 1 '15 at 13:49

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