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What is AC voltage, Vac?

  1. Jul 22, 2013 #1
    So far i have learn Vrms, Vdc and Vac, and this is what i understand

    Vrms is the power of the voltage,
    Vdc is the average voltage.

    And what is Vac actually? The sine wave? another word for R.M.S, or it is the value of AC voltmeter can measured out, I am not sure where i find this equation,

    Vac = √(Vrms^2 - Vdc^2)
    And in a stimulation of Multisim software, i found that the AC voltmeter give almost the same answer computed from the equation above, and i hardly found a source mention about this Vac stuff
  2. jcsd
  3. Jul 22, 2013 #2
    Hello NV...

    Sorry to say these terms are NOT exact - and often cause confusion and real world errors even with experienced engineers, technicians and electricians!

    First Vrms is NOT the power of the voltage. V RMS (root mean square) is a way we measure / represent voltage - that provides a little more meaning, and easier to calculate with than Peak, or Peak-to-Peak. Since V RMS x I RMS x Cos Phi= Power ( where cos Phi is the CoSine of the phase angle between the voltage and the current) - I know that may be TMI for now.

    Also -- RMS is one of many types of "Average" --- Average is very ambiguous. ( Median, Norm, Mean, RMS = all forms of averages) - confused yet?

    As a generalization the RMS value ( for power ) can the thought of as an average representing the useful portion of the waveform - wordy I know. RMS also can be applied to any waveform, and is a pretty specific calculation. However, to confuse the issue, when making measurements any reading that is not listed as "TRUE RMS" is suspect, because a metering device may state RMS - but is not calculating RMS properly... ugh... getting off track.

    So Vrms, Vdc, Vac -- Actually only the Vrms in this case is specific.

    Vac - is a generic ( ambiguous ) term for Voltage of an Alternating "Current" Waveform - the Current term is misleading as well - but that is what the world has settled on!

    -- V ac does not define how this is measured. In an industry - power for example, Vac is essentially ALWAYS - RMS, but ONLY for the primary carrier if 50 or 60 Hz ( eg not including harmonics ), or if listed otherwise.

    The best way to clarify is to include the RMS, P-P (peak to Peak) in the units - examples:

    Household Power in the USA would be Vac = 120 V RMS ( ideally a pure sine wave, no harmonics or DC offsets etc.)

    A voltage signal for audio might be Vac = 2 V P-P

    Vdc - a little more defined than Vac, but still can cause confusion particularly when the overall voltage(signal) is really the SUM of Direct Current ( non-alternating) and AC (Alternating) signals.

    As for the formula it is really an RMS issue, and would be good to try to separate RMS from Voltage, it applies to the voltage case-but it is really a Mathematical Construct ref Wikipedia ( https://en.wikipedia.org/wiki/Root_mean_square )
  4. Aug 18, 2016 #3
    Vrms is .707 x V peak.
    It is the dc working voltage of an ac signal.
    In other words 10 volts peak draws the same amount of power as 7.07 vdc across the same load.
    Vdc is a direct current voltage while Vac is an alternating current voltage that may be expressed in several ways. Vrms, V peak, V peak to peak.
    If undefined then Vac normally means Vrms, but that the electricians fault.
    V ave is V peak x .636
    Last edited by a moderator: Aug 18, 2016
  5. Aug 18, 2016 #4


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    That is only true for a pure sinusoid. RMS is a more general measurement that applies to any waveform.
  6. Aug 18, 2016 #5


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    To give the OP and other posters some examples, the ratio of peak-to-RMS is different for a triangle wave than it is for a sine wave. For a triangle wave, it's ##V_{pk} = \frac{1}{\sqrt{3}} V_rms##. For a pulse waveform, the conversion factor depends on the duty cycle, and is ##\frac{1}{\sqrt{D}}##. For gaussian noise, there is no peak voltage (it's infinite), but the RMS voltage is given the by the standard deviation of the distribution. See the "Noise Characteristics" section of this white paper by TI for a better explanation. This should convince you that RMS and peak measurements are only interchangeable in AC analysis, not in general.
  7. Aug 19, 2016 #6


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    The only way to find the real RMS value of many signal is to actually measure the Volts at a high enough sample rate (faster than the Nyqvist rate) and doing the calculation with with the samples, averaging over a suitable (long enough) period of time. Vrms will give you, the Power delivered into a Resistive load using
    That's a fairly trivial operation for frequencies below 'RF frequencies', using available digital processors.
  8. Aug 24, 2016 #7
    oh. it is so important to know about Vrms, Vdc and Vac.
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