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Power Calculation

Discussion in 'General Electronics Discussion' started by eagle76, Aug 23, 2010.

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  1. eagle76


    Aug 23, 2010
    I need some help determining the power from two different (but similar) wave forms. One is a sine wave with valley points at zero and peak points at 25V with a frequency of 1.25 MHz. The other is a sine wave with valley points at 10V and peaks at 35V with a frequency of 1.25 MHz. What is the easiest way to calculate the power and compare these two signals from the scope? Way back I was taught to use the area under the curves to calculate these values but I have forgotten a lot of my math since then.
  2. trobbins


    Jun 15, 2010
    eagle - first things first - describe how you would calculate power when you know voltage? Start with a general case - say a 25VDC voltage.

    Ciao, Tim
  3. davenn

    davenn Moderator

    Sep 5, 2009
    hi there

    have a look along this path....

    take the frequency out of the equation. other than your peak voltage the other thing you need is the load resistance

    Power avg. can be shown by the same method that for a time-varying voltage, V(t), with RMS value VRMS,

    Pavg = (VRMS)squared / Rload

    This equation can be used for any periodic waveform, such as a sinusoidal or sawtooth waveform, allowing us to calculate
    the mean power delivered into a specified load.

    see what figures you come up with

  4. Militoy


    Aug 24, 2010
    It appears to me that Eagle76 is comparing 2 identical sine signals (25 Vpp) – but with a DC offset in one signal. Davenn’s power formula will work for both – as long as they are coupled to a load with unity power factor – and as long as true RMS voltage can be determined.

    For the sine signal with DC offset, the RMS voltage is:

    Vrms = √ (Vdc^2 + Vpk^2/2)

    If the load draws reactive power, the VA product (or V^2/R) needs to be multiplied by the cosine of the phase angle (θ) between the voltage and current in the load.
    Hope this helps…
  5. eagle76


    Aug 23, 2010
    Thank you all for providing me some answers. The question was poorly worded on my part but Militoy has hit closest to what I needed. Thanks again. The value I was looking for is Vrms on the curves I described. From there the power derivation is pretty straight forward.
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