RE: [CAD_CAM_EDM_DRO] Re: What is Rms?

Graham:

I believe that the only voltage that will give you correct "effective" voltage number by averaging it to get the "effective" voltage, is a square wave, or strauight DC, or changing DC which changes as if it were a square wave(very quick transitions from voltage to voltage).

What I think most miss in the RMS method is that it does NOT just "invert the negative portion of the wave". By squaring the instantaneous voltage, averaging all of the squares, and THEN taking the square root, you get a larger number than you would get by, say, rectifying a sine wave to get it all "positive" and then averaging the sine wave.

Andy Wander

-----Original Message-----
From: CAD_CAM_EDM_DRO@yahoogroups.com on behalf of Graham Stabler
Sent: Thu 11/24/2005 3:10 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Cc:
Subject: [CAD_CAM_EDM_DRO] Re: What is Rms?



My short version

If a voltage is DC or even if it is always positive and pulses then
you can get its effective voltage by taking an average. The average
of a sinewave (AC) which spends half of its time positive and half of
its time negative is zero but it does carry power!

The RMS is the DC voltage you would need to supply to provide the same
power as the given AC signal. The RMS of a sinewave is the amplitude
divided by sqrt(2). The amplitude is half of the peak to peak
voltage. sqrt(2) is 1.41. Conversely if you want to know the peak
(say if you intend to recify it then multiply the RMS by 1.41.

So if you have a sinewave of 100v it has an rms of 100/1.41 = 70.9v as
this is the equivalent DC voltage you can find the power disipated in
a load by doing P = IV or I^2.R or V^2/R

And if you were building a PSU and your transformer halved the applied
voltage you might get 120v(rms)/2 after the transformer and once
recified 60X1.41 = 84.6v assuming you have a nice big smoothing
capacitor anyway.

Graham





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