Re: [CAD_CAM_EDM_DRO] Re: What is Rms?

On Sat, 2005-11-26 at 07:05 +0000, Joel Hagen wrote:

> RMS stands for root mean square, it is used to compute the real value
> of what you are measuring.
> If you run 100% for 50% of the time and 0 for the other 50% you have a
> effective rms of 50% of what you are measuring.

Actually, that's not accurate. The RMS value of a 50% duty cycle square
wave is 0.7071 (sqrt(1/2)). You forgot to take the square root :) The
DC average voltage is 50% of peak though.

> It is used in math analysis to determine the real effective value of
> what you are measuring, power, voltage, amps etc.
> That is why there are voltage meters that are true rms, and those that
> are not. In ACV it makes a big difference what the freqency is, ie
> 60hz or 400hz. Ever wonder about the great stuff from the airforce
> that you can't use.

Also, not quite right. It makes no difference what frequency a sinewave
source is, the RMS value is still 1/sqrt(2)* peak. The reason that
cheap meters aren't accurate for other frequencies is that they assume a
60Hz sinewave, so they average several readings with timing appropriate
for a 60Hz source. True RMS meters can either use much faster sampling
(and actually do RMS calculations, rather than assuming a sinewave and
taking the 1/sqrt(2) shortcut), or can use an analog front end that
gives RMS output (to either the analog or digital front end).

- Steve