Re: Average, RMS and Power Factor made easy!

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "35045 by way of Terry Fritz <twftesla-at-uswest-dot-net>" <free0076-at-flinders.edu.au>



On Thu, 18 Jan 2001, Tesla list wrote:

> Original poster: "David Sharpe by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <sccr4us-at-erols-dot-com>
> 
> Terry
> The 0.7071 factor for pk to rms value is ONLY VALID for sinusoidal periodic
> waveforms.  Square waves are a different fish to fry...

The 0.7071 factor is valid for a very small subset of waveforms. You are
right to warn people who try to use it for non-sinusoids since this is a
very common mistake. You are unlucky however, since the 50% squarewave
with the low-level set to 0 V will belong to that subset for which this
factor is correct. You will find this to be true if you calculate the RMS
value for yourself like I did in my earlier post but you will also find
that if you look it up.

> I'll look in my Yokogawa references and see what the rms equations are for
> a periodic square wave...  I believe the HP meter over the Tek any day of the
> week in this case.

I think that's a bit unfair as I have used Tektronix DSO equipment and I
have the utmost respect for it. In this case it was correct too.. It turns
out (from a later post) that the HP meter was AC coupled and if you
calculate the RMS value after subtracting the 2.5 V DC component you end
up with what the HP meter reads. Both meters are correct if you take
coupling into account.

> Regards
> 
> Dave Sharpe, TCBOR
> Chesterfield, VA. USA
> 

Have fun,
Darren Freeman