FreeLists / si-list / [SI-LIST] Re: AC Waveform question

["Doug McKean" <dmckean@xxxxxxxxxxxxxxx>]

Doug,

The meter on the ac setting should read an rms value
of the peak value or 0.707 volts ac.  This is true of all
pure sinusoidal waveforms.  Non- sinusoidal waveforms
do not follow the 0.707 rule.

Fluke makes a "true rms" meter which means it's made
to measure the rms value of sinusoids and non-sinusoids.
So, you can accurately use the "true rms" fluke (or any
other fluke) to measure the rms value of any type of
waveform with confidence.

Other meters assume a sinusoidal waveform by using
diodes to rectify the incoming signal to grab the peak
value. So, measuring a non-sinusoidal waveforms with
a meter that's not true rms will produce inaccurate rms
results.

In the for what it's worth department, in a perfect world,
measuring an ac signal with a meter on the dc setting
would read the dc bias of the ac signal.

For future reference ....

If T=period of waveform, Vm=max voltage of signal,
and (Vm)^2 is Vm squared, then rms is calculated by

Yrms = sqrt[ 1/T * Integral (from 0 to T) (Vm)^2*dt]

Effective values of Vm*sin(w*t) is a/(sqrt2) = 0.707*Vm.
From this equation it should be seen that non-sinusoids
do not reduce to 0.707*rms but reduce to something else.
For instance, a sawtooth waveform reduces to something
like Vm/sqrt(3) = 0.577*Vm (if I remember correctly).

Regards, Doug McKean


----- Original Message -----
From: "Doug Brooks" <doug@xxxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Monday, October 07, 2002 9:04 AM
Subject: [SI-LIST] AC Waveform question


>
> Hi all,
>
> I know I must have learned this in school, but now I'm not sure of the
answer!
>
> Assume I have a full-wave rectified sine wave of low frequency. Assume the
> magnitude is 1.0 Volt peak (zero to peak). Assume I apply this wave form
> across an ordinary 1 Volt analog meter. What does the meter read?
> The RMS value (.707 Volts)  or
> The average value (.636 Volts)
>
> Doug
>
>
>
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