Re: [CAD_CAM_EDM_DRO] Re: Question on Power Supply

----- Original Message -----
From: "Jon Elson" <elson@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Thursday, June 09, 2005 1:08 PM
Subject: Re: [CAD_CAM_EDM_DRO] Re: Question on Power Supply


> Polaraligned wrote:
>
>>1.414 is the square root of 2, and is how we get the RMS value of
>>a peak to peak voltage. The RMS value is also called the "effective
>>voltage". Your 120vac household outlet is really 170v peak to peak.
>>
> (minute quibble here - it is 170 V peak (measuring from zero to EITHER
> the positive or negatuve peak, thus 340 V Peak-to-Peak.)
>
>>This RMS value is what most meters read.
>>
> And, most cheap meters take the peak voltage and then multiply by a
> scaling factor
> of 0.707 This works fine for true sine waves, but if the wave shape is
> distorted, or
> not at all sinusoidal (like square waves, pulses, etc.) the cheap meter
> can give wildly
> erroneous readings. For instance, the TRUE RMS value of a 120 V perfect
> square
> wave is 120 V. A cheap meter will tell you it is about 85 V! (120 V
> peak * .707)
> Worse, a narrow 120 V pulse will ALSO read about 85 V, although the true
> RMS
> value would be, say, 12 V if it was a 10% duty cycle pulse, or 1.2 V for
> a 1% pulse.
> Way wrong! A true RMS meter will give the correct reading all these
> cases.
>
> Jon

There is a further complication. Cheap digital meters read peak voltage
and
apply the necessary multiplication factor to get the desired RMS reading.

For movement meters it is different. The mechanical mechanism of movement
meters measures the average current flowing through the movement's coil.
The average
is not the same thing as RMS. Thus the multiplication factor is different
than for digital
meters.

In the situation where you are measuring a narrow pulse. The digital meter
will read the peak value of the pulse and apply the multiplication factor
and
give it's reading. The movement meter will average the pulse and apply
it's multiplication factor. The two readings will be very different. The
movement meter will give a better approximation of the RMS value than
the digital meter.

This site discuss the difference;
http://www.testproductsintl.com/rms.html

Pete.