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From: Jeff Barfett (no email)
Date: Tue May 29 2007 - 20:39:54 EDT
Gil,
Hopes this helps explain it - not my words but worked for me.
Peak RMS Voltage:
RMS voltage is absolutely the most common way to measure/quantify AC
voltage. It is also the most useful. Because AC voltage is constantly
changing and is at or near the highest and lowest points in the cycle for
only a tiny fraction of the cycle, the peak voltage is not a good way to
determine how much work can be done by an AC power source (e.g. your
amplifier, a wall outlet in your house...). DC voltage is constant. Its
voltage level can be plugged directly into the formulas for power and you
will get an accurate image of its ability to do work. RMS voltage will give
you the same ability to predict how much work will be done by an AC voltage.
The RMS voltage of a pure sine wave is approximately .707*peak voltage. If
you read voltage with a voltmeter you are generally given the RMS voltage of
the wave form. Some meters display an 'average' voltage which is very close
to RMS. When reading voltage with a voltmeter, the display indicates the RMS
or average voltage not the peak or peak-peak voltage.
If the waveform isn't a pure sine wave (like a square wave or a signal with
mixed sine waves of different frequencies), multiplying the peak times .707
will not give an accurate RMS value and therefore will not give an accurate
indication of the work that the waveform can produce when driving a load.
For more complex signals, you need a meter that will calculate the RMS value
from a set of samples taken at regular intervals.
-----Original Message-----
From: Gil Johnson [mailto:]
Sent: Tuesday, May 29, 2007 5:52 PM
To: TWL
Cc:
Subject: T&T: 12 volt Charging options
Jeff, I'm confused. 110VAC is 110-120VAC, meaning that it's peak to peak
voltage is 110. Use any AC voltmeter and you'll likely read something
between 110 and 125 VAC. If they are suggesting a different method of
measuring the voltage, it would be helpful to understand. Not having that
point of reference would make it impossible to troubleshoot a suspected
voltage problem.
"In order to meet its ratings, 164 peak volts are required. A battery
charger uses only the top portion of the input sinewave. Therefore, small
variations in peak voltage results in large variations in the amount of the
waveform that the charger has to work with. Standard public power of 117V
has a peak voltage of 164V."
Gil
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