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If someone measures the
value of the AC voltage
coming out of a
transformer using an
oscilloscope and says it
is 20 volts peak to peak
and we use a voltmeter
to confirm this we will
find that the meter
reads only 7.07 volts.
This is because the
scope measures peak to
peak values and the
meter measures RMS
values.
In figure 1 the 'scope
displays the peak value.
The peak to peak voltage
is twice this. For
example if the peak is
10 volts then the peak
to peak is 20 volts.
When using a meter to
measure the same AC
voltage a different
value is obtained. This
is because, as we said,
meters measure RMS
values.
A Root Mean Square (RMS)
voltage gives the same
heating effect as a DC
voltage of the same
value. See figures
2 and 3. Both
thermometers show the
same temperature when
the resistors are heated
by the current passing
through them.
RMS values can be
converted to peak to
peak values and
vice-versa.
RMS values times 1.414
equals the Peak value.
Peak to Peak is twice
this. 7.07 volts RMS
times 1.414 and then
doubled is 20 volts, the
Peak to Peak value.
Peak values times 0.707
gives the RMS value.
Don't forget that Peak
is half the Peak to
Peak.
20 volts Peak to Peak is
10 volts Peak. 10 volts
Peak times 0.707 equals
7.07 volts RMS.
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