325
EN-17
equivalent DC value that causes the same amount of
heat to be dissipated in a resistor. True RMS
measurement greatly simplifies the analysis of
complex AC signals. Since the RMS value is the DC
equivalent of the original waveform, it provides a
reliable basis for comparing dissimilar waveforms.
By contrast, many meters use average-responding
AC converters rather than true RMS converters. The
scale factor in these meters is adjusted so that they
display the RMS value for a harmonic-free sine wave.
However, However, if a signal is not sinusoidal,
average-responding meters do not display correct
RMS readings.
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WAVEFORM COMPARISON
Table 1.illustrates the relationship between AC
and DC components for common waveforms, and
compares readings for true RMS meters and average-
responding meters. For example, consider the first
waveform, a 1.414V (zero-to-peak) sine wave. Both
the 325 and RMS-calibrated average-responding
meters display the correct RMS reading of 1.000V(the
DC component equals 0). However, consider the 2V
(peak-to-peak) square wave. Both types of meter
correctly measure the DC component (0V), but your
TM-28E also correctly measures the AC component
(1.000V) The average-responding meter measures
1.111V, which amounts to an 11% error.
Table 1. WAVEFROM COMPARISON CHART
Metered Voltages
Peak Value
AC Components only
Total RMS
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