2.9 Measuring Power in
Electronic Wattmeter
Chapter 2
Although the previous circuit is the preferred connection the
error
associated with a more normal connection is
comparatively
small.
For example with a 3-ohm loudspeaker or load the expected
error due to normal connection of the current measuring circuit
of
60mQ is:
6
~~Q
X 1
00%=
2%
This is probably low compared with errors introduced by normal
load or loudspeaker connection cables.
For higher impedance loads the error is even
smaller.
Audio Frequency Circuits
Power outputs associated with audio amplifiers are often quoted
as 'Watts rms'. If it is assumed that 'Watts rms' has the same
meaning
in
relation to watts as volts rms does to volts, then
'Watts
rms'
means:
the 'Square root of the mean value of the square of the
instantaneous watts'.
It
is
by
no
means certain that this
is
always intended when 'watts
rms' is used.
It may sometimes
be
employed to denote 'real' power
as
opposed to 'reactive' or 'imaginary' power but,
as
such, is a
misnomer.
It
may
be
noted that
in
the strict sense Watts rms does not
correspond to any useful physical
reality and its relationship
to
normal Watts is dependent
on
the waveform.
The table
in
fig 2.9 shows various basic voltage waveforms
all
drawn to the same scale for the 1 volt
rms
and hence for 1 watt
in
1 ohm together with the associated values of Volts peak
and
Watts
rms.
1 8
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