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552 ⏐⏐⏐ SINUSOIDAL ALTERNATING WAVEFORMS
V m
v movement
� 2�
V dc = 0.318V m
FIG. 13.64
Half-wave rectified signal.
FIG. 13.65
Electrodynamometer movement. (Courtesy of
Weston Instruments, Inc.)
revealing that the scale indication is 1.11 times the dc level measured
by the movement; that is,
Meter indication � 1.11 (dc or average value)
full-wave (13.38)
Some ac meters use a half-wave rectifier arrangement that results in
the waveform of Fig. 13.64, which has half the average value of Fig.
13.63(b) over one full cycle. The result is
Meter indication � 2.22 (dc or average value)
half-wave (13.39)
A second movement, called the electrodynamometer movement
(Fig. 13.65), can measure both ac and dc quantities without a change in
internal circuitry. The movement can, in fact, read the effective value of
any periodic or nonperiodic waveform because a reversal in current
direction reverses the fields of both the stationary and the movable
coils, so the deflection of the pointer is always up-scale.
The VOM, introduced in Chapter 2, can be used to measure both dc
and ac voltages using a d’Arsonval movement and the proper switching
networks. That is, when the meter is used for dc measurements, the dial
setting will establish the proper series resistance for the chosen scale
and will permit the appropriate dc level to pass directly to the movement.
For ac measurements, the dial setting will introduce a network
that employs a full- or half-wave rectifier to establish a dc level. As discussed
above, each setting is properly calibrated to indicate the desired
quantity on the face of the instrument.
EXAMPLE 13.24 Determine the reading of each meter for each situation
of Fig. 13.66(a) and (b).
d’Arsonval
movement
rms scale
(full-wave
rectifier)
Voltmeter
Electrodynamometer
movement
rms scale
Voltmeter
(1)
(a)
dc
+
(1)
(b)
20 V
–
FIG. 13.66
Example 13.24.
(2)
ac
+
Vm = 20 V
–
dc
+
+
25 V
–
e = 15 sin 200t
–
(2)