The a.c. voltmeter using half wave rectifier is
achieved by introducing a diode in a basic d.c. voltmeter. This is shown in the
Fig. 1.
The diode D conducts only during positive half
cycle. Let us compare the sensitivities of d.c. and a.c. voltmeters.
The sensitivity of d.c. voltmeter is,
Let Ifsd be 1 mA, hence the sensitivity becomes
1 kΩ /volt. The series resistance Rs is 10 kΩ hence the 10 V d.c. input
would cause exactly the full scale deflection, when connected with prpoper
polarity.
Let purely sinusoidal input of 10 V r.m.s is applied.
Now the rectified d.c. is pulsating d.c. hence
meter will deflect proportional to the average value.
.
But the diode conducts only for half cycle and
meter movement is bypassed for another cycle. Hence it responds to half the
average value of the a.c. input.
Thus pointer will deflect for full scale if 10
V d.c. is applied and 4.5 V when 10 V r.m.s. sinusoidal input is applied.
Note: thus the a.c. voltmeter is less sensitive
than d.c. voltmeter.
Thus the value of series multiplier can be
obtained for a.c. voltmeter as,
Where Idc is the full scale deflection current..