A.C. Voltmeter using Half Wave Rectifier

       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 I_fsd  be 1 mA, hence the sensitivity becomes 1 kΩ /volt. The series resistance R_s 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.

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       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 I_dc  is the full scale deflection current..