Describe working of A.C. voltmeter using half wave rectifier with neat circuit diagram.

Describe working of A.C. voltmeter using half wave rectifier with neat circuit diagram. 
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1 Answer

Answer :

AC 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.

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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,

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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.

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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.

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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 

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Thus the value of series multiplier can be obtained for a.c. voltmeter as,

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Where Idc is the full scale deflection current..

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