Basic Rectifier Type A.C. Voltmeter

     
        A general rectifier type a.c. voltmeter is shown in the Fig. 1.
  
       The diodes D1 an D2 are used for the rectifier circuit. The diodes show the nonlinear bahaviour for the low currents hence to increase the current though diode D1, the meter is shunted with a resistance Rsh. This ensures high current through diode and its linear behaviour.
       When the a.c. input is applied, for the positive half cycle, the diode Dconducts and causes the meter deflection proportional  to the average value of the half cycle.
       In the negative cycle, the diode D2 conducts and Dis reverse biased. The current through the meter is in opposite direction and hence meter movement is bypassed.
       Thus due to diodes, the rectifying action produces pulsating d.c. and the meter indicating the average value of the input.

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