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 R_sh. This ensures high current through diode and its linear behaviour.

       When the a.c. input is applied, for the positive half cycle, the diode D₁ conducts and causes the meter deflection proportional  to the average value of the half cycle.

       In the negative cycle, the diode D₂ conducts and D₁ is 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.