RECTIFER TYPE VOLTMETER

 Rectifier type instruments generally use a PMMC movement along side a rectifier arrangement. Silicon diodes are preferred due to their low reverse current and high forward current ratings. Figure 4.14 gives an ac voltmeter circuit consisting of a multiplier, a bridge rectifier and a PMMC movement.

Figure 4.14 AC Voltmeter

Figure 4.15 Average and RMS Value of Current

The bridge rectifier provides a full wave pulsating dc. Due to the inertia of the movable coil, the meter indicates a steady deflection proportional to the average value of the current figure 4.15. The meter scale is usually calibrated to give the RMS (Root mean square) value of an alternating sine wave input.

Practical rectifiers are non liner devices particularly at low values of forward current figure 4.16.

Hence the meter scale is non linear and is usually crowded at the lower end of a coffee range voltmeter. In this part the meter has low sensitivity due to the high forward resistance of the diode. Also, the diode resistance depends on the temperature.

 

      

                                Figure 4.16 Diode Characteristics(Forward)

The rectifier exhibits capacitance properties when reverse biased and tends to bypass higher frequencies. The meter reading may be in error by as much 0.5% decrease for every 1 kHz rise in frequency.

A general rectifier type ac voltmeter is given in figure 4.17. Diode D₁ conducts during the positive half of the input cycle and causes the meter to deflects according to the average value of this half cycle. The meter movement is shunted by a resistor, R_sh, so as to draw more current through the diode D₁ and move the operating point into the linear portion of the characteristics curve. In the negative half cycle, diode D₂ conducts and the current through the measuring circuit, which is in an opposite direction, by passes the meter movement.