Page 862 - Basic Electrical Engineering

An analog multimeter is basically a permanent magnet moving coil
galvanometer, For measurement of current, a selector switch is operated in
the current measurement mode. As shown in Fig. 11.33 (a),

Figure 11.33 (a) Multi-range ammeter; (b) multi-range voltmeter

a number of low-value resistors, called shunts, are connected in parallel with
the instrument through a range selection selector switch. The required range

can be selected by moving the selector switch to a particular position as
shown in Fig. 11.33 (a).

The use of the multimeter as a multi-range voltmeter has been shown in
Fig. 11.33 (b). The rectifier diode D conducts during the positive half cycle
1
of the input waveform and causes the meter to give deflection according to
the average value of the half cycle. Diode D prevents a reverse voltage
2
appearing across the diode D during the negative half cycle of the input
1
voltage. The calibration of the scale is such that the same scale is used for

both dc and ac measurements. The rms value of the sine wave is = .

The rms value of the half-rectified wave = . The rms value of the half-

rectified wave is 0.45 times the rms value of the full sine wave

voltage. In order to have the same deflection on dc and corresponding ac