(a) Need for a Rectifier:
Moving coil meter is sensitive. It can accurately measure direct current. When alternating current is applied to this meter it will not give indication.
It is worthwhile to remember that the average value of a pure sine wave is zero. Hence there will be no deflection in the moving coil ammeter when alternating current is applied to it. If we can by some arrangement circulate some average current proportional to the alternating current applied, through the meter it will indicate a reading. A rectifier is used for this purpose. Hence moving coil ammeter provided with a rectifier to rectify the alternating current, is called a Rectifier Type of Ammeter.
(b) The Rectifier :
A diode can be used as a rectifier. Early instruments used copper oxide and selenium diodes for rectification. Germanium and Silicon diodes are now used as the rectifier elements. The copper oxide and selenium diodes have low peak inverse voltages of 2 V, and 10 V respectively. They can handle only limited amounts of current. So the copper oxide and selenium diodes have become obsolete.
In the case of germanium diodes peak inverse voltage is about 300 V. They have a current rating of about 100 mA. Silicon diodes of the low current type have a peak inverse voltage of about 1000 V. Their current rating is of the order of 500 mA. Germanium diodes are preferred when low voltage drop in the conducting direction is required. Silicon diodes are preferred when large peak inverse voltage and low temperature coefficient are required.
The voltage vs current characteristic of an ideal rectifier element along with its resistance variation during forward and reverse bias conditions is shown in Figure.
Figure (a) Volt amp. Characteristic of a Rectifier Under Ideal Conditions
Figure (b) Variation of Resistance Under Forward and Reverse Bias (IDEAL)
From the characteristics shown above the diode or the rectifier allows current in the forward direction and allows no current in the reverse direction. Also from the characteristics the resistance of the rectifier is constant in the forward direction. In the reverse direction the resistance is infinite. (There will be high resistance; approximately it is assumed as infinite).
The practical characteristics of the rectifier diode are shown below under Figure (a) & (b).
Figure (a) Volt.amp. Characteristic of Rectifier Diode
Figure (b) Variation of Resistance Under Forward Diode, and Reverse Bias Conditions (IDEAL)
From the practical characteristics it can be found that the resistance of the rectifier will not be constant during the forward bias. It can also be seen that the current in the reverse biased condition is also not zero. Hence the resistance of the rectifier is a function of current. It is also to be noted that the resistance of the rectifier changes with temperature.
A half wave or full wave rectifier can be used to give the current to the moving coil ammeter used as a rectifier type of ac ammeter.
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Multimeters