The conversion of alternating current into the direct current is called rectification. Semiconductor diodes are extensively used for this purpose.
There are three common types of rectification:
An alternating voltage of Time period T is called input voltage is applied to a diode D which is connected in series with a load resistance R. In this method only one half of alternating current cycle is converted into direct current.
During the positive half cycle of the input alternating voltage during the time interval 0 → T/2, the diode D is forward biased, so it offers very low resistance and current flows through R. The flow of current through R causes a potential drop across it which varies in accordance with the alternating input.
During the negative half cycle of the input alternating voltage during the time interval T/2 → T, the diode D is reverse biased, so it offers very high resistance and practically no current flows through R and the potential drop across R is almost zero. The same events repeat during the next cycle and so on. The current through R flows in only one direction which means it is a direct current. However this current flows in pulses. The voltage which appears across load resistance R is known as output voltage.
The circuit consists of two diodes and a center tap transformer. When the center tap is grounded the voltage at opposite ends of the secondary coil is 180° out of phase with each other. During the positive half cycle at point 1, there is a negative half cycle at point 2. Therefore diode D1 is forward biased and allows the current to flow through the junction while diode D2 is reverse biased and acts as an open circuit. As a result, the positive half cycle appears across the output. During the negative half-cycle at point 1, there is a positive half cycle at point 2. Therefore diode D1 is reverse biased and stops conducting, while diode D2 is forward biased and conducts, hence we get another positive half cycle across the output, through D2. Thus during these half of A.C. input, the current flows in the same direction through the load resistance. The output voltage across the load resistance is rippled DC containing both the half cycles. To get smooth DC a suitable capacitor is connected in parallel with the resistance R L.
We have seen that in a half-wave rectification, we get the output by only one half of the alternating output voltage. The other half cycle is blocked and we get no output. However, both halves of the output voltage cycle can be utilized using full-wave rectification. Its circuit consists of four diodes connected in such a way to form a bridge.
During the positive half cycle i.e., during the time 0 → T/2, the Terminal A of the bridge circuit is positive with respect to terminal B. Now diodes D1 and D3 become forward biased and conduct. The current flows through the circuit shown by the arrow.
During the negative half cycle i.e., during the time interval T/2 → T, the terminal A of the bridge circuit is negative and the terminal B is positive. Now diodes D2 and D4 become forward biased and conduct. The current flows through the circuit shown by arrows. If we compare fig (a) and (b), it can be observed that the direction of current flow through the load resistance R is the same in both halves of the input cycle. Thus both halves of the alternating input voltage send a unidirectional current through the load resistance. The input and output voltages are shown in the above figure. The output voltage is not smooth, it has pulses. The output can be made smooth by using a circuit known as a filter.
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