What is Rectification?
Conversion of alternating current into direct current is called rectification. Semiconductor diodes are extensively used for this purpose. There are two common types of rectification:
- Half wave rectification
- Full wave rectification
Half Wave Rectification:
An alternating voltage of Time period T is called input voltage is applied to a diode D which is connected in series with 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 a very low resistance and current flows through R. The flow of current through R causes a potential drop across it which varies is 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 a very high resistance and practically no current flows through R and 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.
Full Wave Rectification:
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 are 180° out of phase with each other. During positive half cycle at point 1, there is 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 like an open circuit. As a result, positive half cycle appears across the output. During the negative half cycle at point 1, there is positive half cycle at point 2. Therefore diode D1 is reverse biased and stop 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.
Full Wave Bridge Rectification:
We have seen that in a half wave rectification, we get the output by only one 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 becomes forward biased and conduct.The current flows through the circuit shown by 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 direction of current flow through the load resistance R is same in both halves of the input cycle. Thus both halves of the alternating input voltage send a undirectional current through the load resistance. The input and output voltages are shown in above figure. The output voltage is not smooth, it has pulses. The out put can be made smooth by using a circuit known as filter.