Question

With the help of a circuit diagram, explain the working of a full wave rectifier. Depict the input and output waveforms.

Hint:

In a full-wave rectifier:
• Both half cycles of AC are utilized.
• Current through the load always flows in the same direction.
• Output frequency is twice the input AC frequency.
• Efficiency is much higher than that of a half-wave rectifier.

Answer 1

Concept: A rectifier is an electronic device used to convert alternating current (AC) into direct current (DC). In a half-wave rectifier, only one half cycle of the input AC signal is utilized. As a result, the efficiency is low and a large amount of input power is wasted. To overcome this drawback, a full-wave rectifier is used. A full-wave rectifier utilizes both the positive and negative half cycles of the AC input signal and converts them into pulsating DC. Consequently, the output current flows through the load resistor in the same direction during both half cycles of the input voltage. Hence, a full-wave rectifier is more efficient than a half-wave rectifier and produces a higher average output voltage.

Step 1: Draw the circuit diagram of a full-wave rectifier. A full-wave rectifier consists of: 
• A centre-tapped transformer. 
• Two p-n junction diodes \(D_1\) and \(D_2\). 
• A load resistance \(R_L\). 
The circuit diagram is shown below:
 
where 
• \(A\) and \(B\) are the ends of the secondary winding. 
• \(C\) is the centre tap. 
• \(R_L\) is the load resistor. 

Step 2: Explain the working during the positive half cycle. During the positive half cycle of the AC input: 
• End \(A\) becomes positive with respect to the centre tap \(C\). 
• End \(B\) becomes negative with respect to \(C\). 
• Diode \(D_1\) becomes forward biased. 
• Diode \(D_2\) becomes reverse biased. Therefore, only \(D_1\) conducts. The current path is \[ A \rightarrow D_1 \rightarrow R_L \rightarrow C. \] Hence current flows through the load resistor in a particular direction. 

Step 3: Explain the working during the negative half cycle. During the negative half cycle: 
• End \(B\) becomes positive with respect to \(C\). 
• End \(A\) becomes negative with respect to \(C\). 
• Diode \(D_2\) becomes forward biased. 
• Diode \(D_1\) becomes reverse biased. Therefore, only \(D_2\) conducts. The current path is \[ B \rightarrow D_2 \rightarrow R_L \rightarrow C. \] Again the current through \(R_L\) flows in the same direction as in the previous half cycle. Thus, both half cycles contribute to the output current. 

Step 4: Explain why the output becomes pulsating DC. Since current flows through the load resistor in the same direction during both half cycles: 
• The output voltage never changes polarity. 
• Both positive and negative half cycles are utilized. 
• The output consists of successive positive pulses. Therefore, the output is a pulsating direct voltage. 

Step 5: Draw the input and output waveforms. 
Input AC waveform The waveform contains both positive and negative half cycles. 

Output waveform of full-wave rectifier All portions of the waveform remain above the time axis, indicating pulsating DC. 


Step 6: State the advantages of a full-wave rectifier. Compared with a half-wave rectifier: 
• Both half cycles of AC are utilized. 
• Average output voltage is higher. 
• Rectification efficiency is greater. 
• Ripple content is lower. 
• Smoother DC output is obtained after filtering. 

Final Answer: A full-wave rectifier uses two diodes and a centre-tapped transformer to convert both half cycles of an AC signal into pulsating DC. During the positive half cycle diode \(D_1\) conducts, while during the negative half cycle diode \(D_2\) conducts. In both cases, current through the load resistor flows in the same direction. Therefore, the output obtained is a pulsating DC voltage, as shown in the output waveform.