Question

Draw a circuit diagram of a full-wave rectifier using p-n junction diodes. Explain its working and show the input-output waveforms.

Hint:

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

Answer 1

Concept: A rectifier is an electronic device that converts alternating current (AC) into direct current (DC). A full-wave rectifier utilizes both half cycles of the alternating input voltage. As a result, the output obtained is a pulsating DC whose frequency is twice the frequency of the input AC signal. A centre-tapped transformer and two p-n junction diodes are commonly used to construct a full-wave rectifier.

Step 1: Circuit diagram of a full-wave rectifier.

Here,
• \(D_1\) and \(D_2\) are p-n junction diodes,
• \(R_L\) is the load resistance,
• the transformer provides two equal secondary voltages that are \(180^\circ\) out of phase.

Step 2: 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,
• diode \(D_1\) becomes forward biased,
• diode \(D_2\) becomes reverse biased. Therefore, current flows through \[ A \rightarrow D_1 \rightarrow R_L \rightarrow \text{Centre Tap}. \] A voltage is developed across the load resistance.

Step 3: Working during the negative half cycle. During the next half cycle,
• end \(B\) becomes positive with respect to the centre tap,
• diode \(D_2\) becomes forward biased,
• diode \(D_1\) becomes reverse biased. Current now flows through \[ B \rightarrow D_2 \rightarrow R_L \rightarrow \text{Centre Tap}. \] The direction of current through the load resistance remains the same as during the previous half cycle.

Step 4: Explain why the output becomes DC. Since current through the load resistor flows in the same direction during both half cycles of the input AC signal,
• both halves of the AC waveform contribute to the output,
• the output voltage never reverses polarity,
• a pulsating DC output is obtained. Thus, the alternating input is rectified into direct current.

Step 5: Input and output waveforms. Input AC waveform: \[ \begin{array}{c} V | \;\;\;\; /\backslash \;\;\;\; /\backslash | \;\;\; / \;\;\backslash \;/ \;\;\backslash |__/____\_/____\_______ t | \;\;\backslash \;\;/ \backslash \;\;/ | \;\;\;\;\backslash/\;\;\;\;\backslash/ \end{array} \] Output waveform of full-wave rectifier: \[ \begin{array}{c} V | \;\;\;\; /\backslash \;\;\;\; /\backslash | \;\;\; / \;\;\backslash / \;\;\backslash |__/____\_/____\_/____\_____ t \end{array} \] All portions of the waveform remain above the time axis, indicating pulsating DC. Conclusion: A full-wave rectifier uses both half cycles of the input AC signal. Hence it produces a larger average DC output and has a higher efficiency than a half-wave rectifier.