(a) Characteristics of a p-n Junction Diode for Rectification
Step 1 (Governing Principle): [1 Mark]
A p-n junction diode acts as a unidirectional switch. Its suitability for rectification is based on its asymmetric conduction property:
- Forward Bias: When the p-region is at a higher potential than the n-region, the depletion layer narrows, offering low resistance, allowing current to flow ($I \approx mA$).
- Reverse Bias: When the p-region is at a lower potential than the n-region, the depletion layer widens, offering very high resistance, effectively blocking current flow ($I \approx \mu A$).
Step 2 (Working Mechanism): [1 Mark]
Because the diode conducts only when forward-biased, it converts alternating current (AC) into unidirectional pulsating direct current (DC). The mathematical condition for conduction is:
$$V_{applied} > V_{barrier}$$Where $V_{barrier} \approx 0.7V$ for Silicon. This allows the diode to "clip" the negative half-cycles of an AC input signal.
(b) Full Wave Rectifier
Step 1 (Circuit Diagram & Components): [1 Mark]
A full-wave rectifier uses two diodes ($D_1, D_2$) and a center-tapped transformer. The input AC voltage is applied to the primary coil, and the output is taken across the load resistor $R_L$ connected between the center tap and the common junction of the diodes.
[Diagram Description: Center-tapped transformer connected to two diodes $D_1$ and $D_2$ at the ends of the secondary coil, with $R_L$ connected to the center tap.]
Step 2 (Step-by-Step Working): [1.5 Marks]
- Positive Half-Cycle: Point A is positive relative to the center tap. Diode $D_1$ is forward-biased and conducts, while $D_2$ is reverse-biased. Current flows through $R_L$ in a specific direction.
- Negative Half-Cycle: Point B is positive relative to the center tap. Diode $D_2$ is forward-biased and conducts, while $D_1$ is reverse-biased. Current flows through $R_L$ in the same direction as the first half-cycle.
Step 3 (Final Conclusion): [0.5 Marks]
The output voltage across $R_L$ is unidirectional. The frequency of the output ripple is twice the input frequency ($f_{out} = 2f_{in}$). The output is given by:
$$V_{out}(t) = |V_m \sin(\omega t)|$$This confirms the conversion of AC to pulsating DC.
💡 EL10 Concept Insight:
Think of a p-n junction diode like a "one-way turnstile" at a stadium gate; it lets people (electrons) walk through in one direction but locks up if they try to push back the other way. A full-wave rectifier uses two of these turnstiles so that no matter which way the "crowd" (AC current) tries to move, they are always funneled into the same exit path, creating a steady flow in one direction.