Q.(a) Explain the construction and working of full wave rectifier. OR

(a) A full-wave rectifier uses two diodes (with a centre-tapped secondary) that conduct on alternate half-cycles so both halves of the AC input contribute current to the load in the same direction; (b) a transformer uses mutual induction between two windings on a common core, with output-to-input voltage ratio equal to the turns ratio. Both alternatives answered below.

(a) Full wave rectifier — construction and working

Construction. A step-down transformer with a centre-tapped secondary winding is used. The two ends of the secondary are connected to the anodes of two diodes, $D_1$ and $D_2$; the centre tap is grounded (this is the reference/common terminal). The cathodes of both diodes are joined together and connected to one end of the load resistor $R_L$, the other end of which is connected back to the centre tap (ground).

Working. During one half-cycle of the AC input, the upper end of the secondary is positive relative to the centre tap: $D_1$ becomes forward biased and conducts, sending current through $R_L$ (from the cathode junction, through $R_L$, back to the centre tap); $D_2$ is reverse biased and does not conduct.

During the next half-cycle, the polarity reverses: the lower end of the secondary is now positive relative to the centre tap, so $D_2$ becomes forward biased and conducts, again sending current through $R_L$ in the same direction as before (since current always flows from the diode cathode junction, through $R_L$, to the centre tap); $D_1$ is now reverse biased.

Result. Since one diode or the other conducts during every half-cycle, current flows through the load during the entire AC cycle, in a single (unidirectional) direction — giving a pulsating DC output with two humps per input cycle (ripple frequency $=2\times$ input frequency). This gives full-wave rectification a much higher efficiency (theoretical maximum $\approx81.2\%$) than a half-wave rectifier ($\approx40.6\%$), and less ripple.

(A bridge rectifier, using four diodes and no centre tap, achieves the same full-wave rectification without needing a centre-tapped transformer.)

(b) Transformer — construction and working

Construction. A transformer consists of two coils of insulated copper wire — the primary (with $N_p$ turns) and the secondary (with $N_s$ turns) — wound on a common soft-iron core, laminated (thin, insulated sheets) to reduce eddy-current losses. The two windings are electrically isolated from each other but magnetically linked through the shared core.

…