Q4. Why must the reverse-voltage rating exceed the peak secondary voltage in an unfiltered half-wave rectifier?
In the half-cycles when it is reverse biased the diode faces the peak voltage; if that reached breakdown, the diode would conduct backwards and could be destroyed. With a smoothing capacitor, the peak reverse voltage can approach twice the secondary peak (NCERT page 338).
Q5. What is the role of the centre tap in a full-wave rectifier?
It is the return point for the load current: the output is taken between the joined n-sides of the diodes and the centre tap, so each diode works on half the secondary voltage in alternate half-cycles (NCERT page 338).
Q6. Assertion: a full-wave rectifier is more efficient than a half-wave rectifier. Reason: it gives an output in both halves of the ac cycle. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion (NCERT page 339).
Q7. A half-wave rectifier on 60 Hz mains is replaced by a full-wave rectifier. What are the output frequencies before and after?
60 Hz before and 120 Hz after (worked as NCERT Exercise 14.6, page 342).
Q8. A filter capacitor of 470 μF is across a 2 kΩ load. Find the time constant.
τ = CRL = 470 × 10−6 × 2000 = 0.94 s (time constant as on NCERT page 339).