A sinusoidal alternating voltage is to be rectified. (a) Suggest one advantage of full-wave rectification as compared with half-wave rectification.

Question 5

A sinusoidal alternating voltage is to be rectified.

(a) Suggest one advantage of full-wave rectification as compared with half-wave rectification. [1]

(b) The rectification is produced using the circuit of Fig. 7.1.

Fig. 7.1

All the diodes may be considered to be ideal.

The variation with time t of the alternating voltage applied to the circuit is shown in Fig. 7.2 and in Fig. 7.3.

(i) On the axes of Fig. 7.2, draw a graph to show the variation with time t of the potential difference across diode A. [1]

(ii) On the axes of Fig. 7.3, draw a graph to show the variation with time t of the potential difference across diode B. [1]

(c)

(i) On Fig. 7.1, draw the symbol for a capacitor, connected into the circuit so as to provide smoothing. [1]

(ii) Fig. 7.4 shows the variation with time t of the smoothed potential difference across the resistor R in Fig. 7.1.

1. State how the amount of smoothing may be increased. [1]

2. On Fig. 7.4, draw the variation with time t of the potential difference across resistor R for increased smoothing. [2]

Reference: Past Exam Paper – November 2009 Paper 41 Q7

Solution:

(a) Example:

More (output) power is available

There is less ripple for same smoothing capacitor

(any sensible suggestion)

(b)

(i) The curve should be drawn showing half-wave rectification

{Diode A conducts only when the upper terminal of the supply is positive. That is, when the line of the alternating voltage applied is positive, current flows and the p.d. across the resistor is positive. This means that the p.d. across the diode is ZERO. Here, we want to plot the graph of p.d. across the diode, not the resistor.}

(ii) The curve is similar to (i) but with a phase shift of 180°

{Diode B conducts only when the lower terminal of the supply is positive. That is, when the line of the alternating voltage applied is negative, current flows through the resistor and the p.d. across the resistor is positive. This means that the p.d. across the diode is ZERO. Again, here want to plot the graph of p.d. across the diode. So, the p.d. across the diode is positive when the applied alternating voltage is positive (that is, when the diode is reverse-biased).}

(c)

(i) A correct symbol, connected in parallel with R    

(ii)

1. By using a larger capacitor / second capacitor in parallel with R

(not increase R)

2. The curve has the same peak values, with a correct shape giving less ripple