Full Wave Rectification. Filter Circuit

In a.c. mains transistor receivers, diodes are used to rectify the alternating mams voltage and to produce steady or d.c. voltage for the circuit of the receiver.

Figure 39.7 (i) shows how two diodes, D₁ and D₂, can produce full-wave rectification. The secondary PQ of the mains transformer is centre-tapped at T, so that a.c. voltages of opposite polarity are applied simultaneously to d₁ and D₂ on one half of the input cycle. Thus in Fig. 39.7 (i), d₁ conducts well but D₂ does not. On the other half of the same cycle D₂ conducts well but D₁ does not. The output voltage V between A and T, with a resistor joined between them, would hence be that shown in Fig. 39.7 (ii). It is a unidirectional voltage. Further, compared with half-wave rectification discussed on p. 811, it has a smaller degree of fluctuation and a greater average voltage.

The output voltage У is equivalent to a steady voltage together with varying voltages. In order to filter off the varying voltages, a filter circuit i s used. One form of filter circuit consists of a high inductance L = 50 H say, in series with a large capacitance С — 200 uF say. To a frequency of 50 Hz, the reactance X_L = 2pfL - 2 x 3-14 x 50 x 100 = 31400 Q; the reactance X_c - 1/2 pf С = l/(2 x 3^.4 x 50 x 2000 x 10^-6) = 1^.6 Ohm. Since L and С are in series with V, very little of the varying voltage appears across C; practically the whole of it appears across L. So the output voltage V_xy across a resistance R shown is a fairly steady or d.c. voltage—it has only a small 'ripple' of a.c. voltage, Fig. 39.7 (iii).

Figure 39.8 shows a bridge circuit which produces full-wave rectification without the use of a centre-tapped secondary as in Fig. 39.7 (i). As shown, four diodes are used. On one half of a cycle, when P is +ve relative to Q, only the diodes di conduct. On the other half of the same cycle, only the diodes D₂ conduct. The varying d.c. across А, В is thus similar to that shown in Fig. 39.7 (ii). In this circuit, however, only a 'smoothing' capacitor С is used. Unlike the circuit in Fig. 39.7 (i), С becomes charged to practically the peak value of the varying d.c. voltage. The four-diode bridge rectifier thus provides a greater d.c. output voltage than the circuit in Fig. 39.7 (i).

Zener Diode

When the reverse bias or p.d. is increased across a p-n junction, a large increase in current is suddenly obtained at a voltage Z, Fig. 39.9 (i). This is called the Zener effect, after the discoverer. It is partly due to the high electric field which exists across the narrow p-n junction at the breakdown or Zener voltage Z, which drags more electrons from their atoms and thus increases considerably the number of electron-hole pairs. Ionisation by collision also contributes to the increase in carriers.

Zener diodes are used as voltage regulators or stabilisers in circuits. In Fig. 39.9 (ii), a suitable diode D is placed across a circuit L. Although the battery supply В may fluctuate, and produce changes of current in L and D, if R is suitably chosen, the voltage across D remains practically constant over a reverse current range of tens of milliamperes at the Zener voltage shown in Fig. 39.9 (i). The voltage across L thus remains stable.

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