Rectifiers

Semiconductor diodes are commonly used to convert alternating current (a.c.) to direct current (d.c),in which case they are referred to as rectifiers. The simplest form of rectifier circuit makes use of a single diode and, since it operates on only either positive or negative half-cycles of the supply, it is known as a half-wave rectifier.


Figure .1. shows a simple half-wave rectifier circuit. Mains voltage (220 to 240 V) is applied to the primary of a step-down transformer(T1). The secondary of T1 steps down the 240V r.m.s. to 12 V r.m.s. (the turns ratio of T1 will thus be 240/12 or 20:1). Diode D1 will only allow the current to flow in the direction shown (i.e.from cathode to anode). D1 will be forward biased during each positive half-cycle (relative to common) and will effectively behave like a closed switch. When the circuit current tries to flow in the opposite direction, the voltage bias across the diode will be reversed, causing the diode to act like an open switch (see Figs.2.(a) and 2.(b), respectively).

The switching action of D1 results in a pulsating output voltage which is developed across the load resistor (RL). Since the mains supply is at 50 Hz, the pulses of voltage developed across RL will also be at 50 Hz even if only half the a.c. cycle is present. During the positive half-cycle,the diode will drop the 0.6 V to 0.7 V forward threshold voltage normally associated with silicon diodes. However, during the negative half-cycle the peak a.c.voltage will be dropped across D1 when it is reverse biased.

This is an important consideration when selectiong a diode for a particular application. Assuming that the secondary of T1 provides 12 V r.m.s., the peak voltage output from the transformer’s secondary winding will be given by:

V_pk = 1.414 x V_r.m.s = 1.414 x 12 V = 16.79 V

The peak voltage appilied to D1 will thus be approximately 17 V. The negative half-cycles are blocked by D1 and thus only the positive half-cycles appear aross RL. Note, however, that the actual peak voltage across RL will be the 17 V positive peak being suppliied from the secondary on T1, minus the 0.7 V forward threshold voltage dropped by D1. In other words, positive hafl-cycle pulses having a peak ampilitude of 16.3 V will appear across RL.

POWER SUPPLY

This chapter deals with the unsung hero of most electronic systems, the power supply. Nearly all electronic circuits require a source of well regulated d.c. at voltages of typically between 5 V and 30 V. In some cases, this supply can be derived directly from batteries(e.g. 6 V, 9 V, 12 V) but in many others it is desirable to make use of a standard a.c. mains outlet. This chapter explains how rectifier and smoothing circuits operate and how power supply output voltages can be closely regulated. The chapter concludes with a brief description of some practical power supply circuits.


The block diagram of a d.c. power supply is shown in Fig.1. Since the mains input is at a relatively high voltage, a step-down transformer of appropriate turns ratio is used to convert this to a low voltage. The a.c. output from the transformer secondary is then rectified using conventional silicon rectifier diodes (see Chapter 5) to produce an unsmoothed (sometimes referred to as pulsating d.c.) output. This is then smoothed and filtered before being applied to a circuit which will regulate (or stabilize) the output voltage so that it remains relatively constant in spite of variations in both load current and incoming mains voltage. Fig.2. shows how some of the electronic components that we have already met can be used in the realization of the block diagram in Fig.1. The iron-cored step-down transformer feeds a rectifier arrangement (often based on a bridge circuit). The output of the rectifier is then applied to a high-value reservoir capacitor. This capacitor stores a considerable amount of charge and is being constantly topped-up by the rectifier arrangement. The capacitor also helps to smooth out the voltage pulses produced by the rectifier. Finally, a stabilizing circuit (often based on a series transistor regulator and a zener diode voltage reference) provides a constant output voltage. We shall now examine each stage of this arrangement in turn, building up to some complete power supply circuits at the end of the chapter.