ABSTRACT
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. 6.1. 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. Figure 6.2 shows how some of the electronic components that we have already met can be used in the realization of the block diagram in Fig. 6.1. The iron-cored step-down transformer feeds a rectifier arrangement (often based on a bridge
Figure 6.1 Block diagram of a d.c. power supply
Figure 6.2 Block diagram of a d.c. power supply showing principal components
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.
