Page 266 - Op Amps Design, Applications, and Troubleshooting

CHAPTER SIX

Power Supply Circuits

6.1 VOLTAGE REGULATION FUNDAMENTALS

Nearly all electronic systems require one or more sources of stable DC voltage. Yet
many systems get their input power from the standard 120-VAC power line. Even
battery-powered units may require stable DC voltages at levels other than those
provided directly by the battery. Figure 6,1 shows the basic role played by a volt-
age regulator circuit and where it fits in the power distribution scheme.
If the system receives its power from the 120-VAC power line, the first step is
usually voltage reduction via a step-down transformer. The output of the trans-
former (there may be more than one) is then rectified to produce pulsating DC.
The rectified waveform is then filtered with large capacitors to produce a rela-
tively smooth but unregulated source of DC voltage. An unregulated voltage
source is one that varies with changes in load current or in applied voltage. All of
the functions just described are represented by the first block in Figure 6.1.
The voltage at the output of the rectifier/filter is smooth DC, but it is not reg-
ulated. Thus, the value of voltage will change with changes in input voltage or
with changes in load current. To eliminate these changes and produce a solid
source of DC voltage, we route the filtered DC to a voltage regulator circuit (the
second block in Figure 6.1).

FIGURE 6.1 A voltage regulator circuit provides constant voltage to a load.

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