Introduction

In order to operate electronic equipment, it is necessary to have electrical power. This power is provided by a variety of power sources. Because of convenience and economy, most instruments are designed to operate from conventional AC line voltages. The instrument power supply must convert the AC line voltage to the DC voltages and other AC voltages necessary to operate the devices and circuits within the specific instrument. Since the operation of the instrument depends on the proper supply of voltages and currents, it is important to learn the characteristics of the power sources most commonly encountered.

Power supplies used for the operation of electronic circuits usually provide only low DC voltages of about 5 to 30 volts. For some instruments it is feasible, desirable, or even necessary to use batteries as sources of DC voltages.

The important methods and problems inherent in the conversion of AC line voltages to useful DC voltages are the major topics for the following discussion. You will read about the individual sections of the power supply, their components, and the purpose of each within the power supply.

An ideal DC power supply would have: (1) a constant output voltage regardless of variations in the current required by the load (good regulation or low output resistance); (2) a constant output voltage regardless of variations in temperature, AC line voltage, age of power supply, etc. (good stability); and (3) no noise voltage of line or other frequency superimposed on the DC output (low ripple). In addition to these characteristics, the DC output voltage and the current capability of the power supply must meet the operational requirements of the electronic devices.