2 minute read

Electrical Power Supply

The Requirement For Power Supplies, Plug-in Power Supplies, Power Supply Voltage Regulation, Voltage-regulation CircuitsBatteries as power supplies, Power supplies and load interaction, Simple transformer power supplies

An electrical power supply is a device that provides the energy needed by electrical or electronic equipment to perform their functions. Often, that energy originates from a source with inappropriate electrical characteristics, and a power supply is needed to change the power to meet the equipment's requirements. Power supplies usually change alternating current into direct current, raise or lower the voltage as required, and deliver the electrical energy with a more constant voltage than the original source provides. Power supplies often provide protection against power source failures that might damage the equipment. They may also provide isolation from the electrical noise that is usually found on commercial power lines.

An electrical power supply can be a simple battery or may be more sophisticated than the equipment it supports. An appropriate power supply is an essential part of every working collection of electrical or electronic circuits.

Two basic types of chemical cells are used in batteries that supply power to electronic equipment. Primary cells are normally not rechargeable. They are intended to be discarded after their energy reserve is depleted. Secondary cells, on the other hand, are rechargeable. The lead-acid secondary cell used in an automobile's battery can be recharged many times before it fails. Nickel-cadmium batteries are based on secondary cells.

When a single power supply serves several independent external circuits, changes in current demand imposed by one circuit may cause voltage changes that affect the operation of the other circuits. These interactions constitute unwanted signal coupling through the common power source, producing instability. Voltage-regulators can prevent this problem by reducing the internal resistance of the common power source.

Alternating current is required for most power lines because AC makes it possible to change the voltage to current ratio with transformers. Transformers are used in power supplies when it is necessary to increase or decrease voltage. The AC output of these transformers usually must be rectified into direct current. The resulting pulsating direct current is filtered to create nearly-pure direct current.

Electrical power supplies are not the most glamorous part of contemporary technology, but without them many electronic products that we take for granted would not be possible.



Cannon, Don L. Understanding Solid-State Electronics. 5th ed. Upper Saddle River, NJ: SAMS division of Prentice Hall Publishing Company, 1991.

Giancoli, Douglas C. Physics: Principles With Applications. 3rd ed. Upper Saddle River, NJ: Prentice Hall, 1991.

Donald Beaty


. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Alternating current

—Electric current that flows first in one direction, then in the other; abbreviated AC.

Direct current (DC)

—Electrical current that always flows in the same direction.


—Electrical circuitry designed to smooth voltage variations.


—Whole-number multiple of a fundamental frequency.


—SI abbreviation for Hertz, the unit of frequency.

Internal resistance

—Fictitious resistance proposed to explain voltage variation.


—Analysis of a complicated device with a simpler analogy.


—Unit of electrical resistance, equal to 1 Volt per Ampere.


—Side-by-side electrical connection.


—Changing alternating current (AC) to direct current (DC) by blocking reverse flow of charge.


—Repetitive voltage variation from inadequate filtering.

Additional topics

Science EncyclopediaScience & Philosophy: Dysprosium to Electrophoresis - Electrophoretic Theory