Rectifiers

Type of physical science: Classical physics

Field of study: Electromagnetism

Rectifiers are devices incorporated into electronic systems to convert alternating current to direct current, to switch current on or off, and to control the level of current within an electronic system. They can also be used to transmit, detect, amplify, and otherwise control a radio signal. Rectifiers come in many forms, including semiconductors, commutators, and vacuum tubes.

Overview

During the late nineteenth century, at a time when technical standards were being set that would pave the way for widespread deployment of electrical transmission facilities to provide electric power to industry and homes across the United States, a debate raged over whether electricity should be distributed as alternating current (AC) or direct current (DC).

Eventually, those who supported alternating current won despite passionate pleas by Thomas Alva Edison for adoption of the direct current model. Alternating current had advantages at the time that led to its adoption, even though it had to be transformed into direct current to run electric motors and other electrical systems. That transformation process is called rectification.

Rectification is a means of creating a one-way flow of electricity known as direct current. Alternating current is different. First, alternating current flows in one direction, then the opposite, then back again, then the opposite again, repeating the cycle sixty times per second. A device used to rectify, or transform, electric current from AC to DC is called a rectifier.

Rectifiers take many forms that include vacuum tubes, metallic rectifiers, and semiconductors.

There are half-wave rectifiers, full-wave rectifiers, and bridge rectifiers.

The semiconductor is today's most popular rectifier platform. Semiconductors can be manufactured in very small sizes, and recent advances in electronic circuitry have led to miniaturization of most sophisticated electronic systems. To understand how semiconductors work, it is necessary to look at how matter is structured.

Matter is made up of atoms, and atoms contain varying numbers of electrons, protons, and neutrons. Each element contains unique configurations of these charged particles so that different substances conduct electricity in different ways, exhibiting varying degrees of conductivity. Simply stated, substances made up of atoms that contain fewer than four free electrons--electrons that travel freely from one like atom to another--make good conductors of electricity, and those with more than four free electrons make poor conductors. Poor conductors, on the other hand, make good insulators. Atoms with exactly four free electrons are semiconductors, neither good conductors nor good insulators. Perhaps the best known semiconducting material is silicon, which, when combined with some other elements containing fewer than four or more than four free electrons, can be transformed into materials with predictable conductive properties that are ideal for use as rectifiers.

Semiconductor devices contain two crystals, one made of silicon mixed with a substance such as aluminum that contains fewer than four free electrons, the other made of a substance such as phosphorus that contains more than four free electrons. The silicon and aluminum mixture creates a positive charge when electrons move through it, while the silicon and phosphorus mixture creates a negative charge. Combining the two creates a diode, which contains a cathode and an anode.

In a rectifier, voltage transmitted from the cathode to the anode (also called the plate) is positive during one-half of the cycle and negative during the other half. That means the anode is alternately positively charged and negatively charged. Current flows when the anode is positively charged but does not flow when the anode is negatively charged. It can be said, then, that the diode is alternately a conductor and an insulator, allowing current to pass during the positive side of the cycle and stopping the flow on the negative side. When that happens, the flow of electrons is one-way (direct current), the life blood of electric motors and electronic devices of every kind.

Applications

Since the development of the modern semiconductor, many earlier devices used for rectification are seldom used today. In the early 1880's, Edison discovered that vacuum tubes exhibited rectifying properties. These tubes, or diodes, were able to rectify large amounts of electricity in industrial machinery and provide small amounts of direct current to household appliances, such as radio receiving sets. Soon, different kinds of rectifiers were developed to perform different kinds of tasks.

Today's common silicon-controlled rectifier (SCR) is often used as a switch to turn on direct electric current to a device or several devices very quickly. Simple applications include sensors that trigger alarms, open or close valves automatically, or activate displays. SCRs are also used to turn alternating current power on and off as they have a number of advantages over mechanical switches, particularly where large amounts of power are present. The lack of moving parts and speed of operation are desirable, as is the fact that the device can be activated by light, heat, pressure, speed, and any number of other measures that can be converted to electric pulses.

Rectifiers can also be used to vary the amount of AC power being applied to an electronic circuit. The pulse used to control the level of power can be set to occur at any point in the AC cycle, from full on to full off. This is particularly useful in systems that require variations in power in relation to values determined by sensors.

Rectifiers are also used to provide direct current to radio-transmitting devices that send radio frequency signals throughout the atmosphere. High-powered radio and television transmitters often use hot cathode mercury-vapor rectifier tubes for that purpose. Edison discovered that by heating a cathode with a filament in a vacuum tube containing mercury vapor, the cathode would display a positive potential. A vacuum tube or series of tubes could then be used to control frequency output as well as power levels of electrical energy, which suggested to Edison and others that applications for rectifiers existed in the emerging scientific experimentation with wireless transmission, the earliest form of radio. These vacuum tubes were called diodes because they contained cathodes and anodes, and they could be used to detect and amplify radio signals. Thus, the technology was beginning to emerge that would employ rectifiers to transmit radio waves on specific frequencies within the electromagnetic spectrum at controllable power output levels. Variations of these same rectifier tubes could detect and amplify the transmitted signals, thus forming the basic elements of early radio sets, or receivers.

Eventually, similar techniques were devised to transmit and receive television signals at different locations in the electromagnetic spectrum. Still others were developed to provide private and public two-way radio transmission, radar, satellite telemetry, and many other uses.

A variation on the vacuum tube diode--the triode--was developed by Sir John Ambrose Fleming as a major advance in radio amplification. The triode used vacuum technology, most of which has been supplanted by sophisticated semiconductors that are more efficient, less costly, and more convenient to use. These semiconductors are solid-state devices, requiring no special catalytic environment, such as those created in vacuum tubes. Also, they do not require filaments to heat that environment. They are not susceptible to distortions or other disturbances that plague vacuum tubes, and they require no warm-up period to operate. These devices are virtually instantaneous in operation, which makes them inherently more desirable for virtually all applications requiring rectifiers.

Finally, it is interesting that rectifiers, first used to transform alternating current into direct current, can also be used to transform direct current into alternating current. Their usefulness, it seems, is unlimited.

Context

Electricity has been the subject of scientific study for centuries. Archaeological discoveries suggest that ancient civilizations dating back thousands of years may have developed crude batteries that could create and store electrical energy. It was not until the latter part of the nineteenth century, however, that it became possible to distribute electricity widely in a controlled fashion. Once the deployment of electrical transmission facilities achieved momentum, new electrical applications emerged that required specialized controls. While Edison's incandescent light bulb represented perhaps the most elementary application of electrical power--building simple resistance into a circuit to create light--electric motors and switches that operate on automatic sensing technology required a higher level of sophistication in control technology. Rectifiers represent one of the most important components of virtually all electronic systems. They perform a multitude of functions required to transform standard alternating current that is delivered to the customer by the power company at standard power output levels into forms necessary to operate other components in those systems.

Rectifiers have also served as the launch platform for many electronic innovations over the years. Wireless, for example, was the forerunner of radio and television broadcasting, satellite communication, microwave transmission, and cellular telephones, among many other radio transmission systems. Wireless was first proposed by scientists intrigued by the unusual behavior of electricity when applied to Edison's vacuum tube. Using vacuum-tube technology, they were able to transmit, receive, tune, and amplify radio signals. Further refinements of Edison's vacuum tube led to more efficient and higher quality signal transmission and reception, eventually making it possible to establish broadcast stations in communities across the country and around the world, each operating on discrete frequencies within prescribed areas of the electromagnetic spectrum, and each operating at prescribed power output levels. Later, new applications for radio appeared, including radar and aeronautical communications. Communication satellites use the latest and most sophisticated radio technologies that have become available only since the invention and refinement of solid-state semiconductors. These semiconductors are more efficient than vacuum tubes in the way they use power and transmit signals and also in the range of frequencies in which they can function. Modern-day communications satellites operate in the ultrahigh electromagnetic frequency spectrum well beyond the capabilities of even the most sophisticated vacuum tubes. Where early wireless measured frequency range in the hundreds of cycles per second, or hertz, commercial satellite frequencies are often measured in gigahertz.

The other major technological advance spawned by rectifiers came with the emergence of solid-state semiconductors. Since rectifiers can be designed to turn on electric current instantaneously with little power and no mechanical operation, the whole realm of sensing devices in use today and proposed for the future have become possible. Thermostats can be set to react to temperature variations; smoke detectors can detect changes in the composition of local atmosphere; satellites can detect the presence of heat, cold, environmental gases, moisture, and any number of other conditions from high above the earth through variations in light reflections from the surface of the earth. Also, AC to DC conversion, solar panel generated electrical production, instrument monitoring, and electrical system operation aboard space vehicles and platforms can be accomplished automatically with little or no manual control. These systems can also adjust to a whole range of environmental conditions through computer analysis of data supplied by sensors.

Perhaps most important, advancements in miniaturization of elecronic circuitry have made many of these systems available to virtually anyone at low cost. Computers, calculators, household appliances, television remotes, and electronic gadgets of every description are now providing untold benefits to society. Rectifiers have provided the key that continues to unlock the awesome potential of electricity.

Principal terms

ALTERNATING CURRENT: electrical current in which electrons alternately flow in opposite directions as the charge goes from positive to negative and back in continuing cycles

ANODE: the positive electrode, also called the plate, through which electrons move out

CATHODE: the primary source of electrons in an electron vacuum tube

CONDUCTOR: a material containing atoms with fewer than four free electrons in the outer orbit that allows an electric current to pass through it with a high degree of efficiency

DIODE: electron flow is from cathode to anode, in that direction only; flow in the opposite direction, as happens with alternating current, is highly resisted

DIRECT CURRENT: electrical current in which electrons flow in one direction or are caused to flow in one direction through use of a rectifier.

INSULATOR: a material containing atoms with more than four free electrons in the outer orbit and that does not allow electric current to pass through it with acceptable efficiency

MERCURY-VAPOR RECTIFIER: the first vacuum tube that was capable of detecting, transmitting, and amplifying electronic radio signals

SEMICONDUCTOR: a material containing atoms with four free electrons in the valence, or outer, orbit and whose resistivity is between that of conductors and insulators

VACUUM TUBE: a device resembling an incandescent light bulb that contains a cathode, an anode, a filament, and a vapor cloud; also called a diode, it has spawned variations

Bibliography

Albert, Arthur Lemuel. ELECTRONICS AND ELECTRON DEVICES. New York: Macmillan, 1956. This volume contains a lucid and comprehensible discussion of basic electronic theory that includes a particularly useful discussion of rectifiers and their place in electronics. Other areas covered are amplifiers, capacitors, oscillators, semiconductors, and photoelectric devices. Contains illustrations, diagrams, and an index.

Lurch, E. Norman. FUNDAMENTALS OF ELECTRONICS. New York: John Wiley & Sons, 1981. In addition to a good discussion of fundamental electronic principles, this volume contains an excellent in-depth discussion of rectifiers, especially the concept of amplification in electronic circuitry. Contains diagrams, illustrations, and an index.

Lytel, Allan. ABC'S OF SILICON CONTROLLED RECTIFIERS. Indianapolis: Howard W. Sams, 1972. This slim book is a concise overview of rectifiers since the invention of the transistor and the integrated semiconducting device. Describes the theory and operation of rectifiers, with emphasis on switching and electric motor control.

U.S. Bureau of Naval Personnel. BASIS ELECTRONICS. Washington, D.C.: U.S. Bureau of Naval Personnel, Navy Training Course, 1968. Yet another good introduction to electronics, this volume contains a good discussion of antennas, receivers, and an introduction to early computers and their electronics. Contains an excellent glossary of electrical terminology.

Wilson, Alan H. SEMI-CONDUCTORS AND METALS. Cambridge, England: Cambridge University Press, 1939. For those wishing to explore the various conductive characteristics of semiconducting materials, this volume contains an exhausting look at the potentials associated with hundreds of materials commonly found in nature and which are categorized as metals.

Charges and Currents

Electrons and Atoms

Forces on Charges and Currents