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H.I.V Aids

2008-10-21T04:26:00.001-07:00

ARTICLES
African Leader in AIDS Fight Wins $5 Million Prize
By THE ASSOCIATED PRESS
Festus Gontebanye Mogae has received a prize worth more than $5 million that recognizes good governance in Africa.October 20, 2008
Circumcision Benefit In AIDS Is Divided
By REUTERS
There is not enough evidence to show that circumcision reduces the risk of AIDS in sex between men, researchers are reporting, even though previous studies in Africa have shown its pronounced benefit in reducing AIDS from heterosexual sex. ''Over all, we're not finding a protective effect associated with circumcision for gay and bisexual men,'' said Gregorio A. Millett of the Centers for Disease Control and Prevention, the lead author of a report that appears Wednesday in The Journal of the Ame... October 8, 2008
Discoverers of AIDS and Cancer Viruses Win Nobel
By LAWRENCE K. ALTMAN
The Nobel Prize in Medicine was awarded Monday to three European scientists who had discovered viruses behind two devastating illnesses, AIDS and cervical cancer. Half of the award will be shared by two French virologists, Françoise Barré-Sinoussi, 61, and Luc A. Montagnier, 76, for discovering H.I.V., the virus that causes AIDS. Conspicuously omitted was Dr. Robert C. Gallo, an American virologist who vied with the French team in a long, often acrimonious dispute over credit for the discovery ... October 7, 2008
H.I.V. Spreads in China, Affecting New Populations
By DONALD G. MCNEIL JR
Infection with the AIDS virus in China is spreading beyond the country’s original high-risk groups and the virus has spread to all provinces.October 7, 2008
More on AIDS and: MEDICINE AND HEALTH, CHINA
Detailed Study on Spread of H.I.V. in U.S.
By GARDINER HARRIS
A study of people newly infected with H.I.V. has confirmed that the majority of new cases occur among gay and bisexual men and that blacks are most at risk.September 12, 2008
More on AIDS and: BLACKS, HOMOSEXUALITY, CENTERS FOR DISEASE CONTROL AND PREVENTION
RWANDA; Orphaned by Genocide and AIDS, A Generation Poor and Depressed
By DONALD G. MCNEIL JR.
Rwanda, a country that suffered 100 days of tribal genocide in 1994 and has also been hit hard by the AIDS epidemic, is believed to have the highest percentage of orphans in the world. Now a survey finds that depression is alarmingly common among teenage and young adult orphans there who head households and care for younger children.September 9, 2008
H.I.V. Is Spreading in New York City at Three Times the National Rate, a Study Finds
By SEWELL CHAN
The virus that causes AIDS is spreading in New York City at three times the national rate an incidence of 72 new infections for every 100,000 people, compared with 23 per 100,000 nationally.August 28, 2008
More on AIDS and: NEW YORK CITY
Teenagers Changing Sexual Behavior
By NICHOLAS BAKALAR
Compared with their peers in 1991, high school students today are less likely to be sexually active, and when they are, more likely to use condoms.August 26, 2008
More on AIDS and: SEX, SEXUALLY TRANSMITTED DISEASES, CONDOMS, CHILDREN AND YOUTH, MEDICINE AND HEALTH
Foreclosures Mean Crises for H.I.V. Positive Renters
By APRIL DEMBOSKY
At least 50 H.I.V.-positive renters have complained to city housing organizations in the past months of being forced out or threatened with eviction because of foreclosures.August 26, 2008
More on AIDS and: HOUSING, FORECLOSURES, NEW YORK CITY
Lifting the Veil on AIDS in a Mexican Prison
By MARC LACEY
An American organization has set up an AIDS awareness program inside Mexican prisons, since the myths associated with AIDS are pronounced among the prisoners.August 26, 2008
More on AIDS and: SEX, PRISONS AND PRISONERS, MEXICO CITY (MEXICO)
first recognized 25 years ago. Nicholas D. Kristof travels to the country hardest hit by the virus today.
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Battle Over the AIDS Memorial Quilt
The creator of the AIDS Memorial Quilt is locked in a legal tug of war with the quilt’s caretaker.
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Tracking the Spread of HIV
A U.N. report indicates that the number of people infected with H.I.V. may have stabilized globally since the 1990s, but the disease is still proliferating in several individual countries.

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Television Production

2008-09-09T07:53:00.000-07:00

Television Production
I

INTRODUCTION

Television Production, techniques used to create a television program. The entire process of creating a program may involve developing a script, creating a budget, hiring creative talent, designing a set, and rehearsing lines before filming takes place. After filming, the post-production process may include video editing and the addition of sound, music, and optical effects.
The three basic forms of television programs are fictional, nonfictional, and live television. Fictional programs include daytime soap operas; situation comedies; dramatic series; and motion pictures made for television, including the mini-series (a multiple-part movie). The basic nonfictional, or reality, programs include game shows, talk shows, news, and magazine shows (informational shows exploring a variety of news stories in an entertainment format). Live television is generally restricted to sports, awards shows, news coverage, and several network daily talk shows.

Most television programs are produced by production companies unrelated to the television networks and licensed to the networks. The network creates the financing for the production by selling commercial time to sponsors.

II

THE PRODUCTION TEAM

The personnel involved in the production of a television program include creative talent such as actors, directors, writers, and producers as well as technical crew members such as camera operators, electrical technicians, and sound technicians.

The executive producer is responsible for the complete project and is usually the person who conceives the project and sells it to the network. The executive producer bears final responsibility for the budget and all creative personnel, including the writer, line producer, director, and major cast members. The line producer reports to the executive producer and is responsible for the shooting schedule, budget, crew, and all production logistics.

The writer or writers develop the script for each show. They often work during preproduction and rehearsals to correct problems encountered by the actors or directors, or to revise for budgetary or production considerations.

Reporting to the executive producer, the director helps choose actors, locations, and the visual design of the production, such as the style of sets and wardrobe. In addition, the director is responsible for the performances of the actors as well as all camera movements. After filming, the director edits the videotape to create what is known as a director's cut.

Actors work under the direction of the director to portray a character. Performers include talk-show hosts, newscasters, and sports announcers. Actors and performers are chosen by the producer, and most audition to earn their part. Once they are hired, actors memorize their lines from a script and usually participate in a rehearsal before the program is filmed, or shot. Performers may provide live commentary, or in the case of newscasters, they may read their lines from cue cards or a TelePrompTer—a machine that displays words on a screen.
The production manager is responsible for all physical production elements, including equipment, crew, and location. The assistant directors report to the director and are responsible for controlling the set, managing the extras, and in general carrying out the director's needs. The cinematographer, who operates the camera, is responsible for lighting the set and the care and movement of the camera.

The production designer, also called the art director, is responsible for the design, construction, and appearance of the sets and the wardrobe. Often the makeup artists and hair stylists report to the production designer. The key grip is responsible for the camera dolly (the platform that holds and moves the camera) and all on-set logistical support, such as camera mounts, which are used to affix the camera to a car or crane.

Videotape production involves a technical director, who is responsible for video recording, and video engineers, who are responsible for the maintenance and quality of the electronic equipment and their output.

III

PRODUCING A PROGRAM

The creation of a television show begins with an idea for a program and the development of a script. A television network may also require a commitment from one or more well-known actors before financially committing to film a show. Producing a show involves three main stages: pre-production, principle photography, and post-production.
A

Pre-production Activities

Pre-production activities involve the planning, budgeting, and preparation needed before shooting begins. The pre-production period can last as long as a month or more for a movie, or just a week for a single episode of a situation comedy. Productions of great complexity, such as a telethon or a live-awards ceremony, may take months of pre-production. Three key people involved in pre-production are the production manager, director, and casting director. The production manager's first tasks are to produce a preliminary budget, hire the location manager, and locate key crew department leaders. The first essential production decisions are the location of shooting and a start-of-production date. The director's first activities are to review the script for creative changes, begin the casting process, and select assistant directors and camera operators. Subsequently, every decision involving cast, creative crew, location, schedule, or visual components will require the director's consultation or approval.

The culminating activity of the pre-production process is the final production meeting, attended by all crew members, producers, director, and often, the writer. Led by the director, the pre-production team reviews the script in detail scene by scene. Each element of production is reviewed and any questions answered. This meeting can last from two hours to a full day depending on the complexity of the shoot.

B

Principle Photography

Principle photography is the period in which all the tape or film needed for the project is shot. All television programs are shot using one of two basic methods of photography: single camera film production and multiple camera tape production. The single camera method is used to produce movies for television and most dramatic series. Multiple camera tape production is used to produce most situation comedies, soap operas, talk shows, game shows, news magazines, and live programs such as sports, awards shows, and the news. Some forms of programming such as music videos or reality programs (special interest news presented in an entertaining format) employ both methods, using single camera shooting for field pieces and multiple camera for in-studio footage.

The single camera film mode of production is virtually identical to the method of making theatrical movies. The script is broken down into individual scenes. Each scene is shot from a number of angles. The widest shot, which includes all the action, is called the master. Additional shots include closer angles of the characters, sometimes in groups of two or more, and almost always at least one angle of each actor alone. That shot can be either a medium shot (from waist to head), close-up (only head and shoulders), or extreme close-up (of the face only). Many times a scene includes insert shots (such as a close-up of a clock or a gun) or cutaways (a shot of the sky or tree or other visual that relates to the scene). Scenes are scheduled to be filmed according to production efficiency, not story progression. The film is pieced together in sequential order during post-production.

The multiple camera tape method is most suitable for shooting inside a studio. Three or four videotape cameras are focused on the action taking place on the set, and scenes are shot in sequence. Each camera operator works from a list of camera positions and framing requirements for the full scene. Together the cameras cover all required camera angles.
Using headsets to communicate with the camera crew, the director asks for camera adjustments during the filming of the scene and indicates to the technical director which cameras to use at each moment. The technical director ensures the selected shot is recorded on a master tape. The result is a fully edited, complete show, needing only sound effects, music, optical effects, and titles to be complete.

C

Post-Production Activities

Post-production begins with the completion of filming and continues until the project is delivered to the network for airing. The two main activities of post-production are the editing, or assembling, of video footage and the creation of a complete sound track.
Editing may begin during production. In single-camera shoots, the film from each day is reviewed at a later time by the director, producer, and network in the order in which it was shot. These films, called dailies, are then broken down and assembled into scenes by the editors. The first full assemblage is shown to the director, who makes further editing changes and creates the director's cut. Thereafter, the producer and the network make changes until a final cut is created.

The final cut is given to the sound department, which is responsible for preparing the music tracks, or recordings; sound effects; and dialogue tracks for final combination into one track. The final mixing of all the sound is called dubbing. During this period, the sound engineers will spot the music—that is, select the points at which music will be inserted—and musicians will write and record the music. Sound engineers also adjust dialogue recording for production quality and record new or replacement dialogue in a process called looping. Sound effects are also added at this time. The resulting dubbing session, which can take several days for a movie or just a few hours for a multiple camera tape production, can involve the combination of 5 to 25 separate sound tracks.

The final stage of post-production is the addition of optical effects, such as scene fade-outs or dissolves, insertion of titles and credits; creation of special visual effects, such as animations; and color correction.

The post-production process can take as long as eight weeks for a movie to three days for a situation comedy. Commonly, all optical effects, titles, and music are rolled in during the production of soap operas, game shows, or talk shows—greatly reducing post-production.

IV

TECHNOLOGICAL ADVANCES

Prior to the advent of videotape in the 1950s, original programming for television was produced live or shot on film for future airing. Variety shows, such as “The Texaco Star Theatre” (1950-1951) with Milton Berle, “Your Show of Shows,” (1950-1954) and “The Ed Sullivan Show,” (1948-1971) and game shows were the most popular forms. “I Love Lucy” (1951-1957) pioneered the multiple camera style of shooting comedy. But television forms were still limited by the technology. The development of videotape made most live entertainment programming unnecessary and not worth the risk of making mistakes on the air.

The 1960s witnessed great advances in film production technology, including smaller cameras, mobile units, and low-light film. Producing quality film programming became possible, and the film studios entered television production, utilizing their own stages and equipment. The 1970s and the advent of government network regulation of production and distribution opened production possibilities to entrepreneurs and individual creative people. Television producers, including Aaron Spelling, Norman Lear, and Mary Tyler Moore, formed their own companies, and the studio control of production and programming disappeared.

The 1980s and 1990s brought cable and satellite television. As audiences became more fragmented, programming that reached special interest groups, such as community news magazine programs, became profitable. Yet, because of the small audience size, low-cost production became an absolute necessity. In the 1990s advances in technology brought the video camera out of the studio and into the field, expanding television's visual possibilities and making today's magazine show economically possible.

Revolution in Electronics

2008-09-09T07:30:00.001-07:00

A Revolution in Electronics

The invention of the transistor in 1948 was a turning point in the history of electronics. Transistors are composed of a semiconducting material—that is, a substance that can act as either a conductor or an insulator. Transistors quickly replaced vacuum tubes for amplifying electronic signals in devices ranging from radios to telephone lines to military targeting devices. Without the bulky heat-generating vacuum tubes, electronic devices became much more compact and powerful. A 1951 Scientific American article heralds the opening of the age of solid-state electronics.

A Revolution In Electronics

The transistor, a superior new means of controlling the flow of electrons, has now been developed in a practical form; it liberates electronics from the limitations of the vacuum tube

Electronics was born 45 years ago when Lee De Forest invented the first three-electrode vacuum tube—the 'audion.' The two world wars gave electronics the impetus that made it the huge industry it is today. World War I, during which radio telephony was developed from a laboratory curiosity into a reasonably practical means of communication, gave birth to commercial radio: the first broadcasting station, KDKA, was licensed in 1920. World War II, which opened a new horizon in electronics through the development of radar and its attendant technology, produced television. There had been experiments with television before the war, but it was the wartime advances achieved on behalf of radar that made commercial TV immediately realizable. The story of television's postwar rise is dramatically told in production figures: the number of television receiving sets made in the U. S. rocketed from 6,500 in 1946 to 7,500,000 in 1950. Besides television, World War II produced another major electronic advance: high-speed digital computing machines, which also seem destined to have a profound social impact, though this is still largely in the future.

Paradoxically the vacuum tube, which made all this possible, has now become a bottleneck impeding further progress in electronics. The electronics art has reached a stage where a fundamental new development is needed to rescue it from the limitations of the vacuum tube. Just such a development has recently arrived on the scene. It is the transistor, which can perform most of the functions of the vacuum tube and escapes most of its limitations. The transistor promises to revolutionize electronics; indeed, the revolution is already beginning.
To understand the causes and nature of this revolution, we must first look into what is wrong with the vacuum tube. To begin with, the vacuum tube is inherently rather short-lived and unreliable. This difficulty has been exacerbated by the endemic price competition in the commercial electronics industry. Competing on a price basis for the mass market for radio and television sets, manufacturers have achieved prodigies of economy in design and of efficiency in production. But this has exacted its inevitable toll in quality. Most commercially available vacuum tubes are not as reliable as they could be if more care, meaning more money, were put into their design and manufacture. This shortcoming is inconvenient enough in a 6-tube or 8-tube radio set; it is considerably more inconvenient in a television receiver using 25 to 35 tubes, which quadruples the chances of set failure for a given rate of tube failure. As electronic equipment becomes more complex, the reliability of individual components becomes more and more important.

The telephone system affords an excellent illustration. A transcontinental phone call requires the proper operation of 12,300 vacuum tubes, 112,000 resistors, 97,000 capacitors and large numbers of other components. Hence in the telephone system the reliability of electronic equipment is far more important than its initial cost; a cheap device that requires much maintenance will be much more expensive in the end than a more carefully built apparatus that can work for years with no attention. Because of its extreme concern with reliability, the telephone system has gone to great pains to make the components of its electronic equipment as dependable and long-lived as the state of the art will permit. For the Bell Telephone System the Western Electric Company manufactures equipment to exacting specifications worked out by the Bell Telephone Laboratories. The vacuum tubes it produces are the most durable in the world. In a submarine cable recently laid on the bottom of the sea between Key West and Havana are six repeater amplifiers, each containing three vacuum tubes. They are of course inaccessible for replacement. But if any of those 18 tubes sealed into the cable should fail in the next 20 years, the designers of the cable will be surprised.

Yet there are limits to the reliability that can be built into vacuum tubes even with great care, ingenuity and cost. The telephone system has never been able to use electronic equipment for two important functions—central-office switching and signal amplification on local lines—partly because no vacuum tube is sufficiently dependable.

Reliability in electronic equipment is especially important to the military services. The cost of field maintenance of their electronic gear at present is about 10 times the initial cost of the equipment. The Army, Navy and Air Force are using increasingly complicated electronic devices (e.g., radar bombsights) in increasing numbers and putting increasing reliance on them; the Navy, for example, has a bomber aircraft that is not built for an optical bombsight at all—even has no transparent window for one. In the Air Force's B-36 the electronic equipment accounts for about 10 per cent of the total cost of that very expensive airplane, and in the newest all-weather fighters electronic devices make up 20 per cent of the cost of the plane. Hence to the military, as well as to civilians, the reliability of electronic equipment is a matter of urgent concern.

A further problem in electronic equipment is that of size. As the equipment grows in complexity, it grows more and more bulky. This has led to an attempted solution rather inelegantly called 'miniaturization.' Vacuum tubes have been designed in miniature and sub-miniature models with essentially the same electrical capabilities as the old standard sizes. Resistors, capacitors, transformers and other circuit elements have been greatly reduced in total volume by radical redesign. 'Printed circuits' and other compact schemes for prefabricated wiring have been devised. And the utmost attention has been paid to 'designing the air out of' the equipment as a whole: every nook and cranny in the chassis is filled with wiring and components.
One example of this design trend is…a radio range receiver before and after miniaturization. The volume of the set of standard design is about 300 cubic inches; that of the miniature set is 55 cubic inches—a sixfold reduction. And this was done in the case of a set which had been rather compactly designed, in terms of conventional materials and construction, to begin with.
There is no doubt of the usefulness of such miniaturization for equipment that must be stowed in a small space. But miniaturization intensifies one of the outstanding deficiencies of the vacuum tube: namely, its heavy power requirement and the large amount of heat dissipated in the tube. The vacuum tube, a grandchild of the incandescent lamp, uses heat produced by electric power to boil electrons off the cathode. Generally speaking miniaturization does not reduce the power requirement: the miniature version of the range receiver…takes just about as much power as the larger model—some 30 watts. In the larger model, with a volume of 300 cubic inches, power is liberated at the rate of a tenth of a watt per cubic inch; the components are reasonably well separated from one another, so that ventilation is easy, and the temperature rise of the equipment in operation is modest. On the other hand, in the small model, with components packed as tightly as possible, power is dissipated at the rate of more than half a watt per cubic inch, which is the average rate of power expenditure in the ordinary home electric oven. As a result some spots inside the set reach temperatures as high as 400 degrees Fahrenheit.
This high operating temperature, which is typical of most miniaturized electronic equipment, raises a host of new problems. Connections must be made with special high-melting-point solder. Temperatures must be kept below the softening point of the glass in the tube envelope. And the high temperature greatly reduces the operating life of most of the tube's components.
In short, with the increasing complexity of electronic equipment the relatively low reliability of the vacuum tube and its prodigal use of electric power have become major difficulties.
Thus nearly half a century after De Forest's invention of the tube that gave birth to electronics we are brought to the conclusion that there is nothing wrong with electronics that the elimination of vacuum tubes would not fix!

Until 1948 there seemed no way out of this difficulty. But in that year John Bardeen, W. H. Brattain and William Shockley of the Bell Telephone Laboratories announced the invention of the transistor. At the time the device looked promising but had important practical shortcomings. Transistors could not be produced reliably and with predictable electrical characteristics. They were 'noisy,' would not operate at frequencies higher than a few megacycles per second and had other deficiencies. But three years of hard work at the Bell Laboratories have removed most of the early doubts and objections. Transistors can now be made so that their performance is within 20 per cent of specifications, which is as close production control as has been achieved in half a century of experience in making vacuum tubes. It is likely that further development will lead to methods for producing transistors that conform to specifications within 10 or even 5 per cent.
The transistor is a device made of a crystalline semiconducting material, usually germanium. In this material the number of atomic electrons in the crystal does not quite match the number needed to produce a normal crystal lattice. If there are a few excess electrons, these can travel through the crystal as readily as free electrons traverse the empty space inside a vacuum tube. If there is a deficiency of electrons, the 'holes' representing vacant electron sites also can travel freely through the crystal. At barriers between electron-rich and electron-deficient regions of the germanium, the flow of electrons or holes can be controlled by an applied signal in much the way that the plate current of a vacuum tube is controlled by the voltage applied to the grid. Like the vacuum tube, the transistor can control a larger power than is applied to it—that is, it can amplify.

The important practical difference between the transistor and the vacuum tube is that no power

whatever is needed to set loose the free electrons or holes in the transistor. There is no need to 'boil' electrons out of the solid material; instead, the electrons are controlled as they move inside the solid.

It appears that the durability of transistors, and therefore the reliability of circuits using them, will be excellent. The earliest transistors made had average lifetimes in the vicinity of 70,000 hours, already several times greater than the lifetime of a vacuum tube, and present units are much longer-lived. In principle a properly made transistor should last forever if it is not abused.
Moreover, transistors are highly resistant to mechanical shock and vibration. They survive very severe shock tests without damage. Everyone knows that tapping on the tubes of a radio set produces an unpleasant ringing. This is due to changes in the electrical properties of the vacuum tubes, produced by the mechanical vibration of their elements. Transistors can withstand high accelerations without being subject to such 'microphonic' effects. And in the newer transistors the internal electrical 'noise' has been reduced to a level that compares quite favorably with that of vacuum tubes.

A large part of the improvement in the performance of the device is due to the development of a new design called the 'junction transistor.' The early units consisted of a germanium crystal touched by two closely spaced fine wires—'cat's whiskers.' In the junction transistor this point-contact arrangement has been replaced by a large area contact. A thin layer of electrically positive (electron-deficient) germanium is sandwiched between two electrically negative (electron-rich) ends. The transistor action now takes place over the whole area of contact (junction) between the two types of germanium. It therefore operates more efficiently and consumes far less power.

Obviously the transistor's most important advantage is the modesty of its power requirements. In the first place, it is ready for instant operation without the lengthy warm-up needed by a vacuum tube, so that power does not have to be applied constantly, as it must in vacuum-tube circuits, to equipment that must be ready to operate instantly on demand. Secondly, the total operating power used in a transistor device may be incredibly small—about one-millionth of the power required by a similar vacuum-tube circuit. Ralph Bown, director of research at the Bell Telephone Laboratories, has dramatized this property of the transistor by a vivid analogy. He points out that to amplify the average signal in most electronic equipment, which is in the neighborhood of a microwatt (one-millionth of a watt), a vacuum-tube system requires the expenditure of a watt of power; this is like sending a 12-car freight train, complete with locomotive, to get a pound of butter. The tiniest hearing-aid tubes, in which electrical performance has been considerably degraded in favor of low powerdrain, use about 20 milliwatts. In terms of Bown's simile, this is like sending a 10-ton truck for one pound of butter. The transistor, on the other hand, needs only a microwatt to handle a microwatt.
This million-fold decrease in power requirement is so great that it is hard to appreciate. In a hearing aid a transistor can produce a sound amplification of 45 or 50 decibels at one stage, in contrast to the 25 decibels that can be achieved with low-power-drain vacuum tubes. Yet a transistor hearing aid could run on one set of batteries for about three years, whereas a vacuum-tube hearing aid uses up a set of batteries in a few days. A single transistor amplifier stage could be powered with the energy a flea would expend if it jumped once every eight seconds. We are dealing here not with flea power but rather with lazy-flea power!
In electronic equipment using transistors the problem of high temperature, so serious in the case of vacuum tubes, vanishes altogether. The power required is so small that heating is infinitesimal. The miniaturization of electronic equipment now makes solid sense. The transistor is far more compact than any vacuum tube. A typical transistor occupies only about a two-thousandth of a cubic inch, whereas the smallest subminiature vacuum tube is about one eighth of a cubic inch in volume.

Even at the present very early stage of transistor development it seems certain that transistors will replace vacuum tubes in almost every application. What results can we expect from this major revolution in the techniques and capabilities of electronics? Since the revolution is just beginning, we can only speculate.

We can begin by noting the great gain that electronic machines represent over machines of the strictly mechanical type—the distinction, let us say, between an electronic device such as a television receiver and a mechanical contrivance such as a cash register. The most meaningful difference between electronic machines and mechanical machines is in the level of complexity they can attain. A century ago the ingenious English mathematician Charles Babbage designed an analytical engine which in principle was much the same as the large-scale digital computing machines of today. But Babbage found it impossible to build his proposed machine; it was too complicated to be within the capabilities of an all-mechanical design. The realization of devices of this complexity, including high-speed digital computers and long-distance telephone circuits, became possible only when electronics was developed.

Generally speaking, vacuum-tube electronics permits the building of machines about a hundred times more complicated—and therefore at least a hundred times more competent—than the most complex mechanical machine it is practical to make. Electronic machines enjoy another advantage over mechanism: they can work much faster. The typical unit operation in an electronic machine is accomplished in a time of the order of a millionth of a second; the typical unit operation of a mechanical machine takes about a thousand times longer.

But when we get to such a complex electronic machine as the modern digital computer, the shortcomings of the vacuum tube itself become troublesome. The power the vacuum tube requires and its relatively short lifetime pose substantial obstacles to any further increase in complexity. Now the transistor promises to remove these obstacles. Its lifetime is surely long and may be indefinite. Its power requirement is a million times smaller than that of the vacuum tube. Its electrical performance is at least as good, and in many ways better.

All these favorable properties of the transistor suggest that its use as a substitute for the vacuum tube in electronic machines may permit such machines to grow to levels of complexity now unattainable. Just as vacuum-tube electronics permitted a hundred-fold increase in complexity over that permitted by mechanism, at a conservative estimate transistor electronics may allow a hundred-fold increase in complexity over vacuum-tube machines.

From the general philosophical standpoint, it is of interest to compare the most complicated machine we can make with the machines we observe in nature, such as the human central nervous system. Warren McCulloch of the University of Illinois Medical School has done this in very entertaining terms. He finds that the Eniac computer, containing about 10,000 basic on-or-off elements, is a million times less complex than the brain, which has 10,000 million neurons. The Eniac, indeed, has about the complexity of the nervous system of the flatworm. It has one advantage: its unit operations are accomplished about a thousand times faster than are the unit operations of the brain. Thus if we made a sort of figure of merit for comparing the competence of man-made and natural machines, taking into account both complexity and speed, we should find the Eniac—for those operations fitted to its very low complexity—only about a thousand times less competent than the human brain.

McCulloch remarks that if we made a vacuum-tube computer as complex as the brain, it would require a skyscraper to house it, the power of Niagara to operate it and the full flow of water over the falls to keep it cool. This is altogether a criticism of vacuum tubes. If, as seems reasonable to suppose, the use of transistors will permit a further hundred-fold increase in the complexity of our machines, we shall be able to build, in no greater space and with smaller power requirements than are needed now for vacuum-tube computers, a device only 10,000 times less complicated than the brain. Since it will work a thousand times faster, such a transistor device may be, for those jobs to which its low complexity suits it, as much as one-tenth as competent as the human brain.

This is an exciting prospect, but it has not yet been achieved. Curiously, its achievement seems to rest on the elimination from electronics of the vacuum tubes which gave electronics birth.

Looks of a typical Yoruba man

2008-09-08T08:40:00.000-07:00

The Yoruba of southwestern Nigeria incorporate seven subgroups—the Egba, Ekiti, Ife, Ijebu, Kabba, Ondo, and Oyo—each identified with a particular paramount chief and city. The oni of Ife is the spiritual head of the Yoruba. There is a strong sense of Yoruba identity but also a history of distrust and rivalry dividing the various groups. The majority of Yoruba are farmers or traders who live in large cities of precolonial origin.

Africa! Africa!! Africa!!!

2008-09-05T11:18:00.002-07:00

Africa is often called the cradle of humanity. It was in Africa that humans first arose long, long ago. From Africa, these people migrated into Asia, Europe, Australia, and the Americas.
Today, Africa is home to a remarkable variety of people and cultures. It is a land of striking contrasts and wild beauty. It is also a place facing many problems, including war, starvation, poverty, and disease.

A VAST CONTINENT
Africa is the second largest of Earth’s seven continents, after Asia. It accounts for nearly one-quarter of the world’s land. In the northeast, Africa touches Asia in Egypt. In the northwest, Africa almost touches Europe in Morocco. More than 50 nations are found in Africa. They are home to some 800 million people.

STRADDLING THE EQUATOR
Africa is the only continent that truly straddles the equator, the imaginary line that encircles Earth around its middle. Much of Africa is hot and dry. Only central Africa has a tropical rainy season. Yet few areas of the world are as diverse as Africa.

THE MIGHTY NILE
The longest river in the world, the Nile, empties into the Mediterranean in northeastern Africa. Ancient Egypt, one of the world’s first great civilizations, developed along the mighty Nile more than 5,000 years ago. Today, its magnificent pyramids still tower above the land.

THE SAHARA
The Sahara, the world’s largest desert, reaches across a vast swath of northern Africa. It stretches from the Atlantic Ocean in the west to the Red Sea in the east. In fact, the Sahara covers one-quarter of the entire continent. The Sahara is mainly hot, dry, and empty. But people have used teams of camels to carry goods across this giant desert since ancient times.

EQUATORIAL AFRICA
South of the Sahara is equatorial Africa, lying on either side of the equator. Here we find lush tropical rain forests and tropical grasslands called savannas. On Africa’s west coast are the trading ports of West Africa. These ports ship crops such as coffee, cotton, and cacao beans (from which chocolate is made) to the world.

The Congo River, Central Africa’s largest waterway, empties into the Atlantic Ocean near the equator. The Congo drains a vast basin in Central Africa that receives more rainfall than any other part of the continent. It carries more water than any river in the world except the Amazon River in South America.

GREAT RIFT VALLEY
The highlands of Africa are in the east. They run the length of the continent. Between mountain ranges lies the Great Rift Valley. It runs north to south for more than 3,000 miles (4,830 kilometers). The valley actually begins in the Asian country of Syria and reaches all the way to Mozambique in southeast Africa. The Great Rift Valley is rich with fossils. Here, scientists have found ancient bones that have helped them understand the mystery of human origins.

KILIMANJARO
Africa’s tallest mountains are found in the towering ranges of the east. Kilimanjaro, the highest mountain in Africa, rises in east central Africa. A dormant volcano, Kilimanjaro climbs to 19,341 feet (5,895 meters).

SOUTHERN AFRICA
In southern Africa is the great Namib Desert. This desert lines the southwest coast, reaching inland about 81 miles (130 kilometers). In the center is the fertile High Veld, a large plateau that slopes down to the Indian Ocean in the east. On the southern tip of the continent is South Africa’s famous Cape of Good Hope. This is the place where the Indian and Atlantic oceans meet.

MANY DIFFERENT PEOPLE
The people of Africa may be the most diverse of all the continents. Africa’s 800 million people speak perhaps 2,000 or more languages. Thousands of distinct ethnic groups are found in Africa.
North of the Sahara, people are mainly of Arab origin. South of the Sahara, the people are mostly black Africans. Throughout Africa are scattered people of European ancestry, descendants of colonial settlers.

Most Africans live in rural communities. Many raise livestock or farm. Relatively few people live in cities. But Africa does have many big cities, and they are growing rapidly. They include Cairo, Egypt; Casablanca, Morocco; Lagos, Nigeria; and Cape Town, South Africa.
From the 1500s to the 1800s, millions of Africans were forcibly removed to North and South America to work as slaves. Their descendants are still prominent among the people of both of those continents.

A WILD PLACE
Africa is famous for its stunning wildlife. The continent is home to thousands of species of mammals, birds, reptiles, amphibians, fishes, and insects.
South of the Sahara, Africa teems with animal life. Wild herds of antelope, zebras, giraffes, and impalas roam across the savanna. Lions, cheetahs, and leopards feed on them. Elephants live in some forests and grasslands. Gorillas live in the rain forests of Central Africa.
Today, many of these animals are endangered. Farms and cities have replaced much of the land they called home. They have been hunted for trophies, meat, and sport. Many countries in Africa try to protect endangered animals, but their numbers continue to dwindle.

MANY CHALLENGES
Today, Africa faces many challenges. The majority of its people remain poor. Droughts are frequent in many areas, leading to terrible starvation. Wars within and between countries have killed millions of people and forced millions of others to leave their homes and live elsewhere as refugees.

Diseases such as tuberculosis and malaria kill millions of Africans each year. In recent years, acquired immunodeficiency syndrome (AIDS) has spread rapidly, especially south of the Sahara. It has infected millions of people and devastated some regions.Despite these troubles, Africa remains a magnificent place. Perhaps no other part of Earth is as varied in its geography, wildlife, and people

Zoo

2008-09-05T11:13:00.000-07:00

Zoos

You don’t have to go to Africa to see an elephant. You don’t have to go to the North Pole to see a polar bear. You can see elephants, bears, lions, crocodiles, hawks, and all kinds of other wild animals at a zoo.

Zoos are places where animals are kept so that you can learn about them. Zoos are also places where scientists study animals. Some zoos help preserve endangered animals.

WHAT DOES A ZOO LOOK LIKE?
Zoos once kept animals in cages. Today, they try to keep animals in larger, open places. The places look like the home of each animal that lives there. They have rocks and trees, bushes, and other plants that the animal likes. Some places might look like deserts or prairies or rain forests. Polar bears have ponds of ice-cold water to play in. Monkeys have trees they can climb. Pandas have lots of bamboo to eat.

The places where the animals live have fences and other borders. But the fences are hidden and hard to see. Part of the San Diego Wild Animal Park in California looks like a plain in Africa. It seems like the animals are free to go anywhere.

Many zoos have places with closed-in tops where hundreds of birds can fly around. Some zoos have places you can walk through to see butterflies sipping nectar (juice) from flowers.

WHO WORKS AT A ZOO?
A zoo needs many people to care for the animals. Zoologists are scientists who study animals. They learn about how the animals live. They learn what makes the animals happy.
Veterinarians are animal doctors. They take care of any sick zoo animals. They try to keep all the animals healthy.

Zookeepers take care of everything the animals need. They make sure the animals have plenty of food and water. They watch for any problems with the animals.
Zoos have other workers, too. They have guides who give tours and talk about the animals. They have cooks who work in zoo kitchens to make food for the animals. They even have people who build the homes for zoo animals to live in.

WHERE DO ZOOS GET ANIMALS?
In the past, most animals in zoos were captured wild animals. Today, zoos want their animals to breed (mate and have babies).
It is easy to get some animals to breed at a zoo. Lions living at zoos have many cubs. It is hard to get other animals to breed at a zoo. Cheetahs and giant pandas rarely have babies when they live in a zoo.

PROTECTING ENDANGERED ANIMALS
Zoologists try to save some endangered animals by breeding them at a zoo. Sometimes, they put animals born in a zoo back into the wild. They put the endangered Père David’s deer into an animal preserve in China. They put red wolves in places in North Carolina and Tennessee. These zoo-born animals now live completely in the wild.

Electricity

2008-09-05T10:48:00.000-07:00

Electricity
Watch a bolt of lightning flash across the sky. Flip a switch and light up your bedroom. Click the remote and see the TV come on. What do all of these things have in common? Electricity.
WHAT IS ELECTRICITY?
Electricity is a powerful force of nature. Electricity is everywhere in the universe. Electrical forces hold water, metals, and all other kinds of matter together. You can walk and run because electric signals go through your nerves from your brain to your muscles. The signals tell your muscles where to move.
Electricity makes many machines work. Electricity makes bulbs light up and runs motors in saws, fans, hairdryers, and other appliances. The computer you are using works because of electricity.
WHERE DOES ELECTRICITY COME FROM?
Electricity starts with atoms. Atoms are tiny bits of matter much too small for you to see. Everything in the universe is made up of atoms.
Atoms have two main parts: a center or nucleus, and electrons that orbit or go around the nucleus. Electricity comes from electrons. You cannot see electrons and you cannot see electricity. You can see what electricity does because of electric charge and electric energy.
WHAT IS ELECTRIC CHARGE?
Electric charge comes from the parts inside atoms. There are two kinds of electric charge called positive charge and negative charge. Positive charge comes from the nucleus of an atom. Negative charge comes from electrons. Atoms do not normally have any overall charge because their positive and negative charges cancel each other out. Charge comes when electrons move away from an atom.
Positive charge is just the opposite of negative charge. Positive and negative charges pull toward each other. The pull of positive and negative charges makes two kinds of electricity—static electricity and electric current.
WHAT IS STATIC ELECTRICITY?
Did you ever get a shock after walking across a carpet and touching a metal doorknob? That shock came from static electricity. Huge amounts of static electricity cause lightning.
Electrons that move away from their atoms cause static electricity. You can make static electricity by rubbing certain materials together. Run a plastic comb through your hair. Be sure your hair is clean and dry. Electrons jump from your hair to the comb. This gives the comb a negative electric charge. Your hair loses electrons. This gives your hair a positive electric charge. Hold the comb above your head and watch some of your hairs stand on end. Your hair stands on end because the positive and negative charges are pulling toward one another.
Static electricity also causes lightning. The pull of positive and negative charges between clouds and the ground creates a huge spark. The spark is actually the charges moving very quickly toward each other. Lightning can also be caused by opposite charges inside one cloud, between two clouds, or between clouds and the air.
WHAT MAKES LAMPS LIGHT UP?
Electric current makes lamps and all other electric devices work. Electric current is actually electrons moving in a big loop.
Something must give the electrons a push to get them moving. Batteries can start electrons flowing. Batteries are a source of electric energy. A battery, two wires, and a light bulb can make an electric circuit. The current starts flowing from the battery through a wire to the light bulb. The other wire carries the electric current back to the battery. If you cut the wire, the electric current stops. Switches on an electric circuit turn the current on and off. This is how a wall switch works to turn lights on and off in your home.
The electric energy in your home does not come from batteries. You plug appliances into electric outlets in your walls. The electric energy in the outlets comes from electric power plants.
HOW DO POWER PLANTS WORK?
Huge electric power plants generate or make electricity. Steam or falling water in dams make big machines called turbines turn. The turbine drives another machine called an electric generator. The generator makes electricity.
Long power lines carry electricity from power plants to your home. Wires inside your home bring the electric energy to light bulbs, TVs, microwaves, and your computer.
WHO DISCOVERED ELECTRICITY?
For thousands of years people knew that a material called amber mysteriously pulled on some materials. The ancient Greeks called amber elektron. Scientists in Europe in the 1600s and early 1700s called the materials that amber attracted electrics.
Benjamin Franklin, an American printer, patriot, and inventor, experimented with electricity. He thought lightning and electricity were the same thing. He did a dangerous experiment in the mid-1700s to find out. Franklin flew a kite during a thunderstorm. He attached a metal key to the kite string. An electric charge ran down the wet kite string to the key. The charge made a spark when it hit the key. This showed Franklin that lightning was electricity. He was lucky he was not killed.
Many other scientists have experimented with electricity since Benjamin Franklin. They learned how to make electricity with batteries. They found that electricity would go through wires. An American inventor named Thomas Alva Edison invented many things that use electricity, including the electric light bulb.

Benjamin Franklin
In the mid-1700s, inventor Benjamin Franklin proved that lightning is a form of electricity. In a famous experiment, he flew a kite during a thunderstorm with a metal key attached to the string. An electric charge flowed down the string and caused a spark when it hit the key.

Free ebook manufacturing inverter

2008-09-05T10:38:00.000-07:00

DG 5 SGA
Power Inverters 12V to 230V
contents: construction plans, technical description
Version of Nov.8th,1999
Rev. 3.1
Fig. 1: 1000 VA-Inverter 12 Volt -> 230 Volt
An inverter allowes the use of 230V electrical appliances from a car battery or a solar battery. It must therefor supply a voltage that corresponds to an rms of 230 Volts sine-wave like household main supply or similar. Sine-wave voltages are not easy to generate. The advantage of sine-wave voltages ist the soft temporal rise of voltage and the absence of harmonic oscillations, which cause unwanted counter forces on engines, interferences on radio equipment and surge currents on condensers. On the other hand, square wave voltages can be generated very simply by switches, e.g. electronic valves like mosfet transistors. In former times electromagnetical switches, that operated like a door bell were used for this task. They were called "chopper cartridge" and mastered frequencies up to 200 cycles per second. The efficiency of a square wave inverter is higher than the appropriate sine wave inverter, due to its simplicity. With the help of a transformer the generated square wave voltage can be transformed to a value of 230 Volts (110 Volts) or even higher (radio transmitters e.g.).
Fig. 2: Sine-wave voltage and conventional square wave voltage with both 230 Volt rms
Fig. 2 shows a sine- as well as a square wave voltage with in each case an rms of 230 Volt. In both cases an electric lamp would light with the same intensity. This is, as we know, the definition of rms. As we recognize in Fig. 2, however the peak value of the sine-wave voltage is 325 Volts, i.e. factor Ö 2 more than rms. For electric lamps this is insignificant and electric engines are appropriate for it. Electronic devices were even designed for the peak voltage of sine-wave voltage, because internally they generate DC voltage from the AC supply voltage. A condenser will be loaded on exactly the peak value of the sine-wave voltage. Electronic devices thereby usually cannot be operated on 230 Volt square wave from fig. 2. The industry nevertheless manufactured square wave inverters according to this principle in former times.
Our inverter works with a trick, to obtain the same results from square wave voltage as for sine-wave voltage.
Fig. 3: Square wave voltage with duty cycle 25% for 230 Volt rms ("modified sine")
Square wave voltage in fig. 3 developes the same peak value as sine-wave voltage of 230 Volts, i.e. 230 Volt * Ö 2 = 325 Volts and nevertheless thereby obtains the demanded rms of 230 V. Square wave voltage as shown in fig. 2 (full half wave) with peak value of the corresponding sine-wave voltage would cause double amount of electrical power on electric consumers. The electrical power rises by square of voltage, and square of Ö 2 results in factor 2. The trick is, to switch the output power only for one half of every conducting cycle, thus resulting on a duty cycle of 25% on behalf of the complete oscillation period. If the calculated double amount of electric power will be generated only half the time, effective power remains the same. Industry called this cam shape "modified sine", in order to be able to differentiate the devices from conventional square wave inverters.
The inverter may feed nearly all electrical appliances, designed for 230 Volts, with exception of rotary field engines, that use condensers for generation of an auxiliary phase (condenser engines). Engines of this type are used in most refrigerators, washing machines, dishwashers and some few machine tools. Fluorescent lamps with a series inductivity to limit the operating current also won't work correctly on our inverter. This problem can be solved by increasing the duty cycle on more than 25% while decreasing the peak voltage to 275 Volts. Instead fluorescent lamps with electronics (energy saving lamps) will work very well on the inverter. There may also be problems with some small plug power supplies. An increased magnetizing current results on square wave voltages, while there would be an predominantly inductive load (cosj << 1). Duty cycle 25% and cosj =0 will result in load currents up to factor p /2 (approx. factor 1.5). But don't be confused: Cosj of most electrical appliances is between 0.8 and 1 and would be harmless.
Our inverter is suitable for:
electric drills, fret saws, circular saws, electric chain saws, grinders
Vacuum cleaners, coffee machines,irons, dryers, mixers, sewing machines, electric razors, etc.
lamps, energy-savings lamps
Electronic devices, e.g. music amplifiers, battery chargers
Computers and accessories, UPS
Televisions and radios
ham radio transmitters, high voltage generators, among other things
Schematic diagram:
Description of function:
The inverter chops the 12 Volt DC Battery voltage into a square wave voltage of 50 cycles per second and duty cycle of 25%, transformed by transformer Tr1 to 230 Volt rms. IC1 forms the oscillator with 100 cycles per second (120 cycles per second for 60 cycles output). Frequency is determined by C1 and the resistors R4 and R5. Resistor R6 determins the time of the flyback of the oscillator and affects likewise the frequency. In addition, R6 affects the rms of the output voltage, which must be considered if necessary, if the circuit shall be used for other frequencies. 60 cycles per second can be achieved by alignment, higher frequencies require changes in the frequency-determining parts. For high stability of frequency, special attention must be spent on condenser C1. Ceramic capacitors are not usefull, due to their high sensitivity on temperature. Most foil condensers may keep the frequency quite constant, even against strong temperature variations.
IC2 determines the pulse width and thus rms of the output voltage. The regulater consists of transistor T1, which receives its signal from the diodes D4 and D5, taken from the primary tranformer coil. The regulator adjusts the output voltage by changing the pulse width. It prevents also rising of rms on inductive or capacitive load. The characteristics of regulation can be adapted by changing D4 (important on 24 Volts applications!). Lower voltage level of D4 results in "softer" regulation, i.e. an reduction of the proportional factor.
Against earlier versions of the inverter, IC 8 now will be switched directly by the oscillator signal, thus avoiding errors by unexpected oscillations of the PWM-IC 2. Here the alternate allocation of the impulses for both transistor lines, i.e. for the positive and the negative half wave of the output voltage takes place. The final frequency of 50 cycles per second develops. Flip-flop IC7 stores a switching off instruction of the current limiter for the rest of the half wave. From the gates IC5 (4093-III) and IC6 (4093-IV), the control signal arrives at the complementary MOSFET-driver stage transistors T5/T6 and T7/T8. T6 and T7 are N-channel-enhancement mosfets and T5 and T8 are the complementary P-channel-enhancement mosfets. These transistors correspond to the well-known CMOS basic circuit (CMOS = Complementary MOS), which represents the basic of the CMOS logic family (CMOS inverters). Only the resistors R44 to R47 are new in this circuit. They provide current limitation during shifting process and protect in cases of disturbances. The control unit ist suitable for inverters up to 10 kW output power. The driver stage transistors T5 to T8 provide the signals for the power mosfets, which alternately magnetize transformer Tr1. Inductive idle currents, how they are needed e.g. by electric motors, can be returned to the battery, thanks to the integrated antiparallel recirculating diodes of the transistors. Thus they do not generate unnecessary losses, contrary to early inverters.

The most important task in our inverter is done by the mosfet transistors T13 to T28. They are connected in two groups, each of 8 transistors. They generate alternatingly the positive and negative wave of the output voltage. Each transistor line works on ist own transformer coil. After a transistor line is beeing switched off, the magnetic energy stored in the magnetic field of the transformer returns back to the battery by the integrated recirculation diodes of the second transistor line. The idle current of consumers with inductive load takes the same way. In case of strong heating up of the transistors, which should only happen on defects in the equipment, the bimetal thermal switch F2 shuts off the control electronics. In normal operation, temperature of the heat sink should be as low, that you could touch it by your hands.

The source-currents of the mosfet transistors pass over resistor R20 with the very low value of 0.001 ohms. Load currents of 100 amperes thus produces a voltage drop of only 0.1 Volt, according to an energy dissipation of 10 Watts. The electronic current limiter becomes effective for currents above 350 Amperes, i.e. on voltage drops on R20 of more than 0.35 Volts. Main cause for such high currents are short-circuits or consumers with "large" inductances, e.g. welding transformers or large battery chargers, which exhibit remanence magnetism. Also large electrolytic capacitors from switching power supplies cause immense peak currents (computer screen), just as asymmetrical load of devices with single period rectifiers or thyristor regulaters, which cause a magnetical bias to the transformer of the inverter.
The electronic overload protection by IC9 is a special feature of our inverter. It needs a additional negative supply voltage, which is produced by a charge pump, consisting of IC10 and the transistors T9 and T10. IC9 works as threshold switch (Schmitt trigger). Sensitivity can be affected by change of the value of R22. A value of 1.5 kOhm means e.g. shutdown at lower currents (for inverters with smaller power output.
While starting the inverter, the negative supply voltage from the charge pump will be missing. This leads to immediate shutdown of the power mosfets, indicated by the red LED1. Thus indefinable control signals, that could result in unwanted switching, which would force small batteries to break down, are prevented. Our inverter therefore requests no maximum or minimum battery size - it works on any 12 Volt power supply. If the electronic overload protection becomes active, a positive output signal will be present at pin 6 of IC9. Through resistor R13 the flip-flop IC7 is set, which keeps the blockage upright until the next half wave on pin 11 appeares. IC7 may be closed likewise by transistor T3, which receives its signal from the optional "load detection" . If no load is detected, the inverter will be shutdown by this circuitry in order to save battery power.
Sensitivity of the shutdown circuitry may be tested by disconnecting the lead to resistor R20 and applying variable voltages at connector "C" in the range of 0 ... 1 Volt (important: transformer Tr1 must also be disconnected!). At approx. 0.35 Volts the red LED1 would light up and would get dark again at voltages of scarcely more than 0 Volt. Parallel to resistor R20 a 100 uA measuring instrument may be attached for display of load currents.

The optional "load detection" shall not be described here in detail. It consists of the circuit parts around resistor R33, transformer Tr2, relay1 and the ICs 12 and 13. If this part of the circuit shall not be used, the inverter would work in continuous operation. Thus T3, R10, R9, D6, R15 and D3 would be obsolete. The 230 Volts load would be connected directly to clamp 5 and 6 of transformer Tr1. The "load detection" recognizes an active load by a small DC through the contacts of relay1 and resistor R43. The inverter will be switched on for approx. 5 seconds. If then a load current would appear on R33, the inverter will remain switched on, indicated by LED2 (yellow). The limitation of the output power of 3000 Watts is due to power dissipation of R33. Instead of the "amateur-solution" of R33 and Tr2, a typical current transformer may be used. Some loads do not switch on the inverter, e.g. energy-saving lamps. For this the manual activation at port "G" is intended. A small 1 VA-transformer in parallel to the energy-saving lamp would also cause a DC-current and thus would solve the problem.

Data:
supply voltage: 12 Volt
battery size: depending upon load, otherwise no restriction
output voltage: 230 Volts rms (square wave voltage with duty cycle Tp=25% "modified sine")
good for resistive, inductive and "pseudocapacitive" load (e.g. computers)
efficiency: under full load approx. 95%
quiescent current of control electronics: approx.. 0.05 A ... 0.1 A
total: 0.5A to 2,5 A, depending upon quality and max. induction of the used transformer
pulse width regulation for the stabilization of rms of the output voltage
current limiter in case of short-circuit an thermal protection
option: load detection

Documents for download:
Schematic diagram (approx. 53 kB)
Parts list (approx. 4 kB)
Layout for printing board (approx. 118 kB)
Complement plan (approx. 148 kB)

Transformer:
We can make the transformer by changing the windings of an old transformer.
fig. 4: shell-type transformer core transformer
A transformer will provide best characteristics when the primary coil, that takes over magnetization of the iron core, fits closely around the core. For industrial transformers this would be the 230 Volts coil, on our inverters however it will be the 12 volt coil.
fig. 5: "EI"-sheet metals of a 850 VA-shell-type transformer
For the computation of the numbers of turns the following consideration applies:
The peak value of the primarily generated alternating voltage is given by the battery voltage. This determines the number of the primary windings of the transformer. On the secondary side of the transformer likewise the peak value must be taken also for computation, i.e. 325 Volt. In the case of a fully loaded battery the supply voltage of the inverter amounts to 13.8 Volts. The peak value of the 230 Volts output voltage may not exceed that of the usual supply networks, even if the rms could be held on 230 Volts by reduction of the duty cycle. The following table for the output voltage results (without pulse width regulation):
Battery voltage
Upeak (secondary)
Urms (secondary)
11,8 Volt
297 Volt
210 Volt
12,35 Volt
311 Volt
220 Volt
12,7 Volt (accord. 9 Vrms
320 Volt
227 Volt
12,9 Volt
325 Volt
230 Volt
13,5 Volt
340 Volt
240 Volt
The table applies to fixed duty cycle of 25% and/or sine-wave voltage and without consideration of the magnetization energy. Our inverter will keep the output voltage constant on an rms of 230 Volts, due to its pulse width regulation, even if the peak value will drop or rise, due to the battery voltage. The peak value will not exceed 350 Volts (247 Vrms for sine-wave voltage), critical for electronics, even in case of fully loaded battery. Theoretical, without pulse width regulation, the rms could rise again up to the theoretical factor of 2, according to a duty cycle of 50%, because of the magnetization energy. The recirculating magnetization energy already forms the beginning of the next half wave of the output voltage (see fig. 6). But without load there is no rms by definition, so this consideration is only of theoretical nature, with one exception: A measuring instrument, calibrated on rms would indicate a wrong output voltage and small consumers, who need less than the magnetizing energy of the transformer, could get damaged.
fig. 6: output voltage with no load or inductive load
The table shows, that the transformer needs a ratio of windings of 1 : 25. The schematic diagram shows, that it has two primary windings and one secondary. Both primary windings have the same number of turns and the secondary winding must have by factor 25 more turns (110 Volts: factor 13).
Here a selection of used transformers:
Length
Width
Deep
Power
Primarily
Secondary
idle current
150 mm
125 mm
50 mm
460 VA
2 x 13 W
325 W
1,4 Ampere
150 mm
125 mm
50 mm
460 VA
2 x 14 W
350 W
1,2 Ampere
150 mm
125 mm
67 mm
600 VA
2 x 10 W
250 W
2,2 Ampere
150 mm
125 mm
67 mm
600 VA
2 x 11 W
275 W
1,6 Ampere
150 mm
125 mm
67 mm
600 VA
2 x 12 W
300 W
1,4 Ampere
150 mm
125 mm
95 mm
1000 VA
2 x 9 W
225 W
1,4 Ampere

-
-
2000 VA
2 x 11 W
275 W
2,2 Ampere
170 mm
140 mm
80 mm
850 VA
2 x 12 W
300 W
1,5 Ampere
170 mm
140 mm
75 mm
850 VA
2 x 13 W
325 W
1,3 Ampere
175 mm
140 mm
60 mm
750 VA
2 x 13 W
325 W
1,2 Ampere
Length: length of the "I" from fig. 5
Deep: thickness of pile of all iron sheet metals
Power: rated output
The table shows, that the number of turns is not particularly critical. Only the ratio of primary windings to secondary must be correct. The rms of the output voltage will be finally adjusted by the automatic controller with R16 to the value of 220 or 230 Volts. It is of great importance however, that both primary coils are absolutely symmetrical. They must be wound bifilar, so that they are very close to each other. While one winding will magnetize the core, the corresponding winding will return the magnetizing energy. If there is no close coupling of both primary coils, energy losses will result by overvoltage, causing avalanche effects on the transistors. Despite completely symmetrical structure of the windings, the transformer will show a small magnetical bias (DC biasing), recognizable from the asymmetrical magnetizing currents, which can be watched on R20 with an oscilloscope. This biasing will change on every change of load, in particular with strong inductive loads. This effect is completely normal for square wave voltages at inductances and is connected with the heavy non-linearity of ferro-magnetical materials. The second half wave of the output voltage applies other magnetizing conditions to the ferro-magnetic transformer core due to the remaining remanence. (only with sine-wave voltages an equilibrium can adjust itself after several oscillations, due to hysteresis losses, see "Rush effect"). Critical unbalances, which develop e.g. after an impact short-circuit, are eliminated surely by the electronic shutdown system.

Wire strength:
Current densities from 3.5 A/mm2 to 4 A/mm2 are used on industrial transformers. If our inverter is not beeing used excessivly, current densities may even be higher. A transformer with 1000 VA needs approx. 84 ampere from the 12 Volt battery on nominal load. Since the two primary coils alternate mutually, we may count from 42 amperes. (This is strictly not correct, since the acceptance applies only if both windings would exhibit double surface for heat emission). For a round wire this would mean a diameter of 4 mm. Such wire is hardly to wind, also automats can't do it perfectly. A solution may be wires with rectangular cross section or several smaller wires in parallel.
After winding the transformer, the sheet metals must be inserted again. With each layer we change the direction of the sheet metals, while in the original condition several sheet metals were probably summarized into packages, in order to increase the air gap and linearize magnetizing currents. This effect isn't needed for our inverter. Magnetizing currents are always extremely nonlinear in square wave transformers, and they are asymmetrical also. This has no effect on the performance of the inverter and the output voltage.
After the transformer has been built, it should first be tested. Therefore we attach its 230 Volts windings to public electricity mains or any other 230 Volts source. Each low-voltage coil should now show 9 Volts. Now we can connect the beginning of one "primary" coil with the end of the other. At the free ends a voltage of 18 V now should appear. If this voltage would be 0 V, the windings have been connected the wrong way.

The making of a transformer is a very laborious work. Nobody likes to take a transformer apart for a second time to correct the windings. With unknown transformers it is advisable to apply first a sample coil of thin and easy to handle wire and test the power input on idle. The windings of the sample coil can be changed without dividing the transformer. For this test the transformer does not need the secondary 230 Volts coil. Only the electronics must be adjusted correctly (tested with another, correct transformer or an oscilloscope: duty cycle 25%).

Transforer computation:
For the first regard, it appears difficult to seize the obscure and for precisive computation not accessible magnetization procedures in the magnetic core of a transformer. I want to show, that in our case this is not necessary. As the table with the numbers of turns shows, a tranformer may be built on different numbers of turns, only the relation to each other must be exact.
We specify the maximum magnetic induction on a value of 1.5 Tesla. For computation now only two simple equations are necessary:
Uind = n x F /t converted: 1') n=Uind x t/F
F = B x A
Uind=induced voltage
n=number of turns
F = magnetic flux
t=transistor switch-on time
B= magnetic induction
A=cross-section area of transformer core
For power electronics resistive load shall not calculate on energy conversion. Thus the whole battery voltage will apply on the transformer coil for the whole switch-on time of the transistor. The switch-on time results in 5 milliseconds, dependend on the period of the 50 cycles / second oscillation and a duty-cycle of 25% (period of a 50 cycle oscillation is 1 / 50 Hz = 20 milliseconds).

Sample calculation for the above described 850 VA transformer:
The cross-section area of the transformer calculates to A = 60 mm x 80 mm = 4,8 x 10-3 m2

Uind = 12.7 Volt
B= 1.5 Tesla = 1.5 Vs/ m2
t= 5 ms
A= 4.8 x 10-3 m2

With equation 2) the magnetic flux calculates to F = B x A = 1.5 Vs/ m2 x 4.8 x 10-3 m2 = 7.2 x 10-3 Vs
set in equation 1') results
Number of turns n=Uind x t/F = 12.7 V x 5 x 10-3 s / (7.2 x 10-3 Vs) = 8.82 (rounded up 9 turns).
By trying I built the transformer with 2 x 12 turns. The losses were clearly smaller thereby. The calculated flux in this case was only 1.1 Tesla.
High load resistor R 20:
fig. 7: resistor 0,001 Ohm made of high-grade steel sheet metal
Resistor R20 takes up the whole load current of the inverter and thus enables the electronic shutdown circuit by evaluation of a small voltage drop. I was very astonished to learn, that different steel grades had different electrical resistance. High-grade steel exhibited a 2.5 higher resistance than conventional steel and that's why I used it. The data for other steel type may differ, so here what I experienced:
value of the resistor: 0,001 ohms
length: 110 mm
Width: 40 mm
Thickness: 1 mm
Distance of the screws for load current: 80 mm
Diameter of screw connections: 6 mm
Distance of solder taps: 55 mm (actual measuring section with 0,001 ohms!)
The actual value of the resistor between the screw connections is little more higher than between the solder taps, the actual measuring section. At the solder points additionally a small 100 uA panel meter may be attached.
Alignment of the resistor:
For an unknown steel grade or the desire for particularly high accuracy of the resistor value an alignment with a defined examining current is recommendet, e.g. by a car lamp at the lab power supply unit. The load current would be adjusted to e.g. 5 amperes. Now we may find with the test prods of a sensitive millivoltage measurement instrument those two points on the resistor, for which the voltage drop will be U= I * R = 5 A * 0,001 Ohm = 5 mV. These points would be marked by a felt-tip pen. At these points the solder taps will be fixed by screws.
Control electronics:
The use of a pre-drilled print board is most comfortable. In the past most inverters have been built on strip hole plates, in small-batch manufacturing.
fig. 8: control electronics on strip hole plate (previous version) and PCB of the "professional edition"
Assembly of the mosfet-transistors on the heat sink:
fig. 10: heat sink, mosfet transistors, connections
Testing:
MOS-FET-transistors:
The transistors on the heat sink may be tested while they are not yet connected to the transformer and the control unit. First we touch with one hand the source connections of the transistors and with the other the gate connections. This will discharge the gates. Now the source / drain connections must behave like a diode, which we can test with an ohm meter. For the next test we connect a car lamp between the drain connections of the transistors and the positive pole of a battery. The negative pole will be connected to the source of the transistors. The gate must be open. If we now touch with one hand the positive pole of the battery and with the other the gates, the lamp will light up. Now we touch the negative pole of the battery and simuntanously the gates and the lamp will be switched off. If this test is positive, the transistors are o.k.
Control unit: For testing the control unit, clamp "G" and clamp "C" must be connected to ground (minus pole). This prevents the load detection circuit from switch-off. The outputs "A" and "B" will show an output voltage between 3.5 and 4 volts. Theoretically the exact value should be 2.5 volts, according to a duty cycle of 25%, but the transformer is not yet connected and so the pulse width regulator will generate maximum value. If a frequency counter and an oscilloscope are available, the control signals may be checked and adjusted to 50 cycles or 20 milliseconds (period of the 50 cycle oscillation) at these outputs. During normal operation the transformer generates peak voltages up to 28 volts on clamp "D". The pulse width regulator may be tested, if variable DC voltages from 12 V to 28 V will be applied to this connection. For testing the current limiter, variable DC voltage may be applied to clamp "C" (0 ... 1 Volt). The switch-off should take place at about 0.35 volts.
The control unit may also be tested in connection with the mosfet transistors. Instead of the transformer, autolamps would be connected. The brightness of the lamps may now be adjusted by turning resistor R16 or connecting a DC voltage to clamp "D" as described above.
The autolamp also makes possible a very simple test to adjust the frequency. Therefore we put in series with the lamp the 12 volts output of a small tranformer, connected primarily to the mains supply. Both alternating voltages will now be added or subtracted, dependent on the phase shift. The lamp will flicker. The goal is, to make this flickering very slowly. Attention: The autolamp must be 24 volts or two lamps in series.
Final assembly:
fig. 11: 1500 VA inverter with 2 parallel transformers and 1000 VA inverter

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