An electrical network is an interconnection of electrical elements such as resistors, inductors, capacitors, transmission lines, voltage sources, current sources and switches. An electrical circuit is a special type of network, one that has a closed loop giving a return path for the current. Electrical networks that consist only of sources (voltage or current), linear lumped elements (resistors, capacitors, inductors), and linear distributed elements (transmission lines) can be analyzed by algebraic and transform methods to determine DC response, AC response, and transient response.
A network that contains active electronic components is known as an electronic circuit. Such networks are generally nonlinear and require more complex design and analysis tools.
TUTORIALS:
History of Electrical Circuits
Early investigations of static electricity go back hundreds of years. Static electricity is a transfer of electrons produced by friction, like when you rub a balloon across a sweater. A spark or very brief flow of current can occur when charged objects come into contact, but there is no continuous flow of current. In the absence of a continuous current, there is no useful application of electricity.
The invention of the battery -- which could produce a continuous flow of current -- made possible the development of the first electric circuits. Alessandro Volta invented the first battery, the voltaic pile, in 1800. The very first circuits used a battery and electrodes immersed in a container of water. The flow of current through the water produced hydrogen and oxygen.
The first widespread application of electric circuits for practical use was for electric lighting. Shortly after Thomas Edison invented his incandescent light bulb, he sought practical applications for it by developing an entire power generation and distribution system. The first such system in the United States was the Pearl Street Station in downtown Manhattan. It provided a few square blocks of the city with electric power, primarily for illumination.
One classification of circuits has to do with the nature of the current flow. The earliest circuits were battery-powered, which made in a steady, constant current that always flowed in the same direction. This is direct current, or DC. The use of DC continued through the time of the first electric power systems. A major problem with the DC system was that power stations could serve an area of only about a square mile because of power loss in the wires.
In 1883, engineers proposed harnessing the tremendous hydroelectric power potential of Niagara Falls to supply the needs of Buffalo, N.Y. Although this power would ultimately go beyond Buffalo to New York City and even farther, there was an initial problem with distance. Buffalo was only 16 miles from Niagara Falls, but the idea was unworkable -- until Nikola Tesla made it possible, as we'll see on the next page.
Tesla's Breakthrough
Engineer Nikola Tesla, aided by theoretical work by Charles Proteus Steinmetz, came up with the idea of using alternating current, or AC. Unlike direct current, AC is always changing and repeatedly reverses direction.
So why was AC the answer to the problem of long-distance power transmission? With AC, it's possible to use transformers to change voltage levels in a circuit. Transformers work on a principle of magnetic induction, which requires a changing magnetic field produced by the alternating current. With transformers, voltages can be increased for long-distance transmission. At the receiving end, the voltage level can decrease to a safer 220V or 110V for business and residential use.
We need high voltages for long distances because wire resistance causes power loss. The electrons bumping into atoms lose energy in the form of heat as they travel. This power loss is proportional to the square of the amount of current moving through the wire.
To measure the amount of power the line transmits, you can multiply the voltage by the current. You can express these two ideas using an equation in which I represents current, V represents voltage and P equals power:
(P = V· I)
Let's consider the example of transmitting 1 megawatt. If we increase the voltage from 100V to 10,000V, we can then decrease the current from 10,000A to 100A. This will reduce the power loss by (100)2, or 10,000. This was Tesla's concept, and from that idea power transmission from Niagara Falls to Buffalo, and ultimately to New York City and beyond, became a reality.
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Edison vs. Tesla
Thomas Edison was a brilliant and intuitive inventor. However, his limited schooling, especially in mathematics, kept him from a true understanding of the theory behind AC electricity. He understood DC well enough, but AC was, strangely, a bit beyond his grasp. He strongly opposed the idea of using AC for long-distance power transmission, but AC gradually replaced DC as the primary means of electric power transmission.
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In the United States and many other countries, the standard frequency for AC power is 60 cycles per second, or 60 Hz (Hertz). This means that 60 times a second, a complete cycle of the current flows in one direction and then in the other. The current flows in one direction for 1/120th of a second and in the other direction for another 1/120th of a second. The time it takes for one cycle to be completed is called a period, which in this case is 1/60th of a second. In Europe and other areas, the standard frequency for AC power is 50 Hz.
Electronic Circuits
You may have heard the term chip, especially when the subject of computer hardware comes up. A chip is a tiny piece of silicon, usually around one centimeter square. A chip may be a single transistor (a piece of silicon that amplifies electrical signals or serves as an on/off switch in computer applications). It can also be an integrated circuit composed of many interconnected transistors. Chips are encapsulated in a hermetically sealed plastic or ceramic enclosure called a package. Sometimes people refer to the whole package as a chip, but the chip is actually inside the package.
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The IC Revolution:
Microelectronics
In the early days of electronic circuits, components like vacuum tubes and transistors were individual devices mounted on a metal chassis or printed circuit boards. Then, in 1959, two researchers, Jack Kilby at Texas Instruments and Robert Noyce at Fairchild Semiconductor (who were working independently), started the microelectronics revolution by developing the first integrated circuit.
They discovered how to combine or integrate several transistors and resistors and connect them to form a circuit, all on the same small chip of silicon. Today, very complex electronic systems --like microprocessors containing millions of transistors -- can fit on a single inch-square silicon chip. These integrated circuits are what make modern-day computers possible.
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There are two basic types of integrated circuit -- monolithic and hybrid. Monolithic ICs include the entire circuit on a single silicon chip. They can range in complexity from just a few transistors to millions of transistors on a computer microprocessor chip. A hybrid IC has a circuit with several chips enclosed in a single package. The chips in a hybrid IC may be a combination of transistors, resistors, capacitors and monolithic IC chips.
A printed circuit board, or PCB, holds an electronic circuit together. The completed PCB with components attached is a printed circuit board assembly, or PCBA. A multilayer PCB may have as many as 10 stacked PCBs. Electroplated copper conductors passing through holes called vias connect the individual PCBs, which forms a three-dimensional electronic circuit.
The most important elements in an electronic circuit are the transistors. Diodes are tiny chips of silicon that act as valves to allow current flow in only one direction. Other electronic components are passive elements like resistors and capacitors. Resistors offer a specified amount of resistance to current, and capacitors store electric charge. The third basic passive circuit element is the inductor, which stores energy in the form of a magnetic field. Microelectronic circuits very rarely use inductors, but they are common in larger power circuits.
Most circuits are designed using computer-aided design programs, or CAD. Many of the circuits used in digital computers are extremely complex and use millions of transistors, so CADs are the only practical way to design them. The circuit designer starts with a general specification for the functioning of the circuit, and the CAD program lays out the complex pattern of interconnections.
The etching of the metal interconnection pattern on a PCB or IC chip uses an etch-resistant masking layer to define the circuit pattern. The exposed metal is etched away, leaving the pattern of connecting metal between components.
Why is AC used in electronic circuits?
In electronic circuits, the distances and currents are very small, so why use AC? First of all, the currents and voltages in these circuits represent constantly changing phenomena, so the electrical representations, or analogs, are also constantly changing. The second reason is that radio waves (like those used by TVs, microwaves and cell phones) are high-frequency AC signals. The frequencies used for all types of wireless communication has steady advanced over the years, from the kilohertz (kHz) range in the early days of radio to the megahertz (MHz) and gigahertz (GHz) range today.
Electronic circuits use DC to provide power for the transistors and other components in electronic systems. A rectifier circuit converts AC power to DC from the AC line voltage.
