Sources of direct current include power supplies, electrochemical cells and batteries, and photovoltaic cells and panels. The intensity, or amplitude, of a direct current might fluctuate with time, and this fluctuation might be periodic. In some such cases the dc has an ac component superimposed on it. An example of this is the output of a photovoltaic cell that receives a modulated light communications signal. A source of dc is sometimes called a dc generator.
Batteries and various other sources of dc produce a constant voltage. This is called pure dc and can be represented by a straight, horizontal line on a graph of voltage versus time. The peak and effective values are the same. The peak to peak value is zero because the instantaneous amplitude never changes. In some instances the value of a dc voltage pulsates or oscillates rapidly with time, in a manner similar to the changes in an ac wave. The unfiltered output of a half wave or a full wave rectifier, for example, is pulsating dc.
History
In 1820, Hans Christian Orsted discovered that electrical current creates a magnetic field. This discovery made scientists relate magnetism to the electric phenomena.In 1879, Thomas Edison invented an electric light bulb. He improved a 50-year-old idea using lower current electricity, an improved vacuum inside the globe and a small carbonized filament, and produced a reliable and long-lasting source of light. At that time, the idea of electric lightning was not new, but nothing had been developed that was practical enough for home use. Edison not only invented an incandescent electric light, but an electric lighting system that contained all the necessary elements to make the incandescent light safe, economical, and practical. Prior to 1879, direct current (DC) electricity had been used in lighting for the outdoors.
It was in the 1880's when the modern electric utility industry began. It was an evolution from street lighting systems and from gas and electric carbon-arc commercial systems. On September 4th, 1882, Edison switched on the world's first electrical power distribution system, providing 110 volts of direct current (DC) to fifty-nine customers, and the first commercial power station began working. It was located in Lower Manhattan, on Pearl Street. This station provided light and electricity to customers in a one square mile range. The electric age had begun. The station was called "Thomas Edison's Pearl Street Electricity Generating Station." This station introduced four elements of a modern electric utility system: Efficient distribution, competitive price, reliable central generation and successful end use.
The War of the Currents-
Edison developed direct current. During the early years of electricity, direct current was the standard in the U.S.
Tesla believed that alternating current (or AC) was the solution to this problem. Alternating current reverses direction a certain number of times per second -- 60 in the U.S. -- and can be converted to different voltages relatively easily using a transformer.
The Chicago World’s Fair -- also known as the World’s Columbian Exposition -- took place in 1893, at the height of the Current War.
General Electric bid to electrify the fair using Edison’s direct current for $554,000, but lost to George Westinghouse, who said he could power the fair for only $399,000 using Tesla’s alternating current.
That same year, the Niagara Falls Power Company decided to award Westinghouse -- who had licensed Tesla’s polyphase AC induction motor patent -- the contract to generate power from Niagara Falls. Although some doubted that the falls could power all of Buffalo, New York, Tesla was convinced it could power not only Buffalo, but also the entire Eastern United States.
On Nov. 16, 1896, Buffalo was lit up by the alternating current from Niagara Falls. By this time General Electric had decided to jump on the alternating current train, too.
It would appear that alternating current had all but obliterated direct current, but in recent years direct current has seen a bit of a renaissance.
Today our electricity is still predominantly powered by alternating current, but computers, LEDs, solar cells and electric vehicles all run on DC power. And methods are now available for converting direct current to higher and lower voltages. Since direct current is more stable, companies are finding ways of using high voltage direct current (HVDC) to transport electricity long distances with less electricity loss.
So it appears the War of the Currents may not be over yet. But instead of continuing in a heated AC vs. DC battle, it looks like the two currents will end up working parallel to each other in a sort of hybrid armistice.
Circuits
A direct current circuit is an electrical circuit that consists of any combination of constant voltage sources, constant current sources, and resistors. In this case, the circuit voltages and currents are independent of time. A particular circuit voltage or current does not depend on the past value of any circuit voltage or current. This implies that the system of equations that represent a DC circuit do not involve integrals or derivatives with respect to time.If a capacitor or inductor is added to a DC circuit, the resulting circuit is not, strictly speaking, a DC circuit. However, most such circuits have a DC solution. This solution gives the circuit voltages and currents when the circuit is in DC steady state. Such a circuit is represented by a system of differential equations. The solution to these equations usually contain a time varying or transient part as well as constant or steady state part. It is this steady state part that is the DC solution. There are some circuits that do not have a DC solution. Two simple examples are a constant current source connected to a capacitor and a constant voltage source connected to an inductor.
In electronics, it is common to refer to a circuit that is powered by a DC voltage source such as a battery or the output of a DC power supply as a DC circuit even though what is meant is that the circuit is DC powered.
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