Direct current is a very important type of electric current in solar energy where the direction of circulation of the flow of electric charges (electrons) does not vary. In many devices the symbol to indicate direct current is DC (direct current), or using the symbol of a continuous line (-) flanked on three shorter lines (---), for the alternating current that is used instead AC (alternating current) or the symbol (~).
The flow of charges occurs through a conductor, such as a metallic wire. The flow could also be established through a semiconductor, an insulator or even under vacuum as occurs in a cathode ray tube. In this type of electric current (direct current) electric charges always flow in the same direction, being a characteristic feature against alternating current.
A synonym in disuse of direct current is galvanic current.
The first commercial power grid, developed by Thomas Edison at the end of the 19th century, used direct current. Nowadays, due to the advantages of alternating current in terms of transformation and transport possibilities, transport and distribution networks use almost exclusively alternating current. In the case of applications that need direct current, as in the case of the railway that uses the third rail system, the alternating current reaches a substation that uses a rectifier to convert it into direct current.
Direct current sources
The first sources of direct current were the sources of chemical current: galvanic cells, then the batteries were invented. The polarity of current chemical sources cannot change spontaneously.
To obtain continuous current on an industrial scale, electric machines are used: direct current generators. Another way to obtain electricity in the form of direct current is by means of photovoltaic solar energy. The photovoltaic cells that make up the photovoltaic modules generate a direct current thanks to the photovoltaic effect.
In the electronic equipment, the AC power supply for the use of DC power supplies. As a general rule, the transformer lowers the alternating current to the desired value and then rectifies it.
In modern electronic equipment, pulse power supplies have become widespread. The soft waves of the output voltage are produced due to the presence of an integrating element that can accumulate electrical energy and transmit it to the load. As a result, an almost constant current can be obtained at the output.
Electricity can accumulate electric capacitors. In general, when a capacitor is discharged, an alternating current flows in the external circuit. If the capacitor discharges through a resistor, a unidirectional alternating current (which gradually decreases) appears. However, if the capacitor is discharged through an inductor, then a bidirectional alternating current appears in the circuit, this device is called an oscillating circuit. Electrolytic capacitors can have a very large electrical capacity (hundreds and thousands of microfarads and more) When these capacitors are discharged through high resistance, the electric current decreases more slowly, and for a short time we can assume that a direct current flows in the external circuit.
Production and conversion of continuous current
The direct current can be produced not only with a dynamo, but also through an alternator and then from an alternating current (AC) with, followed by a straightening process, performed with diodes or rectifier bridges. In reality, these devices eliminate the negative component of alternating current, producing a current that is not continuous but pulsating unidirectional, that is, ideally composed of an alternating current superimposed on a continuous one. A capacitor next to the rectifier allows the signal to be leveled, providing a current as close as possible to a continuous value.
On the contrary, the conversion of a direct current into an alternating current is much more complex, in particular because it is necessary to generate information on the waveform, the frequency and the phase. The operation is carried out by complex electronic devices called inverters.
Direct current applications
Direct current is normally used for applications where we need a low voltage, especially where the energy is produced by batteries or by photovoltaic solar energy systems ( photovoltaic cells), since both only produce direct current.
To a circuit with direct current, it is important not to change the polarity, unless the device has a diode that allows it (most devices do not allow it).