A current source is a piece of equipment that is used to produce or receive an electric current. It is related to the voltage source, which is the dual of the current source. Often, a current source will be used to power various machines, which can derive energy from the electric current produced. There is a wide range current sources available, such as resistor sources, active sources, radio frequency (RF) current sources, and direct current (DC) and alternating current (AC) sources. Each type uses a different mechanism to generate and maintain an electric current. Resistor sources are considered to be the simplest forms of current source. They involve a circuit that contains a resistor in series with a source of voltage. The current generated from this system is equivalent to the amplitude of the voltage, divided by the resistance of the resistor. A resistor current source is generally not considered effective on its own because of the large amount of power lost in the resister, but many types of current source use elements of this setup. For example, active current sources often substitute a different element for the resistor. These use elements of transistors or vacuum tubes to replace the resistor because they can act as current sources when supplied with energy. By substituting these parts in for the resistor, the system will not lose the same amount of power. DC and AC sources are frequently used in many different processes requiring electrical energy. Direct current is simply the flow of an electric current in one direction, while alternating current involves an alternating direction of current flow. AC can be converted to DC with the use of a rectifier, a device that permits only one direction of current flow. Equipment such as batteries, solar cells, and low-voltage devices frequently feature the use of a DC source, while certain motors and types of lighting rely on an AC source. RF current source sends electrical signals at the frequency of radio waves. They differ from DC and AC sources in that they operate at a lower frequency and a higher voltage. Additionally, RF current generally runs along the surface of an electric conductor rather than completely within it, and it has the capacity to travel through insulating elements. It also has a greater capacity to ionize gas and gas plasma materials making it optimal for certain processes, such as electric arc welding and thin film sputtering.
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