Direct current (DC) is electric current that flows in one direction, unlike alternating current (AC), which periodically reverses direction. DC may have a constant magnitude or vary with time while retaining its direction. It is supplied by sources such as batteries and solar cells and is used in electronic equipment and electrical transmission systems. The abbreviation also describes voltage supplies with a fixed polarity, even when no current is flowing. (openstax.org)
Physical meaning and measurement
Electric current measures the rate at which electric charge passes through a cross-section. Its SI unit is the ampere, equivalent to one coulomb per second. Conventional current is defined in the direction positive charge would move. In a metallic conductor, the mobile carriers are negatively charged electrons, whose average drift is opposite to conventional current. For a battery supplying an external circuit, conventional current travels from the positive terminal through the load to the negative terminal. (openstax.org)
Direction and magnitude are separate properties. A steady direct current maintains both, whereas a pulsating direct current changes magnitude without reversing direction. Consequently, “DC” does not necessarily mean perfectly smooth electricity. Likewise, a constant-voltage source does not guarantee constant current: the current depends on the connected circuit and its operating conditions. (openstax.org)
Sources and conversion
A battery maintains a potential difference through electrochemical processes. Connecting its terminals through a suitable conducting path allows it to deliver direct current. A solar cell also produces DC, although its output depends on illumination and operating conditions. Solar installations connected to an AC electrical grid therefore require conversion equipment between the photovoltaic source and the grid. (openstax.org)
A rectifier converts AC into a unidirectional electrical output. Rectification alone does not necessarily produce a constant voltage; the output can contain substantial periodic variation, called ripple. Filtering and regulation reduce this variation. Output capacitors are important in power converters because their capacitance and internal resistance influence the remaining ripple voltage. (ti.com)
An inverter performs the reverse conversion, producing AC from a DC input. In grid-connected solar equipment, electronic switches rapidly change the electrical connections, and filtering produces the required output waveform. A DC–DC converter instead changes one DC voltage level into another. Common switching arrangements include buck converters, which reduce voltage; boost converters, which increase it; and buck–boost converters, which accommodate either relationship between input and output. (energy.gov)
Circuit behavior
In a resistive DC circuit, Ohm’s law relates current, voltage, and resistance:
where is the potential difference, the current, and the resistance. This relation applies to ohmic components under conditions in which their resistance remains approximately constant. It is not a universal description of every electronic device. Circuit analysis also uses conservation of charge at junctions and conservation of energy around closed loops. (opentextbc.ca)
For a resistor, electrical power can be expressed as
This conversion of electrical energy into thermal energy is Joule heating. The relationship explains why reducing current reduces resistive transmission losses. For a specified transmitted power, increasing voltage permits a lower current; this principle benefits both AC and DC transmission. (opentextbc.ca)
A DC source can also produce time-dependent circuit behavior. When an initially uncharged capacitor is connected to a constant-voltage source through a resistor, charging current initially flows and then decreases. In the ideal steady state, the capacitor voltage equals the source voltage and the current becomes zero. The characteristic charging time is determined by the product . Thus, transient behavior must be distinguished from the final steady-state condition. (openstax.org)
Historical development
During the late nineteenth century, competing commercial systems promoted DC and AC distribution in the dispute known as the “War of the Currents.” Thomas Edison advocated DC systems, while George Westinghouse promoted AC systems incorporating Nikola Tesla’s inventions. AC had a decisive practical advantage: an electrical transformer could readily change its voltage, permitting high-voltage transmission followed by lower-voltage distribution. Contemporary DC systems lacked comparably convenient voltage conversion. (energy.gov)
AC consequently became the principal framework for public electricity networks, but DC remained important in batteries and equipment operating internally on DC. Advances in power electronics subsequently made DC voltage conversion and large-scale AC–DC conversion practical. The distinction therefore concerns complementary electrical technologies rather than an absolute replacement of one by the other. (energy.gov)
Applications and transmission
Computers and many other electronic devices operate internally from DC supplies, even when their external input is AC. Battery-powered equipment and photovoltaic systems likewise rely on DC at the source. Power-conversion stages connect these devices to supplies or loads with different voltage and waveform requirements. (energy.gov)
High-voltage direct-current transmission (HVDC) transfers large amounts of power over long distances and can connect AC networks that are not synchronized. A typical link converts AC to DC at one terminal and DC back to AC at the other. Converter stations add substantial cost, so the economic comparison depends on distance, route, and installation type. HVDC is also used for submarine cable connections. (energy.gov)
DC protection presents distinct engineering challenges. Unlike AC, steady direct current does not periodically pass through zero, making interruption more difficult. Fault detection, isolation, and switching equipment must therefore account for the electrical behavior of DC systems rather than simply reproduce AC arrangements. (energy.gov)