What Is the Difference Between AC & DC?

By Chris Blank

In much of the world, and especially in the industrialized western world, electricity is an essential element of daily life, along with appliances and gadgets that run on battery power. Alternating current (AC) and direct current (DC) make modern industrialized life possible. Each type of power-generation system works on its own set of principles. However, unknown to most people, the two types of electric current also often work together.

Edison, Tesla and Westinghouse

In the late 19th century, DC, which was championed by Thomas Edison, inventor of the electric light bulb, appeared to be in the position to become the dominant means of electric power in the United States. However, at the turn of the 20th Century, Nikola Tesla invented AC, along with the means to transmit AC by generators and motors. He then licensed his patents to George Westinghouse, who went head-to-head with Edison. Westinghouse and Tesla eventually prevailed with AC for the electrical grid system, although DC and battery power are still essential for portable electronic appliances and machinery.

How AC Power Works

AC power gets its name because of the way the electrons that form its current flow. AC flows in one direction, then alternates to flow in the opposite direction. AC has the capacity to be carried over long distances via power cords without a significant loss of power. This is the main reason AC prevailed over DC (much like VHS prevailed over Beta because VHS tapes had a longer recording capacity).In the modern electric grid, AC power is transmitted to and from transformers by high-capacity wires, which then connect to the household or commercial establishment that draws from the power through its own internal wiring system. Power cords then connect appliances and electronics to the AC power source by being plugged into outlets. Some businesses and homes also have generators that can be used as a backup system in case of a failure of the main AC power source.

How DC Power Works

DC power works by generating a flow of direct current, or current which flows in a single direction. Sometimes DC is called "dry cell" when referring to batteries, although this is technically incorrect. With a battery, when the stored energy has been depleted, it ceases to generate electron flow. In popular terms, the battery is "dead." With rechargeable batteries, this process can repeat itself several times, although eventually, even rechargeable batteries fail to have the capacity to generate electron flow.With Edison's system, DC power was produced by a generator, which actually initially produced AC power, which was then converted to DC by a device called a commutator. However, DC power cannot be carried over long distances without voltage (power) loss. There is also the danger of long distance wires overheating and catching fire unless they were made of very expensive super-conductive materials. DC power was eventually dismissed as not being commercially viable.

How AC and DC Power Work Together

Most homes convert the AC power that they receive from the electric grid into high voltage DC power. A switching power supply located in the home electrical system then converts the high voltage DC power into lower voltages. These lower voltages are what household appliances actually use. Most subsystems use DC voltages ranging from less than 1 volt to about 48 volts. There are losses involved with the conversion process, but most systems run with at least 80 percent efficiency. Many electronic conversion systems feature more than 90 percent efficiency.

DC Power and the Green Future

Beginning in the early 21st Century, there has been a movement to revive DC power. This movement is motivated by such innovations as electric cars, which are designed to decrease dependency on carbon based fuels. With a DC power gateway located at a home or business, it would be very efficient to recharge an electric car. An additional benefit is that the same DC power gateway could be used to run the appliances in the household or commercial establishment.