Types of direct current

The first commercial electric power transmission (developed in the late nineteenth century) used direct current. Because alternating current is more convenient matt current for electric power distribution and transmission, today nearly all electric rower transmission uses alternating current.

Various definitions

Within Electrical Engineering, the term DC is a synonym for constant. For example, the voltage across a DC voltage source is constant as is the current through J current source. The DC solution of an electric circuit is the solution where all voltages and currents are constant. It can be shown that any voltage or current waveform can decomposed into a sum of a DC component and a time-varying component. The DC component is defined to be the average value of the voltage or current over all time. THE average value of the time-varying component is zero.

Although DC stands for "Direct Current,DC sometimes refers to "constant polarity”

With this definition, DC voltages can vary in time, such as the raw output of a rectifier. Some forms of DC (such as that produced by a voltage regulator) have almost no variations in voltage, but may still have variations in output power and current.

Applications

Direct current installations usually have different types of sockets, switches, and fixtures, mostly due to the low voltages used, from those suitable for alternating current. It is usually important with a direct current appliance to not reverse polarity unless the device has a diode bridge to correct for this. (Most battery-powered devices don't.)

High voltage direct current is used for long-distance point-to-point power transmission and for submarine cables, with voltages from a few kilovolts to approximately megavolt.

DC is commonly found in many low-voltage applications, especially where these are powered by batteries, which can produce only DC, or solar power sys solar cells can produce only DC. Most automotive applications use DC, although the generator is an AC device which uses a rectifier to produce DC. Most electronic circuits require a DC power supply.

Power supplies

Audio Amplifier

Power-line operated power supplies are based on transformers, diodes, 1 and capacitors. Transformers enable these supplies to exchange voltage for current to obtain the large currents and low voltages needed by audio amplifiers and other house-hold electronic devices. Diodes prevent currents from flowing backward through these supplies so that, like batteries, they always pump currents from their negative terminals to their positive ones. Transistors help to control the voltage of the current delivered by a power supply-they cause the output current to experience an adjustable and carefully controlled voltage drop so that the current leaves the supply at a regulated voltage. And capacitors in the supply store separated electric charge and energy for delivery during the times when the supply isn't receiving any power from the alternating current source.

A basic power supply, without any transistor regulation, connects the primary of its transformer to the electric outlet and the secondary of that transformer to a set of diodes. These diodes act as electronic switches, directing current always in one direction through the circuit that the power supply operates. To stop the current from fluctuating so much, capacitors are often attached to the two output wires of the power supply. These capacitors store separated charge during times when the AC power line is delivering more than enough current and release some of that charge with the power line isn't delivering enough.

In more sophisticated modern power supplies, often called "switching power supplies," current from the electric outlet is sent through diodes and into capaci­tors, where it is stored as separated charge. Several transistors then act as electronic

switches to produce a high-frequency alternating current from this stored charge. With a frequency of tens of thousands of cycle* per second, this current passes through the primary of a small transformer. Only a small transformer is needed because the short duration of a cycle reduces the amount of energy that the transformer needs to store in its magnetic field during one cycle. Power passes to a current in the transformer's secondary coil. This secondary coil current is rectified (made DC) by passing it through more diodes. The DC current is filtered with more capacitors and a final transistor stage controls the delivery voltage. But the main delivery voltage control comes from turning on or off the high-frequency AC that is synthesized and delivered to the transformer's primary coil. By starting or stopping this AC current, the power supply can control the flow of power through the transformer and ciently limit the voltage of its overall output.