By David Herres
Thomas Edison, in the waning years of the 19th century, attempted to make a case for his preferred dc electrical system. But George Westinghouse and Nikola Tesla prevailed. AC has endured and with good reason. A simple passive device, the transformer, suffices to change voltage levels for transmission over various distances, and there is less corrosion at terminations because the electrochemical activity is self-cancelling as polarity reverses.The process of converting ac to dc, including filtering and regulation, is known as rectification. A simple, inexpensive rectifier can be built by placing a diode in series with an ac input. The diode conducts when forward biased and does not conduct when reverse biased. So the output of this half-wave rectifier consists only of the portion of the waveform either above the X axis or below it, depending upon how the diode is polarized.
The half-wave rectifier does not make good use of power available at the input. Moreover, this topology places heavy burdens on the diode and filter capacitors.
A full-wave rectifier, in contrast, makes use of the total input waveform. It converts both polarities to a pulsating waveform that can subsequently be filtered to create a high-quality dc. A center-tapped transformer with two diodes suffices to rectify both halves of the ac input waveform. Another circuit configuration that yields the same full-wave output consists of four diodes connected as a “bridge.” This bridge rectifier does not require the transformer to have a center tap, and the power flow is evenly distributed, boosting reliability and reducing maintenance.
In industrial applications, three-phase half-wave and full-wave rectifiers are used, the half-wave rectifier having three diodes and the full-wave rectifier having six diodes.
Many people, including experienced mechanics, are surprised to learn that an automotive alternator is actually three-phase. Six internal diodes provide bridge rectification so the output is dc with reduced ripple, ready for easily accomplished filtering to supply the audio system and other ubiquitous electronics.