A device that changes alternating current (AC), which continuously reverses direction, into direct current (DC), by allowing it to flow in one direction only. Electric lights and motors use alternating current, but in general most electronic equipment needs direct current. Semiconductor diodes can be used as rectifiers.
A rectifier is an electrical device, comprising one or more semiconductive devices (such as diodes) or vacuum tubes arranged for converting alternating current to direct current. When just one diode is used to rectify AC (by blocking the negative or positive portion of the waveform) the difference between the term diode and the term rectifier is merely one of usage, e.g., the term rectifier describes a diode that is being used to convert AC to DC. Rectification is a process whereby alternating current (AC) is converted into direct current (DC). Almost all rectifiers comprise a number of diodes in a specific arrangement for more efficiently converting AC to DC than is possible with just a single diode. Before the development of solid state rectifiers, vacuum tube diodes and copper oxide or selenium rectifier stacks were used. Two metal electrodes in the outer layer of the flame provide a current path and rectification of an applied alternating voltage, but only while the flame is present.
Half-wave rectification
A half wave rectifier is a special case of a clipper. In half wave rectification, either the positive or negative half of the AC wave is passed easily while the other half is blocked, depending on the polarity of the rectifier. Half wave rectification can be achieved with a single diode in a one phase supply.
Full-wave rectification
Full-wave rectification converts both polarities of the input waveform to DC, and is more efficient. This is due to each output polarity requiring 2 rectifiers each, for example, one for when AC terminal 'X' is positive and one for when AC terminal 'Y' is positive. Four rectifiers arranged this way are called a bridge rectifier:
A full wave rectifier converts the whole of the input waveform to one of constant polarity (positive or negative) at its output by reversing the negative (or positive) portions of the alternating current waveform.
For single phase AC, if the AC is center-tapped, then two diodes back-to-back (i.e.
A very common vacuum tube rectifier configuration contained one cathode and twin anodes inside a single envelope;
For three phase AC, six diodes are used. Typically there are three pairs of diodes, each pair, though, is not the same kind of double diode that would be used for a full wave single phase rectifier. Typically, commercially available double diodes have four terminals so the user can configure them as single phase split supply use, for half a bridge, or for three phase use.
Most devices that generate alternating current (such devices are called alternators) generate three phase AC.
Peak loss
An aspect of most rectification is a loss from peak input voltage to the peak output voltage, caused by the threshold voltage of the diodes (around 0.7 V for ordinary diodes and 0.1 V for Schottky diodes). Half wave rectification and full wave rectification using two separate secondaries will have a peak voltage loss of one diode drop. In addition, the diodes will not conduct below this voltage, so the circuit is only passing current through for a portion of each half-cycle, causing short segments of zero voltage to appear between each "hump".
Rectifier output smoothing
While half- and full-wave rectification suffices to deliver a form of DC output, neither produces constant voltage DC. For a given load, a larger capacitor will reduce ripple but will cost more and will create higher peak currents in the transformer secondry and in the supply feeding it.
Three phase bridges provide six peaks per cycle rather than two meaning the capacitor size can be significantly reduced if a 3 phase supply is availible.
If the DC load is very demanding of a smooth supply voltage, a voltage regulator will be used either instead of or in addition to the capacitor-input filter, both to remove the last of the ripple and to deal with variations in supply and load characteristics.
Applications
The primary application of rectifiers is to derive usable DC power from an AC supply. Virtually all electronics requires a DC supply but mains power is AC so rectifiers find uses inside the power supplies of virtually all electronic equipment.
Converting DC voltage from one level to another is much more complicated but rectifiers are usually involved. One method of such DC-to-DC conversion is to first convert to AC (using a device called an inverter), then use a transformer to change the voltage, and finally rectify it back to DC.
High power rectification
Vacuum tubes, metal oxide rectifier stacks and semiconductor diodes are useful in the range of milliamperes to a few hundred amperes of current. For example, to convert AC current into DC current in electric locomotives, a synchronous rectifier may be used. The motor spins in time with the AC frequency and periodically reverses the connections to the load just when the sinusoidal current goes through a zero-crossing. The contacts do not have to switch a large current, but they need to be able to carry a large current to supply the locomotive's DC traction motors.
Another type of rectifier used in high voltage power transmission systems and industrial processing since about 1909 is a mercury arc rectifier or mercury arc valve. These devices can be used at power levels of hundreds of kilowatts, and may be built to handle one to six phases of AC current. The DC generator produces a multiphase alternating current in its windings, but a commutator is used to convert the alternating currents into a direct current output.
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