Rectifier

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. Rectification is commonly performed by semiconductor diodes. Before the development of solid state rectifiers, vacuum tube diodes were used. Early radios called crystal sets used a "cat's whisker" of fine wire pressing on a crystal of galena (lead sulfide) to serve as a point contact rectifier or "crystal detector". In gas heating systems "flame rectification" can be used to detect a flame. 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. Because only one half of the input waveform reaches the output, it is very inefficient if used for power transfer. 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. However, in a circuit with a non-center tapped transformer, four rectifiers are required instead of the one needed for half-wave rectification. 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. The other DC output requires exactly the same, resulting in four individual junctions (See semiconductors/diode). 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. The positive (negative) portions thus combine with the reversed negative (positive) portions to produce an entirely positive(negative) voltage/current waveform.

For single phase AC, if the AC is center-tapped, then two diodes back-to-back (i.e. anodes-to-anode or cathode-to-cathode) form a full wave rectifier. A very common vacuum tube rectifier configuration contained one cathode and twin anodes inside a single envelope; in this way, the two diodes required only one vacuum tube. The 5U4 and 5Y3 were popular examples of this configuration.

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. Instead the pairs are in series (anode to cathode). 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. For example, an automobile alternator has six diodes inside it to function as a full wave rectifier for battery charge applications.

Peak Loss

An important aspect of most full wave rectification is a loss from peak input voltage to the peak output voltage, caused by the threshold voltage of the diodes, often around 0.7 volts. In a diode bridge circuit the peak output will be lower than the peak input by an amount equal to twice this value. 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".

Applications

One of the first applications of rectifiers was detection of amplitude modulated radio signals by a diode. In early Crystal radio receivers the diode was a simple piece of semiconductive mineral.

Power transmission

AC is used for power transmission because it can easily be stepped up or down in voltage by a simple transformer. High voltage power lines are used to transmit electricity long distances at reduced current (which reduces heat and thus energy loss). The power is stepped down at the destination by substation transformers to the more manageable voltages. Converting DC voltage from one level to another is much more complicated. 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. DC is required by the internal circuits of many everyday electrical and electronic items. Computers, telephones, television sets, clocks, solid state lighting, etc., are all designed to run on DC.

Rectifier output smoothing

While half- and full-wave rectification suffices to deliver a form of DC output, neither produces constant voltage DC. In order to produce steady DC from a rectified AC supply, a smoothing circuit is required. In its simplest form this can be what is known as a reservoir capacitor or smoothing capacitor, placed at the DC output of the rectifier. There will still remain an amount of AC ripple voltage where the voltage is not completely smoothed.

To further reduce this ripple, a capacitor-input filter can be used. This complements the reservoir capacitor with a choke and a second filter capacitor, so that a steadier DC output can be obtained across the terminals of the filter capacitor. The choke presents a high impedance to the ripple current.

Peak Load

Another important design consideration is that of peak load. Where a reservoir capacitor is used to store the voltage output of a bridge rectifier, the diodes in the bridge only conduct for a small portion of the input cycle, at the crest of each wave. In extreme cases where many rectifiers are loaded onto a power distribution circuit, it may prove difficult for the power distribution authority to maintain a correctly shaped sinusoidal voltage curve.

Rectification efficiency

Rectification efficiency measures how efficiently a rectifier converts AC to DC. It is defined as the ratio of the DC output power to AC input power, where DC output power is a product of the average current and voltage. A simpler way to calculate efficiency is with

V_{DC}^2\over V_{AC}^2

Without smoothing, full-wave rectifiers have $8\over\pi^2$ or 81% efficiency. Half-wave rectifiers have $4\over\pi^2$ or 40.5% efficiency.

Note: Specialized rectifiers {and functionally analogous devices) can have over 90% efficiency. See Switched-mode power supply, IGBT transistor, Thyristor, MOSFET and Mercury arc valve.

Patents

- Copper oxide rectifiers (L.O. Grondahl ^* and P.H. Geiger developed this device; 1927)

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