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Revision as of 23:20, 1 August 2005 by Light current (talk | contribs) (altered 'Smoothing' heading to refer more to rectifiers)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff)A rectifier is an electrical device, comprising one or more diodes arranged for converting alternating current (AC) to direct current (DC).
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. a rectifier depicts a diode that is being used to convert AC to 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.
Half wave rectifier
As its title implies, a half wave rectifier allows only one half of the input waveform to reach the output. This may be the positive or the negative half depending on the sense in which the diode is connected. Half wave rectification can be achieved by a single diode in a one phase supply.
Full wave 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. If there is no center tap, then four diodes, arranged in a bridge, are needed.
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.
Rectifier Output Smoothing
While half- and full-wave rectification suffices to deliver a form of DC output, neither produces steady 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 steady DC output can be obtained across the terminals of the filter capacitor. The choke effectively presents a high impedance to the ripple current.
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.