The full-wave rectifier is essentially two half-wave rectifiers, and can be made with two diodes and an earthed center tap on the transformer. The positive voltage half of the cycle flows through one diode, and the negative half flows through the other. The center tap allows the circuit to be completed because current can not flow through the other diode. The result is still a pulsating direct current but with just over half the input peak voltage, and double the frequency.
A full-wave rectifier converts the whole of the input waveform to one of constant polarity (positive or negative) at its output. Full-wave rectification converts both polarities of the input waveform to DC (direct current), and is more efficient. However, in a circuit with a non-center tapped transformer, four diodes are required instead of the one needed for half-wave rectification.Four diodes arranged this way are called a diode bridge or bridge rectifier:
For single-phase AC, if the transformer is center-tapped, then two diodes back-to-back (i.e. anodes-to-anode or cathode-to-cathode) can form a full-wave rectifier. Twice as many windings are required on the transformer secondary to obtain the same output voltage compared to the bridge rectifier above.
Full-wave rectifier using a transformer and 2 diodes.
Full-wave rectifier, with vacuum tube having two anodes.
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.
A three-phase bridge rectifier.
3-phase AC input, half & full wave rectified DC output waveforms
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.
Disassembled automobile alternator, showing the six diodes that comprise a full-wave three-phase bridge rectifier.
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 charging applications.
The average and root-mean-square output voltages of an ideal single phase full wave rectifier can be calculated as:
Where:
Vdc,Vav - the average or DC output voltage,
Vp - the peak value of half wave,
Vrms - the root-mean-square value of output voltage.
π = ~ 3.14159
e = ~ 2.71828
Peak loss:
An aspect of most rectification is a loss from the peak input voltage to the peak output voltage, caused by the built-in voltage drop across the diodes (around 0.7 V for ordinary silicon p-n-junction diodes and 0.3 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. Bridge rectification will have a loss of two diode drops. This may represent significant power loss in very low voltage supplies. 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".
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