Most electronic devices need direct current (DC) power, not the alternating current (AC) available from a household electrical socket. Circuits called rectifiers convert AC to DC using components called diodes. A half-wave rectifier uses a single diode, a full-wave rectifier uses two diodes, and a full-wave bridge rectifier uses four diodes. Though it needs more diodes, the full-wave bridge lowers costs by using a simpler transformer.
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Design and Action
The diode in a half-wave rectifier conducts current in the forward direction, passing only the positive halves of an AC wave. A full-wave rectifier's diode pair delivers both halves of the AC wave, reversing the negative half so it comes out positive. The full-wave bridge uses its four diodes two at a time, alternating between diode pairs every time the AC wave changes direction.
A half-wave rectifier delivers only half the available energy in an AC wave. During the negative part of the wave cycle, the circuit produces zero volts. While the design is simple, it is less efficient than a full-wave or full-wave bridge rectifier, which uses both positive and negative wave halves. All rectifiers need a capacitor to remove electrical noise, although the lower noise in the full-wave bridge design improves overall efficiency. Since less noise means it uses smaller capacitors, this saves space and money.
All rectifier circuits use transformers, electronic components that change the 120-volt household AC to a lower value. A full-wave rectifier uses only two diodes, but it needs a transformer with a centre-tapped secondary winding. Since the centre tap connects midway between the two ends of the secondary coil, it can serve as a grounding point. However, adding a centre tap adds expense to the transformer. A full-wave bridge rectifier uses a transformer with a normal secondary: a coil with one output wire connected to each end.
A diode acts as one-way switch, allowing current flow in only one direction. A high voltage, however, can break the diode down, forcing current through the wrong way and damaging the diode. Because of this, diodes are rated for reverse voltage, so the design engineer knows how much voltage the part can safely handle. A full-wave bridge rectifier lowers the breakdown voltage requirement by half. For example, a full-wave rectifier might need to be rated for 100 volts, but a full-wave bridge needs diodes rated for only 50 volts. Diodes with lower ratings cost less, so this reduces the overall circuit cost.
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