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Rectifiers
A diode rectifier circuit converts alternating current -- current that alternates direction in a cycle -- to direct current, which only flows in one direction.
A half-wave rectifier uses one diode and only operates during one half of the AC cycle. The raw output is a pulsing approximation of DC current produced during one half of the AC cycle. A full-wave rectifier uses two diodes and operates during both halves of the AC cycle. Bridge rectifiers require four diodes and operate during both halves of the AC cycle.
Rectifier circuits are commonly found in power supplies and battery charges to convert standard house current to a lower voltage application, such as powering a computer or charging a cell phone or other device.
Circuit Protection
Diodes protect components and circuits by allowing current to flow in just one direction. A diode placed at the output of a circuit allows current to flow out of a circuit, but does not allow current to flow into the circuit at the same point. In this manner, the diode acts as a one-way gatekeeper. This works if the output is DC current, but not if it is AC current.
In a similar fashion, a diode placed on the output of a component allows the output current to flow, but a current flowing in the opposite direction cannot pass the diode.
Voltage Regulation
Zener diodes operate in a manner similar to a standard diode, but the voltage across the terminals stays below a fixed level. A 5-volt zener diode placed across the output of a rectifier circuit prevents the voltage on the output from rising above 5 volts. Used in conjunction with a capacitor, the zener diode smooths the pulsing direct current into a truer form of DC that is regulated at an almost steady 5 volts. Additional filtering may further smooth the output signal.
Other Applications
Diodes were the first electronic semiconductor device and have found their way into many applications far beyond current rectification and protection. Diode logic circuits were once common, but have lost favor to lower-power transistor and integrated circuit logic.
Receivers and transmitters for radios and televisions use different kinds of diodes to produce, tune and demodulate radio waves.
Light-emitting diodes produce light, and laser diodes produce laser beams through similar, but different, processes. Photosensitive diodes sense light and act as a switch.
Perhaps one of the most useful properties of a diode is its adaptability to new applications. Using the basic diode as a model, scientists and engineers modify the diode to fit the needs of new electronic circuits and devices.