optoisolator (optical coupler or optocoupler)

An optoisolator (also known as optical coupler, optocoupler and opto-isolator) is a semiconductor device that uses a short optical transmission path to transfer an electrical signal between circuits or elements of a circuit, while keeping them electrically isolated from each other. These components are used in a wide variety of communications, control and monitoring systems that use light to prevent electrical high voltage from affecting a lower power system receiving a signal.

In its simplest form, an optoisolator consists of a light-emitting diode (LED), IRED (infrared-emitting diode) or laser diode for signal transmission and a photosensor (or phototransistor) for signal reception. Using an optocoupler, when an electrical current is applied to the LED, infrared light is produced and passes through the material inside the optoisolator. The beam travels across a transparent gap and is picked up by the receiver, which converts the modulated light or IR back into an electrical signal. In the absence of light, the input and output circuits are electrically isolated from each other.

Electronic equipment, as well as signal and power transmission lines, are subject to voltage surges from radio frequency transmissions, lightning strikes and spikes in the power supply. To avoid disruptions, optoisolators offer a safe interface between high-voltage components and low-voltage devices.

How an optoisolator works

The optoisolator is enclosed in a single device, and has the appearance of an integrated circuit (IC) or a transistor with extra leads. Optocouplers can be used to isolate low-power circuits from higher power circuits and to remove electrical noise from signals.

Optoisolators are most suited to digital signals but can also be used to transfer analog signals. The isolation of any data rate of more than 1 Mb/sec is considered high speed. The most common speed available for digital and analog optoisolators is 1 Mb/sec, although 10 Mb/sec and 15 Mb/sec digital speeds are also available. Optoisolators are considered too slow for many modern digital uses, but researchers have created alternatives since the 1990s.

In communications, high-speed optoisolators are used in power supplies for servers and telecom applications -- Power over Ethernet (PoE) technology for wired Ethernet LANs, for example. Optoisolator components can also protect Ethernet and fiber optic cables from electrical surges.  In VoIP phones, electrical signals can be isolated using a transistor output optocoupler.

Although no longer common, where modems are used to connect to telephone lines, the use of optoisolators allow a computer to be connected to a telephone line without risk of damage from electrical surges or spikes. In this case, two optoisolators are employed in the analog section of the device: one for upstream signals and the other for downstream signals. If a surge occurs on the telephone line, the computer will be unaffected because the optical gap does not conduct electric current.

Also see diode.

This was last updated in April 2015

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