Optocouplers, also known as Opto-isolators, are devices that provide optical isolation and coupling between two circuits, creating physically- and electrically-isolated signal coupling between them. Optocouplers, which can be assembled using traditional semiconductor packages, contains both a light emitting diode (LED) and a photosensitive semiconductor device in the same housing. 



The LED and the photosensitive device of an optocoupler are assembled in close proximity with each other within the package, arranged in such a way that the light emitted by the LED would strike the photosensitive device and trigger it into conduction.  The photosensitive device is usually a transistor, SCR, or triac in normally non-conducting state. In such an arrangement, therefore, the photo-emitting device is the transmitter and the photo-sensing device is the receiver. 


Figure 1. Block Diagram of an Optocoupler


Optocouplers are excellent isolating devices because their coupling medium is light, allowing very large isolation voltages (several kV's) between circuits.  The coupling light doesn't have to be visible light - many commercially available optocouplers use infrared light or even laser beams as transmission medium.  The emission travels through a transparent gap until it gets picked up by the photosensitive device.  The output waveform is identical to the input waveform, although their amplitudes usually differ.


Opto-isolation is important in applications where 'fragile' digital circuits are at risk of being damaged by large transient voltages or spikes.  Even if damage is not imminent, such spikes can make a circuit malfunction. For instance, digital circuits that are used to activate relays that drive large motors can experience inductive voltage kicks during switching that can produce 'false' triggering pulses, causing the motors to randomly turn on or off.


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Another common application of optocouplers is in modems, allowing a computer to be connected to the telephone line without risk of damage from line transients.  Other applications of optocouplers include: 1) isolated line receivers; 2) computer peripheral interfacing;  3) digital isolation between ADC's and DAC's; 4) switching power supplies; 5) instrument input-output isolation; and 6) ground loop elimination. 


See Also:   MicroprocessorsADC / DAC Analog SwitchesLuminescence




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