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Opto-isolator
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In electronics, an opto-isolator (or optical isolator, optocoupler, photocoupler, or photoMOS) is a device that uses a short optical transmission path to transfer a signal between elements of a circuit, typically a transmitter and a receiver, while keeping them electrically isolated — since the signal goes from an electrical signal to an optical signal back to an electrical signal, electrical contact along the path is broken.
The opto-isolator is simply a package that contains both an infrared LED and a photodetector such as silicon diode, transistor Darlington pair, or SCR.
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Encyclopedia
In electronics, an opto-isolator (or optical isolator, optocoupler, photocoupler, or photoMOS) is a device that uses a short optical transmission path to transfer a signal between elements of a circuit, typically a transmitter and a receiver, while keeping them electrically isolated — since the signal goes from an electrical signal to an optical signal back to an electrical signal, electrical contact along the path is broken.
The opto-isolator is simply a package that contains both an infrared LED and a photodetector such as silicon diode, transistor Darlington pair, or SCR. The wave-length response of each device is tailored to be as identical as possible to permit the highest measure of coupling possible.
Configurations
A common implementation involves a LED and a phototransistor, separated so that light may travel across a barrier but electrical current may not. When an electrical signal is applied to the input of the opto-isolator, its LED lights, its light sensor then activates, and a corresponding electrical signal is generated at the output. Unlike a transformer, the opto-isolator allows for DC coupling and generally provides significant protection from serious overvoltage conditions in one circuit affecting the other.
With a photodiode as the detector, the output current is proportional to the amount of incident light supplied by the emitter. The diode can be used in a photovoltaic mode or a photoconductive mode.
In photovoltaic mode, the diode acts like a current source in parallel with a forward-biased diode. The output current and voltage are dependent on the load impedance and light intensity.
In photoconductive mode, the diode is connected to a supply voltage, and the magnitude of the current conducted is directly proportional to the intensity of light.
The optical path may be air or a dielectric waveguide. The transmitting and receiving elements of an optical isolator may be contained within a single compact module, for mounting, for example, on a circuit board; in this case, the module is often called an optoisolator or opto-isolator. The photosensor may be a photocell, phototransistor, or an optically triggered SCR or TRIAC. Occasionally, this device will in turn operate a power relay or contactor.
For analog isolation, special "analog" optoisolators are used. These devices have two independent, closely matched phototransistors, one of which is typically used to linearize the response using negative feedback.
Application
Among other applications, opto-isolators can help cut down on ground loops, block voltage spikes, and provide electrical isolation.
- One of the requirements of the MIDI (Musical Instrument Digital Interface) standard is that input connections be opto-isolated.
- They are used to isolate low-current control or signal circuitry from transients generated or transmitted by power supply and high-current control circuits. The latter are used within motor and machine control function blocks.
- Most switched-mode power supplies utilise optocouplers for mains isolation.
See also
External links
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