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 an electronic signal between elements of a circuit, typically a transmitter and a receiver, while keeping them electrically isolated—since the electrical signal is converted to a light beam, transferred, then converted back to an electrical signal, there is no need for electrical connection between the source and destination circuits. Isolation between input and output is rated at 7,500V peak for 1 second for a typical component costing less than 1 US$ in small quantities.[1]
The opto-isolator is simply a package that contains both an infrared light-emitting diode (LED) and a photodetector such as a photosensitive silicon diode, transistor Darlington pair, or silicon controlled rectifier (SCR). The wave-length responses of the two devices are tailored to be as identical as possible to permit the highest measure of coupling possible. Other circuitry—for example an output amplifier—may be integrated into the package. An opto-isolator is usually thought of as a single integrated package, but opto-isolation can also be achieved by using separate devices.
Digital opto-isolators change the state of their output when the input state changes; analog isolators produce an analog signal which reproduces the input.
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[edit] Configurations
A common implementation is a LED and a phototransistor in a light-tight housing to exclude ambient light and without common electrical connection, positioned so that light from the LED will impinge on the photodetector. When an electrical signal is applied to the input of the opto-isolator, its LED lights and illuminates the photodetector, producing a corresponding electrical signal in the output circuit. Unlike a transformer the opto-isolator allows DC coupling and can provide any desired degree of electrical isolation and protection from serious overvoltage conditions in one circuit affecting the other. A higher transmission ratio can be obtained by using a Darlington instead of a simple phototransistor, at the cost of reduced noise immunity and higher delay.
With a photodiode as the detector, the output current is proportional to the intensity 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 as 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. This optocoupler type is significantly faster than photo transistor type, but the transmission ratio is very low; it is common to integrate an output amplifier circuit into the same package.
The optical path may be air or a dielectric waveguide. When high noise immunity is required an optical conductive shield can be integrated into the optical path. 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. This device may in turn operate a power relay or contactor.
Analog optoisolators often have two independent, closely matched output phototransistors, one of which is used to linearize the response using negative feedback.
[edit] Application
Among other applications, opto-isolators can help cut down on ground loops, block voltage spikes, and provide electrical isolation.
- switched-mode power supplies use optocouplers for mains isolation. As they work in an environment with much electrical noise and with signals which are not small, optocouplers with low transmission ratio are preferred.
- Where electrical safety is paramount, optocouplers can totally isolate circuitry which may be touched by humans from mains electricity.
- Medical equipment often uses optocouplers.
- One of the requirements of the MIDI (Musical Instrument Digital Interface) standard is that input connections be opto-isolated.
- Optocouplers 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.
[edit] Slotted optical switch
The slotted optical switch uses the same basic components as an opto-isolator, but instead of the optical beam being enclosed to protect the photodetector from ambient light, it is arranged so that an object can interrupt the beam. The photoemitter is always active, and the device changes its output state when the beam is blocked. The distinction between this switch and an opto-coupler is that the photoemitter input is fixed and the light path is manipulated, rather than viceversa. It is used to detect, for example, that the carriage of a printer has reached the end of its travel—the carriage has a projection which breaks the beam.
[edit] See also
[edit] External links
- Optocouplers: When and how to use themPDF (157 KB)
- Simple and Practical Optocoupler CTR Degradation Predictive ModelPDF
- Trends in Optocoupler Radiation DegradationPDF (89.1 KB)
- How Optical Isolation Works - Illustration
