Optocouplers

Optocouplers are capable of transferring an electrical signal between two circuits while electrically isolating the circuits from each other.  They generally consist of an infrared LED emitting section at the input and a silicon photodetector at the output.  In Often, other circuitry is included as part of the device.  The input for optocouplers can be AC or DC.  The output can be a photocell, photodiode, phototransistor, or photodarlington.  Photocells are light-dependent resistors. They are used to detect changes in light intensity.  Photodiodes are a two-electrode, radiation-sensitive junction formed in a semiconductor material in which the reverse current varies with illumination.  A phototransistor is a bipolar transistor used as a photodetector. It provides current at its output that is proportional to light intensity at its input. The low-level input light current is amplified by the current gain (beta) of the transistor.  A photodarlington is a pair of bipolar transistors connected in a Darlington configuration to provide very high current gain and often used as the photodetector section of an optocoupler.

Important performance specifications to consider when searching for optocouplers include isolation voltage, rise time, collector emitter breakdown voltage, resistance on, and operating temperature.  Isolation voltage specifies the input-to-output voltage withstanding capability of an optically coupled isolator.  Rise time is the time that elapses while a pulse waveform increases from 10% to 90% of its maximum value.  Collector emitter breakdown voltage refers to the voltage at which a transistor, biased in the normal direction with no optical or electrical input to the base, will conduct a specified current much higher than the normal leakage currents that occur at lower voltages.  The resistance on refers to the resistance of the optocoupler when activated.  Operating temperature is the temperature range the optocoupler is designed to operate in.

Mounting options for optocouplers include surface mount, flat pack, and through hole (plug-in).  In Surface Mount Technology (SMT), components are mounted on printed circuit boards by soldering the component leads or terminals to the top surface of the board.  A flat pack is an IC package with gull wing or flat leads on two or four sides.  In Through Hole Technology (THT), optocouplers are mounted on printed circuit boards by inserting component leads through holes in the board and then soldering.

• Optocoupler Input-Output Endurance Voltage - App Note
  A major concern of circuit designers is the reliability of an optocoupler when subjected to repeated and long-term, high-voltage stress between its input and output. Most of the technical data on...
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• Thermal Data for Avago Optocouplers - App Note
  This document contains steady state thermal models for optocouplers based on empirical data and theoretical extrapolation. Seven thermal models have been chosen to suit the type of optocoupler: *...
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• ACSL-6xx0 Stacked LED Technology Enables Highly Integrated Optocouplers - White Paper
  Optocouplers are widely used in providing galvanic isolation because of their true isolation characteristics. The heart of a modern optocoupler is an input infrared-emitting LED and an output...
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Part Numbers for Optocouplers