Editing Opto-isolator

{{about|the electronic component|the optical component|optical isolator}}
{{short description|Insulates two circuits from one another while allowing signals to pass through in one direction}}
[[File:Optoisolator Pinout.svg|class=skin-invert-image|thumb|right|Schematic diagram of an opto-isolator showing source of light (LED) on the left, dielectric barrier in the center, and sensor (phototransistor) on the right<ref group=note>Real-world schematic diagrams omit the barrier symbol, and use a single set of directional arrows.</ref>]]

An '''opto-isolator''' (also called an '''optocoupler''', '''photocoupler''', or '''optical isolator''') is an [[electronic component]] that transfers electrical [[Signal|signals]] between two isolated circuits by using light.<ref>Graf, p. 522.</ref> Opto-isolators prevent [[high voltage]]s from affecting the system receiving the signal.<ref name=L2>Lee et al., p. 2.</ref> Commercially available opto-isolators withstand input-to-output voltages up to 10&nbsp;[[Volt|kV]]<ref name=H145>Hasse, p. 145.</ref> and voltage transients with speeds up to 25&nbsp;kV/[[microsecond|μs]].<ref name=J279>Joffe and Kai-Sang Lock, p. 279.</ref>

A common type of opto-isolator consists of an [[Light emitting diode|LED]] and a [[phototransistor]] in the same opaque package. Other types of source-sensor combinations include LED-[[photodiode]], LED-[[thyristor#Photothyristors|LASCR]], and [[lamp (electrical component)|lamp]]-[[photoresistor]] pairs. Usually opto-isolators transfer digital (on-off) signals and can act as an [[electronic switch]], but some techniques allow them to be used with analog signals.

==History==
The value of optically coupling a solid state light emitter to a semiconductor detector for the purpose of electrical isolation was recognized in 1963 by Akmenkalns, et al. (US patent 3,417,249). Photoresistor-based opto-isolators were introduced in 1968. They are the slowest, but also the most [[linear#Electronics|linear]] isolators and still retain a niche market in the audio and music industries. Commercialization of LED technology in 1968–1970 caused a boom in [[optoelectronics]], and by the end of the 1970s the industry developed all principal types of opto-isolators. The majority of opto-isolators on the market use bipolar silicon phototransistor sensors.<ref>Graf, p. 522; PerkinElmer, p. 28.</ref> They attain medium data transfer speed, sufficient for applications like [[electroencephalography]].<ref name=ANA>See Ananthi, pp. 56, 62 for a practical example of an opto-coupled EEG application.</ref> The fastest opto-isolators use [[PIN diode]]s in [[photodiode#Photoconductive mode|photoconductive mode]].

==Operation==
An opto-isolator contains a source (emitter) of light, almost always a [[infrared|near infrared]] [[light-emitting diode]] (LED), that converts electrical input signal into light, a closed optical channel (also called dielectrical channel<ref name=M100/>), and a [[photodetector|photosensor]], which detects incoming light and either generates electric [[energy]] directly, or [[modulation|modulates]] [[electric current]] flowing from an external power supply. The sensor can be a [[photoresistor]], a [[photodiode]], a [[phototransistor]], a [[silicon-controlled rectifier]] (SCR) or a [[TRIAC|triac]]. Since LEDs can sense light in addition to emitting it, construction of symmetrical, bidirectional opto-isolators is possible. An optocoupled [[solid-state relay]] contains a photodiode opto-isolator which drives a power switch, usually a complementary pair of [[MOSFET]]s. A [[slotted optical switch]] contains a source of light and a sensor, but its optical channel is open, allowing [[modulation]] of light by external objects obstructing the path of light or reflecting light into the sensor.

==Electric isolation==
[[File:Optoisolator topologies both.svg|class=skin-invert-image|thumb|right|Planar (top) and silicone dome (bottom) layouts{{dash}}cross-section through a standard [[dual in-line package]]. Relative sizes of LED (red) and sensor (green) are exaggerated.<ref group=note>Based on conceptual drawings published by Basso and by Mims, p. 100. Real-world LEDs and sensors are much smaller; see the photograph in Avago, p. 3 for an example.</ref>]]

[[File:Optocoupler with class Y1 caps.jpg|thumb|Optocoupler on a [[circuit board]]. Note the pair of class Y1 safety [[capacitors]].]]

Electronic equipment and signal and power transmission lines can be subjected to voltage surges induced by [[lightning]], [[electrostatic discharge]], [[Electromagnetic interference|radio frequency transmissions]], switching pulses (spikes) and perturbations in power supply.<ref name=H43>Hasse, p. 43.</ref> Remote lightning strikes can induce surges up to 10&nbsp;[[Volt|kV]], one thousand times more than the voltage limits of many electronic components.<ref name=H60>Hasse, p. 60.</ref> A circuit can also incorporate high voltages by design, in which case it needs safe, reliable means of interfacing its high-voltage components with low-voltage ones.<ref>See Basso for a discussion of such interfacing in [[Switched-mode power supply|switched-mode power supplies]].</ref>

The main function of an opto-isolator is to block such high voltages and voltage transients, so that a surge in one part of the system will not disrupt or destroy the other parts.<ref name=L2/><ref name=HH595>Horowitz and Hill, p. 595.</ref> Historically, this function was delegated to [[isolation transformer]]s, which use [[inductive coupling]] between [[Galvanic isolation|galvanically isolated]] input and output sides. Transformers and opto-isolators are the only two classes of electronic devices that offer ''reinforced protection'' — they protect both the equipment ''and'' the human user operating this equipment.<ref name=H48/> They contain a single physical isolation barrier, but provide protection equivalent to [[Appliance classes#Class II|double isolation]].<ref name=H48>Jaus, p. 48.</ref> Safety, testing and approval of opto-couplers are regulated by national and international standards: [[International Electrotechnical Commission|IEC]] 60747-5-2, [[European Committee for Electrotechnical Standardization|EN (CENELEC)]] 60747-5-2, [[Underwriters Laboratories|UL]] 1577, [[Canadian Standards Association|CSA]] Component Acceptance Notice #5, etc.<ref name=H50>Jaus, pp. 50–51.</ref> Opto-isolator specifications published by manufacturers always follow at least one of these regulatory frameworks.

An opto-isolator connects input and output sides with a beam of light [[modulation|modulated]] by input current. It transforms useful input signal into light, sends it across the [[dielectric]] channel, captures light on the output side and transforms it back into electric signal. Unlike transformers, which pass energy in both directions<ref group=note>A transformer can have as many coils as necessary. Each coil can act as a ''primary'', pumping energy into a common [[magnetic core]], or as a ''secondary'' – picking up energy stored in the core.</ref> with very low losses, opto-isolators are unidirectional (see [[#Bidirectional opto-isolators|exceptions]]) and they cannot transmit ''[[Power (physics)|power]]''.<ref name=J277/> Typical opto-isolators can only modulate the flow of energy already present on the output side.<ref name=J277>Joffe and Kai-Sang Lock, p. 277.</ref> Unlike transformers, opto-isolators can pass [[direct current|DC]] or slow-moving signals and do not require [[impedance matching|matching impedances]] between input and output sides.<ref group=note>The input side circuitry and the LED must be matched, the output side and the sensor must be matched, but there is, usually, no need to match input ''and'' output sides.</ref> Both transformers and opto-isolators are effective in breaking [[Ground loop (electricity)|ground loops]], common in industrial and stage equipment, caused by high or noisy return currents in [[Ground wire#Electronics|ground wires]].<ref>Joffe and Kai-Sang Lock, pp. 268, 276.</ref>

The physical layout of an opto-isolator depends primarily on the desired isolation voltage. Devices rated for less than a few kV have planar (or sandwich) construction.<ref name=M174>Mataré, p. 174</ref> The sensor [[Die (integrated circuit)|die]] is mounted directly on the lead frame of its package (usually, a six-pin or a four-pin [[dual in-line package]]).<ref name=M100/> The sensor is covered with a sheet of glass or clear plastic, which is topped with the LED die.<ref name=M100/> The LED beam fires downward. To minimize losses of light, the useful absorption spectrum of the sensor must match the output spectrum of the LED, which almost invariably lies in the near infrared.<ref>Ball, p. 69.</ref> The optical channel is made as thin as possible for a desired [[breakdown voltage]].<ref name=M174/> For example, to be rated for short-term voltages of 3.75&nbsp;kV and transients of 1&nbsp;kV/μs, the clear [[polyimide]] sheet in the [[Avago Technologies|Avago]] ASSR-300 series is only 0.08&nbsp;mm thick.<ref>[[Avago Technologies]] (2007). ''[http://www.avagotech.com/docs/AV02-0452EN ASSR-301C and ASSR-302C (datasheet)]''. Retrieved November 3, 2010.</ref> Breakdown voltages of planar assemblies depend on the thickness of the transparent sheet<ref name=M174/> and the configuration of bonding wires that connect the dies with external pins.<ref name=M100/> Real in-circuit isolation voltage is further reduced by [[insulator (electrical)#Design|creepage]] over the [[printed circuit board|PCB]] and the surface of the package. Safe design rules require a minimal clearance of 25&nbsp;mm/kV for bare metal conductors or 8.3&nbsp;mm/kV for coated conductors.<ref>Bottrill et al., p. 175.</ref>

Opto-isolators rated for 2.5 to 6&nbsp;kV employ a different layout called ''silicone dome''.<ref name=Basso/> Here, the LED and sensor dies are placed on the opposite sides of the package; the LED fires into the sensor horizontally.<ref name=Basso/> The LED, the sensor and the gap between them are encapsulated in a blob, or dome, of transparent [[silicone]]. The dome acts as a [[mirror|reflector]], retaining all stray light and reflecting it onto the surface of the sensor, minimizing losses in a relatively long optical channel.<ref name=Basso/> In ''double mold'' designs the space between the silicone blob ("inner mold") and the outer shell ("outer mold") is filled with dark dielectric compound with a matched [[thermal expansion#Coefficient of thermal expansion|coefficient of thermal expansion]].<ref name=VI/>

==Types of opto-isolators==

{| class="wikitable"
|-
! Device type<ref group=note>See Horowitz and Hill, p. 597, for an expanded list of opto-isolator types with their schematic symbols and typical specifications.</ref>
! Source of light<ref name=M100>Mims, p. 100.</ref>
! Sensor type<ref name=M100/>
! Speed
! Current transfer ratio
|-
| rowspan=3 | Resistive opto-isolator<br>(Vactrol)
| [[Incandescent light bulb]]
| rowspan=3 | [[cadmium sulfide|CdS]] or [[cadmium selenide|CdSe]] [[photoresistor]] (LDR)
| Very low
| rowspan=3 | <100%<ref group=note>Current through the photoresistor (output current) is proportional to the voltage applied across it. In theory it can exceed 100% of input current, but in practice dissipation of heat according to [[Joule's first law|Joule's law]] limits current transfer ratio at below 100%.</ref>
|-
| [[Neon lamp]]
| Low
|-
| [[Gallium arsenide|GaAs]] [[infrared]] [[light-emitting diode|LED]]
| Low
|-
| Diode opto-isolator
| GaAs infrared LED
| Silicon [[photodiode]]
| Highest
| 0.1–0.2%<ref name=T5/>
|-
| rowspan=2 | Transistor opto-isolator
| rowspan=2 | GaAs infrared LED
| Bipolar silicon [[phototransistor]]
| Medium
| 2–120%<ref name=T5>Mataré, p. 177, table 5.1.</ref>
|-
| [[Darlington transistor|Darlington]] phototransistor
| Medium
| 100–600%<ref name="T5"/>
<!-- |-
| FET opto-isolator
| GaAs infrared LED
| [[Field-effect transistor#Types of field-effect transistors|Photo FET]]
| Medium
|
-->
|-
| Opto-isolated SCR
| GaAs infrared LED
| [[Silicon-controlled rectifier]]
| Low to medium
| >100%<ref name=M177>Mataré, p. 177</ref>
|-
| Opto-isolated triac
| GaAs infrared LED
| [[TRIAC]]
| Low to medium
| Very high
|-
| [[Solid-state relay]]
| Stack of GaAs infrared LEDs
| Stack of photodiodes driving<br>a pair of [[MOSFET]]s or an [[Insulated-gate bipolar transistor|IGBT]]
| Low to high<ref group=note>Low-cost solid-state relays have switching times of tens of milliseconds. Modern high-speed solid-state relays like Avago ASSR-300 series (see [http://www.avagotech.com/docs/AV02-0452EN datasheet]) attain switching times of less than 70&nbsp;nanoseconds.</ref>
| Practically unlimited
|}

===Resistive opto-isolators===
{{main|Resistive opto-isolator}}
<!-- [[File:OEP series optocouples.jpg|thumb|right|Obsolete photoresistor opto-couples (Russian OEP series pictured) retain a niche in modern [[guitar amplifier]] market.]] -->
The earliest opto-isolators, originally marketed as ''light cells'', emerged in the 1960s. They employed miniature [[incandescent light bulb]]s as sources of light, and [[cadmium sulfide]] (CdS) or [[cadmium selenide]] (CdSe) photoresistors (also called light-dependent resistors, LDRs) as receivers. In applications where control linearity was not important, or where available current was too low for driving an incandescent bulb (as was the case in vacuum tube amplifiers), it was replaced with a [[neon lamp]]. These devices (or just their LDR component) were commonly named ''Vactrols'', after a trademark of Vactec, Inc. The trademark has since been [[genericized trademark|genericized]],<ref group=note>According to the [[United States Patent and Trademark Office]], trademark registered in 1969 for "photocell combined with a light source" is now dead ([http://tess2.uspto.gov/bin/showfield?f=doc&state=4005:pnh880.2.4 USPTO database record serial number 72318344]. Retrieved November 5, 2010). The same trademark, registered in 1993 for "medico-surgical tubing connector sold as a component of suction catheters" is now live and owned by Mallinckrodt Inc. ([http://tess2.uspto.gov/bin/showfield?f=doc&state=4005:pnh880.2.2 USPTO database record serial number 74381130]. Retrieved November 5, 2010).</ref> but the original Vactrols are still being manufactured by [[PerkinElmer]].<ref>Weber, p. 190; PerkinElmer, p. 28; Collins, p. 181.</ref><ref group=note>Vactec was purchased by [[EG&G]] (Edgerton, Germeshausen, and Grier, Inc.), a defense contractor, in 1983. In 1999 EG&G purchased formerly independent PerkinElmer, and changed own name PerkinElmer (see [[reverse takeover]]). An unrelated company, Silonex (a division of [[Carlyle Group]]) brands its photoresistive opto-isolators ''Audiohm Optocouplers''.</ref>

The turn-on and turn-off lag of an incandescent bulb lies in hundreds of [[milliseconds]] range, which makes the bulb an effective [[low-pass filter]] and [[rectifier]] but limits the practical modulation frequency range to a few [[Hertz]]. With the introduction of [[light-emitting diode]]s (LEDs) in 1968–1970,<ref>Schubert, pp. 8–9.</ref> the manufacturers replaced incandescent and neon lamps with LEDs and achieved response times of 5&nbsp;milliseconds and modulation frequencies up to 250&nbsp;Hz.<ref>PerkinElmer, pp. 6–7: "at 1 [[Foot-candle|fc]] of illumination the response times are typically in the range of 5&nbsp;ms to 100&nbsp;ms."</ref> The name ''Vactrol'' was carried over on LED-based devices which are, as of 2010, still produced in small quantities.<ref>Weber, p. 190; PerkinElmer, pp. 2,7,28; Collins, p. 181.</ref>

Photoresistors used in opto-isolators rely on bulk effects in a uniform film of [[semiconductor]]; there are no [[p-n junction]]s.<ref name=P3/> Uniquely among photosensors, photoresistors are non-polar devices suited for either AC or DC circuits.<ref name=P3/> Their resistance drops in reverse proportion to the intensity of incoming light, from virtually infinity to a residual floor that may be as low as less than a hundred [[Ohm]]s.<ref name=P3/> These properties made the original Vactrol a convenient and cheap [[automatic gain control]] and [[Dynamic range compression|compressor]] for telephone networks. The photoresistors easily withstood voltages up to 400&nbsp;volts,<ref name=P3>PerkinElmer, p. 3</ref> which made them ideal for driving [[vacuum fluorescent display]]s. Other industrial applications included [[photocopier]]s, industrial [[automation]], professional light measurement instruments and [[Exposure (photography)#Automatic exposure|auto-exposure meters]].<ref name=P3/> Most of these applications are now obsolete, but resistive opto-isolators retained a niche in audio, in particular [[guitar amplifier]], markets.

American guitar and organ manufacturers of the 1960s embraced the resistive opto-isolator as a convenient and cheap [[tremolo]] modulator. [[Fender Musical Instruments Corporation|Fender]]'s early tremolo effects used two [[vacuum tubes]]; after 1964 one of these tubes was replaced by an optocoupler made of a LDR and a neon lamp.<ref>Fliegler and Eiche, p. 28; Teagle and Sprung, p. 225.</ref> To date, Vactrols activated by pressing the [[Effects unit#Stompboxes|stompbox pedal]] are ubiquitous in the music industry.<ref>Weber, p. 190.</ref> Shortages of genuine PerkinElmer Vactrols forced the [[Do it yourself|DIY]] guitar community to "roll their own" resistive opto-isolators.<ref name=C181>Collins, p. 181.</ref> Guitarists to date prefer opto-isolated effects because their superior [[Ground (electricity)#Separating low signal ground from a noisy ground|separation of audio and control grounds]] results in "inherently high quality of the sound".<ref name=C181/> However, the [[distortion]] introduced by a photoresistor at [[line level]] signal is higher than that of a professional electrically-coupled [[Variable-gain amplifier|voltage-controlled amplifier]].<ref>PerkinElmer, pp. 35–36; Silonex, p. 1 (see also distortion charts on subsequent pages).</ref> Performance is further compromised by slow fluctuations of resistance owing to [[light history]], a [[memory effect]] inherent in [[cadmium]] compounds. Such fluctuations take hours to settle and can be only partially offset with [[Feedback#Electronic engineering|feedback]] in the control circuit.<ref>PerkinElmer, pp. 7, 29, 38; Silonex, p. 8.</ref>

===Photodiode opto-isolators===

[[File:Optically isolated.jpg|class=skin-invert-image|thumb|right|A fast photodiode opto-isolator with an output-side amplifier circuit]]

Diode opto-isolators employ LEDs as sources of light and silicon [[photodiode]]s as sensors. When the photodiode is reverse-biased with an external voltage source, incoming light increases the reverse current flowing through the diode. The diode itself does not generate energy; it modulates the flow of energy from an external source. This mode of operation is called [[photodiode#Photoconductive mode|photoconductive mode]]. Alternatively, in the absence of external bias the diode converts the energy of light into [[Electric potential energy|electric energy]] by charging its terminals to a voltage of up to 0.7&nbsp;V. The rate of charge is proportional to the intensity of incoming light. The energy is harvested by draining the charge through an external high-impedance path; the ratio of current transfer can reach 0.2%.<ref name=T5/> This mode of operation is called [[Photodiode#Photovoltaic mode|photovoltaic mode]].

The fastest opto-isolators employ [[PIN diode]]s in photoconductive mode. The response times of PIN diodes lie in the [[nanosecond|subnanosecond]] range; overall system speed is limited by delays in LED output and in biasing circuitry. To minimize these delays, fast digital opto-isolators contain their own LED drivers and output amplifiers optimized for speed. These devices are called ''full logic opto-isolators'': their LEDs and sensors are fully encapsulated within a digital logic circuit.<ref>Horowitz and Hill, pp. 596–597.</ref> The [[Hewlett-Packard]] 6N137/HPCL2601 family of devices equipped with internal output amplifiers was introduced in the late 1970s and attained 10&nbsp;[[Baud|MBd]] data transfer speeds.<ref>Porat and Barna, p. 464. See also full specifications of currently produced devices: ''[http://www.avagotech.com/docs/AV02-0940EN 6N137 / HCPL-2601 datasheet]''. [[Avago Technologies]]. March 2010. Retrieved November 2, 2010.</ref> It remained an industry standard until the introduction of the 50&nbsp;MBd [[Agilent Technologies]]<ref group=note>The former semiconductor division of Agilent Technologies operates as an independent company, [[Avago Technologies]], since 2005.</ref> 7723/0723 family in 2002.<ref name=AVA2002>''[http://www.thefreelibrary.com/TRADE+NEWS%3A+Agilent+Technologies+Introduces+Industry's+Fastest...-a094761414 Agilent Technologies Introduces Industry's Fastest Optocouplers]''. Business Wire. December 2, 2002.</ref> The 7723/0723 series opto-isolators contain [[CMOS]] LED drivers and a CMOS [[buffer amplifier|buffered amplifier]]s, which require two independent external power supplies of 5&nbsp;V each.<ref>[[Agilent Technologies]] (2005). ''[http://www.datasheetcatalog.org/datasheet2/1/03rgplhxdo8wqdacrplq8kjq29fy.pdf Agilent HCPL-7723 & HCPL-0723 50 MBd 2 ns PWD High Speed CMOS Optocoupler (Datasheet)]''. Retrieved November 2, 2010.</ref>

Photodiode opto-isolators can be used for interfacing analog signals, although their [[Diode#Current–voltage characteristic|non-linearity]] invariably [[Amplitude distortion|distorts the signal]]. A special class of analog opto-isolators introduced by [[Burr-Brown Corporation|Burr-Brown]] uses ''two'' photodiodes and an input-side [[operational amplifier]] to compensate for diode non-linearity. One of two identical diodes is wired into the [[feedback|feedback loop]] of the amplifier, which maintains overall current transfer ratio at a constant level regardless of the non-linearity in the second (output) diode.<ref name=HH598/>

A novel idea of a particular optical analog signal isolator was submitted on 3, June 2011. The proposed configuration consist of two different parts. One of them transfers the signal, and the other establishes a negative feedback to ensure that the output signal has the same features as the input signal. This proposed analog isolator is linear over a wide range of input voltage and frequency.<ref>Modern Applied Science Vol 5, No 3 (2011). ''[http://www.ccsenet.org/journal/index.php/mas/article/view/9543/7725  A Novel Approach to Analog Signal Isolation through Digital Opto-coupler (YOUTAB)]''.</ref> However linear opto couplers using this principle have been available for many years, for example the IL300.<ref>Vishay website, IL300 data (accessed 10-20-2015), ''http://www.vishay.com/optocouplers/list/product-83622/'' {{Webarchive|url=https://web.archive.org/web/20161227174819/http://www.vishay.com/optocouplers/list/product-83622/ |date=2016-12-27 }}.</ref>

[[Solid-state relays]] built around [[MOSFET]] switches usually employ a photodiode opto-isolator to drive the switch. The gate of a MOSFET requires relatively small total [[electric charge|charge]] to turn on and its leakage current in steady state is very low. A photodiode in photovoltaic mode can generate turn-on ''charge'' in a reasonably short time but its output ''voltage'' is many times less than the MOSFET's [[threshold voltage]]. To reach the required threshold, solid-state relays contain stacks of up to thirty photodiodes wired in series.<ref name=VI>Vishay Semiconductor.</ref>

===Phototransistor opto-isolators===

Phototransistors are inherently slower than photodiodes.<ref name=B61>Ball, p. 61.</ref> The earliest and the slowest but still common 4N35 opto-isolator, for example, has rise and fall times of 5 [[microsecond|μs]] into a 100 Ohm load<ref>Horowitz and Hill, p. 596. Ball p. 68, provides rise and fall time of 10 μs but does not specify load impedance.</ref> and its bandwidth is limited at around 10 kilohertz - sufficient for applications like [[electroencephalography]]<ref name=ANA/> or [[Pulse-width modulation#Power delivery|pulse-width motor control]].<ref name=B68>Ball, p. 68.</ref> Devices like PC-900 or 6N138 recommended in the original 1983 [[Musical Instrument Digital Interface]] specification<ref>''[http://www.midi.org/techspecs/electrispec.php MIDI Electrical Specification Diagram & Proper Design of Joystick/MIDI Adapter]''. MIDI Manufacturers Association. 1985. Retrieved November 2, 2010.</ref> allow digital data transfer speeds of tens of kiloBauds.<ref>Ball, p. 67.</ref> Phototransistors must be properly [[biasing|biased]] and loaded to achieve their maximum speeds, for example, the 4N28 operates at up to 50&nbsp;kHz with optimum bias and less than 4&nbsp;kHz without it.<ref name=P73>Pease, p. 73.</ref>

Design with transistor opto-isolators requires generous allowances for wide fluctuations of parameters found in commercially available devices.<ref name=P73/> Such fluctuations may be destructive, for example, when an opto-isolator in the [[Feedback loop#Electronic engineering|feedback loop]] of a [[DC-to-DC converter]] changes its [[transfer function]] and causes spurious oscillations,<ref name=Basso>Basso.</ref> or when unexpected delays in opto-isolators cause a [[short circuit]] through one side of an [[H-bridge]].<ref>Ball, pp. 181–182. Shorting one side of an H-bridge is called ''shoot-through''.</ref> Manufacturers' [[datasheet]]s typically list only worst-case values for critical parameters; actual devices surpass these worst-case estimates in an unpredictable fashion.<ref name=P73/> [[Bob Pease]] observed that current transfer ratio in a batch of 4N28's can vary from 15% to more than 100%; the datasheet specified only a minimum of 10%. Transistor [[Bipolar junction transistor#Transistor .27alpha.27 and .27beta.27|beta]] in the same batch can vary from 300 to 3000, resulting in 10:1 variance in [[Bandwidth (signal processing)|bandwidth]].<ref name=P73/>

Opto-isolators using [[field-effect transistor]]s (FETs) as sensors are rare and, like vactrols, can be used as remote-controlled analog potentiometers provided that the voltage across the FET's output terminal does not exceed a few hundred mV.<ref name=HH598>Horowitz and Hill, p. 598.</ref> Opto-FETs turn on without injecting switching charge in the output circuit, which is particularly useful in [[sample and hold]] circuits.<ref name=HH595/>

===Bidirectional opto-isolators===

All opto-isolators described so far are uni-directional. Optical channel always works one way, from the source (LED) to the sensor. The sensors, be they photoresistors, photodiodes or phototransistors, cannot emit light.<ref group=note>Exception: Ternary and quaternary [[gallium arsenide phosphide|GaAsP]] photodiodes can generate light. - Mims, p. 102.</ref> But LEDs, like all semiconductor diodes,<ref group=note>"Even the garden variety signal diodes you use in circuits have a small photovoltaic effect. There are amusing stories of bizarre circuit behavior finally traced to this." - Horowitz and Hill McCoulny, p. 184.</ref> are capable of detecting incoming light, which makes possible construction of a two-way opto-isolator from a pair of LEDs. The simplest bidirectional opto-isolator is merely a pair of LEDs placed face to face and held together with [[heat-shrink tubing]]. If necessary, the gap between two LEDs can be extended with a [[optical fiber|glass fiber insert]].<ref name=M102>Mims vol. 2, p. 102.</ref>

[[Visible spectrum]] LEDs have relatively poor transfer efficiency, thus [[infrared#Regions within the infrared|near infrared spectrum]] [[gallium arsenide|GaAs]], [[gallium arsenide|GaAs:Si]] and [[aluminium gallium arsenide|AlGaAs:Si]] LEDs are the preferred choice for bidirectional devices. Bidirectional opto-isolators built around pairs of GaAs:Si LEDs have current transfer ratio of around 0.06% in either [[photodiode#Photovoltaic mode|photovoltaic]] or [[photodiode#Photoconductive mode|photoconductive]] mode — less than photodiode-based isolators,<ref>Photodiode opto-isolators have current transfer ratios of up to 0.2% - Mataré, p. 177, table 5.1.</ref> but sufficiently practical for real-world applications.<ref name=M102/>

==Types of configurations==
{{Anchor |optosensor}}
{{citations needed|date=December 2019}}
[[File:Gabelkopp.jpg|thumb|A reflective pair (left) and two slotted couplers with the light paths in purple]]

Usually, optocouplers have a ''closed pair'' configuration. This configuration refers to optocouplers enclosed in a dark container wherein the source and sensor are facing each other.

Some optocouplers have a ''slotted coupler/interrupter'' configuration. This configuration refers to optocouplers with an open slot between the source and sensor that has the ability to influence incoming signals. The ''slotted coupler/interrupter'' configuration is suitable for object detection, vibration detection, and bounce-free switching.

Some optocouplers have a ''reflective pair'' configuration. This configuration refers to optocouplers that contain a source that emits light and a sensor that only detects light when it has reflected off an object. The ''reflective pair'' configuration is suitable for the development of tachometers, movement detectors and reflectance monitors.

The later two configurations are frequently referred to as ''optosensors'' or ''[[photoelectric sensor]]s''.

== See also ==
* [[Galvanic isolation]]

==Notes==
{{reflist|group=note}}

==References==
{{reflist|30em}}

==Sources==

<!-- * Don Alfano (2010). ''[http://www.ecnmag.com/blog/2010/06/soapbox/Optocouplers-and-Cigarettes.aspx Optocouplers and Cigarettes]''. ECN, July 1, 2010. Retrieved November 2, 2010. -->
* S. Ananthi (2006). ''[https://books.google.com/books?id=oCm7HJE9DtoC A text book of medical instruments]''. New Age International. {{ISBN|81-224-1572-5}}.
* [[Avago Technologies]] (2010). ''[http://www.avagotech.com/docs/AV02-1909EN Safety Considerations When Using Optocouplers and Alternative Isolators for Providing Protection Against Electrical Hazards]''. January 2010. Retrieved November 5, 2010.
* Stuart R. Ball (2004). ''[https://books.google.com/books?id=TziS7WMcIDQC Analog interfacing to embedded microprocessor systems]''. Elsevier. {{ISBN|0-7506-7723-6}}.
* Christophe Basso (2009). ''[https://web.archive.org/web/20101123124116/http://eetweb.com/applications/dealing-lowcurrent-optocouplers-20091001/ Dealing with Low-Current Optocouplers]''. Energy Efficiency and Technology, September 1, 2009. Retrieved November 2, 2010.
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== External links ==
* {{commons category inline|Opto-isolators}}

{{Authority control}}

[[Category:Optoelectronics]]
[[Category:Electrical components]]
[[Category:Solid state switches]]
[[Category:Semiconductor devices]]