Editing Opto-isolator (section)

==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/>