An opto-isolator (also called an optocoupler, photocoupler, or optical isolator) is an electronic component that transfers electrical signals between two isolated circuits by using light.Graf, p. 522. Opto-isolators prevent high voltages from affecting the system receiving the signal.Lee et al., p. 2. Commercially available opto-isolators withstand input-to-output voltages up to 10 kVHasse, p. 145. and voltage transients with speeds up to 25 kV/μs.Joffe and Kai-Sang Lock, p. 279.
A common type of opto-isolator consists of an LED and a phototransistor in the same opaque package. Other types of source-sensor combinations include LED-photodiode, LED-LASCR, and 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 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.Graf, p. 522; PerkinElmer, p. 28. They attain medium data transfer speed, sufficient for applications like electroencephalography.See Ananthi, pp. 56, 62 for a practical example of an opto-coupled EEG application. The fastest opto-isolators use PIN diodes in photoconductive mode.
Operation
An opto-isolator contains a source (emitter) of light, almost always a near infrared light-emitting diode (LED), that converts electrical input signal into light, a closed optical channel (also called dielectrical channel), and a photosensor, which detects incoming light and either generates electric energy directly, or 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. 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 MOSFETs. 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
Electronic equipment and signal and power transmission lines can be subjected to voltage surges induced by lightning, electrostatic discharge, radio frequency transmissions, switching pulses (spikes) and perturbations in power supply.Hasse, p. 43. Remote lightning strikes can induce surges up to 10 kV, one thousand times more than the voltage limits of many electronic components.Hasse, p. 60. 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.See Basso for a discussion of such interfacing in switched-mode power supplies.
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.Horowitz and Hill, p. 595. Historically, this function was delegated to isolation transformers, which use inductive coupling between 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. They contain a single physical isolation barrier, but provide protection equivalent to double isolation.Jaus, p. 48. Safety, testing and approval of opto-couplers are regulated by national and international standards: IEC 60747-5-2, EN (CENELEC) 60747-5-2, UL 1577, CSA Component Acceptance Notice #5, etc.Jaus, pp. 50–51. 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 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 directionsA 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. with very low losses, opto-isolators are unidirectional (see exceptions) and they cannot transmit power. Typical opto-isolators can only modulate the flow of energy already present on the output side.Joffe and Kai-Sang Lock, p. 277. Unlike transformers, opto-isolators can pass DC or slow-moving signals and do not require matching impedances between input and output sides.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. Both transformers and opto-isolators are effective in breaking ground loops, common in industrial and stage equipment, caused by high or noisy return currents in ground wires.Joffe and Kai-Sang Lock, pp. 268, 276.
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.Mataré, p. 174 The sensor die is mounted directly on the lead frame of its package (usually, a six-pin or a four-pin dual in-line package). The sensor is covered with a sheet of glass or clear plastic, which is topped with the LED die. 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.Ball, p. 69. The optical channel is made as thin as possible for a desired breakdown voltage. For example, to be rated for short-term voltages of 3.75 kV and transients of 1 kV/μs, the clear polyimide sheet in the Avago ASSR-300 series is only 0.08 mm thick.Avago Technologies (2007). ASSR-301C and ASSR-302C (datasheet). Retrieved November 3, 2010. Breakdown voltages of planar assemblies depend on the thickness of the transparent sheet and the configuration of bonding wires that connect the dies with external pins. Real in-circuit isolation voltage is further reduced by creepage over the PCB and the surface of the package. Safe design rules require a minimal clearance of 25 mm/kV for bare metal conductors or 8.3 mm/kV for coated conductors.Bottrill et al., p. 175.
Opto-isolators rated for 2.5 to 6 kV employ a different layout called silicone dome. Here, the LED and sensor dies are placed on the opposite sides of the package; the LED fires into the sensor horizontally. The LED, the sensor and the gap between them are encapsulated in a blob, or dome, of transparent silicone. The dome acts as a reflector, retaining all stray light and reflecting it onto the surface of the sensor, minimizing losses in a relatively long optical channel. 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 coefficient of thermal expansion.
Types of opto-isolators
Device typeSee Horowitz and Hill, p. 597, for an expanded list of opto-isolator types with their schematic symbols and typical specifications. Source of lightMims, p. 100. Sensor type Speed Current transfer ratio Resistive opto-isolator(Vactrol) Incandescent light bulb CdS or CdSe photoresistor (LDR) Very low <100%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 law limits current transfer ratio at below 100%. Neon lamp Low GaAs infrared LED Low Diode opto-isolator GaAs infrared LED Silicon photodiode Highest 0.1–0.2% Transistor opto-isolator GaAs infrared LED Bipolar silicon phototransistor Medium 2–120%Mataré, p. 177, table 5.1. Darlington phototransistor Medium 100–600% Opto-isolated SCR GaAs infrared LED Silicon-controlled rectifier Low to medium >100%Mataré, p. 177 Opto-isolated triac GaAs infrared LED TRIAC Low to medium Very high Solid-state relay Stack of GaAs infrared LEDs Stack of photodiodes drivinga pair of MOSFETs or an IGBT Low to highLow-cost solid-state relays have switching times of tens of milliseconds. Modern high-speed solid-state relays like Avago ASSR-300 series (see datasheet) attain switching times of less than 70 nanoseconds. Practically unlimited
Resistive opto-isolators
The earliest opto-isolators, originally marketed as light cells, emerged in the 1960s. They employed miniature incandescent light bulbs 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,According to the United States Patent and Trademark Office, trademark registered in 1969 for "photocell combined with a light source" is now dead (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. (USPTO database record serial number 74381130. Retrieved November 5, 2010). but the original Vactrols are still being manufactured by PerkinElmer.Weber, p. 190; PerkinElmer, p. 28; Collins, p. 181.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.
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 diodes (LEDs) in 1968–1970,Schubert, pp. 8–9. the manufacturers replaced incandescent and neon lamps with LEDs and achieved response times of 5 milliseconds and modulation frequencies up to 250 Hz.PerkinElmer, pp. 6–7: "at 1 fc of illumination the response times are typically in the range of 5 ms to 100 ms." The name Vactrol was carried over on LED-based devices which are, as of 2010, still produced in small quantities.Weber, p. 190; PerkinElmer, pp. 2,7,28; Collins, p. 181.