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Resistive opto-isolator (RO), also called photoresistive opto-isolator, vactrol (after a genericized trademark introduced by Vactec, Inc. in the 1960s), analog opto-isolator〔In PerkinElmer literature.〕 or lamp-coupled photocell,〔EG&G Corporation (1971). ''(Instruction Manual for EG&G Model 196 Aircraft Hygrometer System )''. November 1971. p. 18.〕 is an optoelectronic device consisting of a source and detector of light, which are optically coupled and electrically isolated from each other. The light source is usually a light-emitting diode (LED), a miniature incandescent lamp, or sometimes a neon lamp, whereas the detector is a semiconductor-based photoresistor made of cadmium selenide (CdSe) or cadmium sulfide (CdS). The source and detector are coupled through a transparent glue or through the air. Electrically, RO is a resistance controlled by the current flowing through the light source. In the dark state, the resistance typically exceeds a few MOhm; when illuminated, it decreases as the inverse of the light intensity. In contrast to the photodiode and phototransistor, the photoresistor can operate in both the AC and DC circuits and have a voltage of several hundred volts across it. The harmonic distortions of the output current by the RO are typically within 0.1% at voltages below 0.5 V. RO is the first and the slowest opto-isolator: its switching time exceeds 1 ms, and for the lamp-based models can reach hundreds of milliseconds. Parasitic capacitance limits the frequency range of the photoresistor by ultrasonic frequencies. Cadmium-based photoresistors exhibit a "memory effect": their resistance depends on the illumination history; it also drifts during the illumination and stabilizes within hours, or even weeks for high-sensitivity models. Heating induces irreversible degradation of ROs, whereas cooling to below −25 °C dramatically increases the response time. Therefore, ROs were mostly replaced in the 1970s by the faster and more stable photodiodes and photoresistors. ROs are still used in some sound equipment, guitar amplifiers and analog synthesizers owing to their good electrical isolation, low signal distortion and ease of circuit design. ==History== In 1873, Willoughby Smith discovered the photoconductivity of selenium. In the early 1900s, the studies of the external photoeffect in vacuum tubes resulted in the commercial production of photoresistors. In 1918, American and German engineers independently suggested the use of vacuum photocells for reading optical phonograms in the film projectors in cinemas, and Lee de Forest, Western Electric and General Electric produced three competing systems using such photocells.〔 In 1927, the first commercial sound film, ''The Jazz Singer'', was produced in the United States, and by 1930 sound films had replaced silent films.〔 The success of sound films stimulated the search for new applications of photocells. Various types of photocells were considered: vacuum, gas-discharge, photovoltaic and photoresistive, but the industry favored slow yet cheap selenium devices. By the mid-1930s, selenium photocells controlled assembly lines, elevators〔 and looms. Fire alarms with selenium sensors came into mass production in the UK and then in the US. Norbert Wiener proposed, and Truman Gray built an optical scanner for inputting and processing data in analog computers. Kurt Kramer introduced a selenium photocell to medical research. In 1940, Glenn Milliken built the first practical selenium-based oximeter to monitor the physical condition of the Royal Air Force pilots. It was a RO where the light source and detector were separated by the ear lobe of the pilot.〔〔 In the early 1950s, Teletronix used the "T4" optical attenuator in the famous LA-2 compressor,furthermore, for their unique sound, they are still used today by Universal Audio in their reproductions of the LA-2. After the 1950s selenium in photocells was gradually replaced by CdS and CdSe. By 1960, ROs based on incandescent lamps and CdS/CdSe photoresistors were used in feedback circuits in the industry, for example, for controlling rotation speed and voltage. In the early 1960s, the introduction of sensitive and compact CdS/CdSe photoresistors resulted in the mass production of cameras with automatic exposure.〔〔 However, these photoresistors were not adopted in medicine because of their memory effect and rapid aging〔 – they required regular recalibration that was not acceptable for medical practice.〔〔 In the early 1960s, Gibson and Fender started using ROs to modulate the tremolo effect in guitar amplifiers. Both companies were assembling their ROs from discrete lamps, photoresistors and coupling tubes. While Gibson used cheap but slow incandescent lamps as light sources, Fender replaced them with neon lamps, which increased the maximum frequency to tens of Hz and reduced controlling currents, but resulted in a nonlinear modulation. Therefore, other producers preferred incandescent lamps for their linearity. In 1967 Vactec introduced a compact RO branded as Vactrol.〔 Unlike the tube-coupled ROs of Fender and Gibson, Vactrols were sealed and sturdy devices. In the early 1970s, Vactec replaced incandescent bulbs by LEDs. This increased the switching speed, but not to the level required for digital devices. Therefore, the introduction of the faster photodiodes and phototransistors in the 1970s pushed out ROs from the market.〔〔 ROs retained narrow application niches in sound equipment and some industrial automation devices that did not require high speeds.〔〔 Vactec did not extend their rights to the trademark Vactrol,〔 and it has become a household word in the English language for any RO used in audio equipment, including the ROs of Fender and Gibson.〔 As of 2012, the Vactrol ROs were produced by PerkinElmer, the successor Vactec. In the European Union, the production and distribution of Cd-based photoresistors is banned since January 1, 2010. The initial version of the EU Directive on the restriction of hazardous substances (RoHS), adopted in 2003, allowed the use of cadmium in the devices that had no Cd-free counterparts.〔 However, in 2009 the European Commission excluded Cd-based ROs used in professional audio equipment from the list of permitted devices.〔 "From 2 January 2013, the use of cadmium is permitted in photoresistors for analogue optocouplers applied in professional audio equipment. ... However, the exemption is limited in time, since the Commission considers that the research for cadmium-free technology is in progress and substitutes could become available by the end of 2013."〔(Commission adopts RoHS exemption for cadmium ). European Union. 5 February 2013〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Resistive opto-isolator」の詳細全文を読む スポンサード リンク
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