Resistive opto-isolator (RO), also called photoresistive opto-isolator, vactrol (after a genericized trademark introduced by Vactec, Inc. in the 1960s), analog opto-isolatorIn 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 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 to 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 phototransistors. 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 Millikan 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 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 with 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 2010, the Vactrol ROs were produced by PerkinElmer, the successor of Vactec until its illumination and detection solutions businesses were divested as an independent company Excelitas Technologies in November 2010.Excelitas Technologies, Excelitas ended the production of ROs in December 2015.Excelitas Technologies, Product EOL announcement, 1 June 2015, As of 2022 Vactrol type ROs are still manufactured by successor of Silonex the Advanced Photonix.Photonics Media, 4 March 2013,Advanced Photonix, Optocouplers, and at least two factories in Shenzhen, China.Optocoupler products, Linear optocouplers,
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
Physical properties
Light sources, detectors and their coupling Most ROs use CdS or CdSe as light-sensitive material.
The spectral sensitivity of CdS photoresistors peaks for red light (wavelength λ = 640 nm) and extends up to 900 nm. These devices can control a few mA, and have a quasilinear dependence of photocurrent on the light intensity at a constant voltage. Their high dark resistance, reaching tens GOhm, provides a high dynamic range with respect to the light intensity and low signal distortions. However, their reaction time to a change in the light intensity is long, about 140 ms at 25 °C.
CdSe photoresistors are 5–100 times more sensitive than CdS devices; their sensitivity peaks in the red to near-infrared region (670–850 nm) and extends up to 1100 nm. They have an inferior dynamic range and linearity than their CdS counterparts, but are faster, with a time constant of less than 20 ms.
The optimal light sources for CdS/CdSe photoresistors are AlGaAs heterostructures (emission wavelength ~660 nm) or GaP LEDs (λ = 697 nm). The luminosity of the LED is nearly proportional to the controlling current. The emission spectrum depends on the temperature of LED, and thus on the current, but this variation is too small to affect the spectral matching of the LED and photoresistor.
For mechanical stability, the LED and photoresistor are glued with a transparent epoxy, glue or an organic polymer. The glue also functions as a diffuser spreading the light beam – if it would fall near the border of the semiconductor and electrical contacts then a slight shift of the LED position might significantly alter the RO response.
Transfer characteristic
The transfer characteristic of RO is usually represented as the electrical resistance of the photoresistor as a function of the current through the light source; it is a convolution of three main factors: the dependence of light source intensity on its current, optical coupling and spectral matching between the light source and photoresistor, and the photoresponse of the photoresistor. The first dependence is almost linear and temperature independent for LEDs in the practical range of the controlling current. On the contrary, for incandescent lamps the light-current curve is not linear, and the emission spectrum varies with temperature, and thus with the input current. As to the light detector, its properties depend on temperature, voltage and the history of utilization (memory effect). Therefore, the transfer characteristic takes a range of values.
The equivalent circuit of the photoresistor consists of three components: RD – dark resistance, which is determined by the semiconductor and can range from a few MOhm to hundreds GOhm; RRL – residual resistance of illuminated but non-loaded photoresistor, typically between 100 Ohm and 10 kOhm; RI – ideal photoresistance, which is inversely proportional to the light intensity. Because of the large value of RD, the total resistance is mainly determined by RI. The dynamic range of the photoresistor with respect to illumination is equal to the ratio of the critical illumination Φcr to the sensitivity threshold Φth.
RD and RI, but not RRL, decrease with increasing voltage that results in signal distortions. At low illumination levels, the resistance of cadmium-based ROs increases by about 1% upon heating by 1 °C. At higher light intensities, the thermal coefficient of resistance can change its values and even its sign.
Memory effect
Cadmium-based photoresistors exhibit a pronounced memory effect, that is, their resistance depends on the history of illumination. It also shows characteristic overshoots, with the values reaching a temporary minimum or maximum after application of light. These resistivity changes affect the RO temperature, bringing additional instabilities. The stabilization time nonlinearly increases with the light intensity and can vary between hours and days; by convention, it is assumed that the output of an illuminated RO reaches equilibrium within 24 hours.
The memory effect is evaluated using the ratio of Rmax to Rmin (see figure). This ratio increases with decreasing light intensity and has a value of 1.5–1.6 at 0.1 lux and 1.05–1.10 at 1000 lux for PerkinElmer devices. In some low-resistive RO models this ratio was as high as 5.5, but by 2009 their production has been discontinued. High-resistance photoresistors typically have a less pronounced memory effect, are less sensitive to temperature, and have a more linear response, but are also relatively slow. Some devices designed in the 1960s had a negligible memory effect, but exhibited unacceptably high signal distortions at high current levels.
Operating frequencies