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, but the original Vactrols are still being manufactured by
PerkinElmer. 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, the manufacturers replaced incandescent and neon lamps with LEDs and achieved response times of 5 milliseconds and modulation frequencies up to 250 Hz. The name
Vactrol was carried over on LED-based devices which are, as of 2010, still produced in small quantities. Photoresistors used in opto-isolators rely on bulk effects in a uniform film of
semiconductor; there are no
p-n junctions. which made them ideal for driving
vacuum fluorescent displays. Other industrial applications included
photocopiers, industrial
automation, professional light measurement instruments and
auto-exposure meters. To date, Vactrols activated by pressing the
stompbox pedal are ubiquitous in the music industry. Shortages of genuine PerkinElmer Vactrols forced the
DIY guitar community to "roll their own" resistive opto-isolators. Guitarists to date prefer opto-isolated effects because their superior
separation of audio and control grounds results in "inherently high quality of the sound". 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 in the control circuit.
Photodiode opto-isolators Diode opto-isolators employ LEDs as sources of light and silicon
photodiodes 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
photoconductive mode. Alternatively, in the absence of external bias the diode converts the energy of light into
electric energy by charging its terminals to a voltage of up to 0.7 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%. The
Hewlett-Packard 6N137/HPCL2601 family of devices equipped with internal output amplifiers was introduced in the late 1970s and attained 10
MBd data transfer speeds. It remained an industry standard until the introduction of the 50 MBd
Agilent Technologies 7723/0723 family in 2002. The 7723/0723 series opto-isolators contain
CMOS LED drivers and a CMOS
buffered amplifiers, which require two independent external power supplies of 5 V each. Photodiode opto-isolators can be used for interfacing analog signals, although their
non-linearity invariably
distorts the signal. A special class of analog opto-isolators introduced by
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 loop of the amplifier, which maintains overall current transfer ratio at a constant level regardless of the non-linearity in the second (output) diode. However linear opto couplers using this principle have been available for many years, for example the IL300.
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
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.
Phototransistor opto-isolators Phototransistors are inherently slower than photodiodes. The earliest and the slowest but still common 4N35 opto-isolator, for example, has rise and fall times of 5
μs into a 100 Ohm load and its bandwidth is limited at around 10 kilohertz - sufficient for applications like
electroencephalography Devices like PC-900 or 6N138 recommended in the original 1983
Musical Instrument Digital Interface specification allow digital data transfer speeds of tens of kiloBauds. Phototransistors must be properly
biased and loaded to achieve their maximum speeds, for example, the 4N28 operates at up to 50 kHz with optimum bias and less than 4 kHz without it. Design with transistor opto-isolators requires generous allowances for wide fluctuations of parameters found in commercially available devices. or when unexpected delays in opto-isolators cause a
short circuit through one side of an
H-bridge. Manufacturers'
datasheets typically list only worst-case values for critical parameters; actual devices surpass these worst-case estimates in an unpredictable fashion. Opto-FETs turn on without injecting switching charge in the output circuit, which is particularly useful in
sample and hold circuits. But LEDs, like all semiconductor diodes, 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
glass fiber insert.
Visible spectrum LEDs have relatively poor transfer efficiency, thus
near infrared spectrum GaAs,
GaAs:Si and
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
photovoltaic or
photoconductive mode — less than photodiode-based isolators, but sufficiently practical for real-world applications. ==Types of configurations==