What Are Analog Optical Isolators?
PerkinElmer Optoelectronics has been a leading manufacturer of
analog optical isolators for over twenty years and makes a broad range
of standard parts under its trademark VACTROL®.
They must be protected from excessive forward current due to the low
dynamic resistance in the forward direction. The forward characteristic
of an LED typically used in VACTROLs is shown below.
There are many kinds of optical isolators, but the most common is the
LED/phototransistor type. Other familiar types use output elements
such as light sensitive SCRs, Triacs, FETs, and ICs. The major
application for these silicon based devices is to provide electrical
isolation of digital lines connected between different pieces of
equipment. The principle of operation is very simple. When an input
current is applied to the LED, the output phototransistor turns on. The
only connection between the LED and phototransistor is through
light—not electricity, thus the term optical isolator. These optical
isolators are primarily digital in nature with fast response times suitable
for interfacing with logic gates. Rise and fall times of a few
microseconds, faster for some isolators, are typical.
LED Forward Characteristics
Output Element
The analog optical isolator (AOI) also uses an optical link between
input and output. The input element is an LED and the output element
is always photoconductive cell or simply photocell. Together, the
coupled pair act as an electrically variable potentiometer. since the
output element of the AOI is a resistor, the voltage applied to this
output resistor may be DC and/or AC and the magnitude may be as
low as zero or as high as the maximum voltage rating. Because the
input will control the magnitude of a complex waveform in a
proportional manner, this type of isolator is an analog control element.
AOIs may be used in the ON-OFF mode but the fastest response time
is only in the millisecond range. A level sensitive Schmitt trigger is
required between the AOI and logic gates when used in digital circuits.
The figure below shows the circuit diagram of a standard AOI.
The output element in all PerkinElmer’s AOIs is a light dependent
resistor (LDR), also called a photoconductor or photocell. Photocells
are true resistors.
These passive resistors are made from a light sensitive polycrystalline
semiconductor thin film which has a very high electron/photon gain.
There are no P/N junctions in a photocell, making it a bilateral device.
The resistance of the photocell depends on the amount of light falling
on the cell. For a given illumination, the amount of electrical current
through the cell depends on the voltage applied. This voltage may be
either AC or DC. Thus, the photocell is the ideal low distortion output
element for an analog optoisolator.
A complete discussion of photoconductive cells can be found in the
first section of this book.
AOI Circuit Diagram
Input Element
Light emitting diodes used in AOIs are usually visible LEDs best
matching the sensitivity spectrum of the photocell output element.
LEDs are the ideal input element in most applications. They require
low drive current and voltage, respond very fast and have virtually
unlimited life. They are very rugged and are unaffected by shock and
vibration. Since the LED is a diode, it conducts in one direction only.
28
PERKINELMER [ PERKINELMER OPTOELECTRONICS ]
