There are many situations where signals and data need to
be transferred from one subsystem to another within a
piece of electronics equipment, or from one piece of
equipment to another, without making a direct .ohmic.
electrical connection. Often this is because the source
and destination are (or may be at times) at very different
voltage levels, like a microprocessor which is operating
from 5V DC but being used to control a triac which is
switching 240V AC. In such situations the link between
the two must be an isolated one, to protect the
microprocessor from overvoltage damage.
Relays can of course provide this kind of isolation, but
even small relays tend to be fairly bulky compared with
ICs and many of today.s other miniature circuit
components. Because they.re electro-mechanical, relays
are also not as reliable . and only capable of relatively
low speed operation. Where small size, higher speed and
greater reliability are important, a much better
alternative is to use an optocoupler. These use a beam of
light to transmit the signals or data across an electrical
barrier, and achieve excellent isolation.
Optocouplers typically come in a small 6-pin or 8-pin IC
package, but are essentially a combination of two distinct
devices: an optical transmitter, typically a gallium arsenide
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LED (light-emitting diode) and an optical receiver such as a
phototransistor or light-triggered diac. The two are
separated by a transparent barrier which blocks any
electrical current flow between the two, but does allow the
passage of light. The basic idea is shown in Fig.1, along with
the usual circuit symbol for an optocoupler.
Usually the electrical connections to the LED section are
brought out to the pins on one side of the package and
those for the phototransistor or diac to the other side, to
physically separate them as much as possible. This usually
between 500V and 7500V between input and output.
Optocouplers are essentially digital or switching devices, so
they.re best for transferring either on-off control signals or
digital data. Analog signals can be transferred by means of
frequency or pulse-width modulation.
The most important parameter for most optocouplers is
their transfer efficiency, usually measured in terms of their
current transfer ratio or CTR. This is simply the ratio
between a current change in the output transistor and the
current change in the input LED which produced it. Typical
values for CTR range from 10% to 50% for devices with an
output phototransistor and up to 2000% or so for those
with a Darlington transistor pair in the output. Note,
however that in most devices CTR tends to vary with
absolute current level. Typically it peaks at a LED current
level of about 10mA, and falls away at both higher and
lower current levels.
Other optocoupler parameters include the output
transistor.s maximum collector-emitter voltage rating
VCE(max), which limits the supply voltage in the output
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circuit; the input LED.s maximum current rating IF(max),
which is used to calculate the minimum value for its series
resistor; and the optocoupler.s bandwidth, which
determines the highest signal frequency that can be
transferred through it . determined mainly by internal
device construction and the performance of the output
phototransistor. Typical opto-couplers with a single output
phototransistor may have a bandwidth of 200 – 300kHz,
while those with a Darlington pair are usually about 10
times lower, at around 20 – 30kHz.
How They.re Used
Basically the simplest way to visualise an optocoupler is in
terms of its two main components: the input LED and the
output transistor or diac. As the two are electrically
isolated, this gives a fair amount of flexibility when it comes
to connecting them into circuit. All we really have to do is
work out a convenient way of turning the input LED on and
off, and using the resulting switching of the phototransistor/
diac to generate an output waveform or logic
Electus Distribution Reference Data Sheet: OPTOCOUP.PDF (1)
Fig.1: Construction of a typical
optocoupler and the usual circuit symbol.
Fig.2: Typical ways of driving an opto.s LED.
WHEN & HOW TO USE THEM
Fig.3: Protecting the LED against reverse voltage.