Optoelectronics

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
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...