Priniciples of Electromechanical Relay Operation
3.31 Optoisolator Relay
The optoisolator, sometimes called an optocoupler, is an assembly that
contains a light emitting diode and a solid state photosensitive device. These
are placed in close proximity to each other so that light generated by the LED
will be impressed upon the photosensitive device, which may be a transistor,
SCR or triac that is normally non-conducting. An input signal fed to the LED
causes it to glow, emitting light. When the light energy is impressed upon
the solid state device it becomes conductive, allowing the output circuit to
be energized. Since the coupling medium is light, the optoisolator can be
designed to attain an isolation voltage rating of several thousands of
volts.