OPTICALLY COUPLED ISOLATORS.

A coupler can be thought of as a signal-transfer device that has its input and output electronically isolated from each other through an optical link. Each coupler consists of an input light-emitting diode, usually one that emits in the infrared or near-infrared region, and an output sillcon photodetector, which can be a photodiode, a phototransistor, a photoDarlington, or a photoSCR. The LED emitter and the light sensor are optically linked together but dielectrically isolated from each other. In addition to being available in the popular integral version, in which the LED, the sensor, and the gap between them are housed in one package, couplers can be purchased in air-gap versions. This type of device is also supplied in a single package, but the user has access to the gap between the emitter and the sensor. Such a coupler is switched by interrupting the LED beam. Air-gap couplers are primarily used as sensing devices in business machines, fluid-flow monitors, and engine controls. Since the gap must be fairly large, compared to that of the integral type, the current gain of these devices tends to be low. Also, their maximum isolation voltage is limited to the dielectric strength of air. An integral type of coupler, on the other hand, has a plastic or glass in the gap to increase the isolation voltage rating.