the optoelectronic coupler as an isolating element is show in fig..
a light-emitting diode and phototransistor are mounted very close to one another in a single package. Light from the diode, caused by current i, falls on the phototransistor giving rise to a current iz. The information contained by current therefore trans ferred to the photodetecting element(the phototransistor) without any electrical cou pling path. Light is the coupling link. Typically, the coupling capacitance that exists between the LED and phototransistor of optoelectronic couplers is 1 pF. Optoelec- tronic couplers have the advantage over transformer isolation couplers of being able to operate from de up to about 10 kHz. However, they do not provide as much electrical isolation or linearity