A: Optocouplers are well known as optoisolators providing an isolated galvanic barrier between the input and output utilizing infrared light. On the input side an infrared light emitting diode is used with all optocoupler types. Unlike transformers or capacitors, which can only transfer AC signals across the isolation barrier, optocouplers can. In isolated power supplies, optocouplers pass the feedback signal across the isolation boundary. Optocouplers contain both a light-emitting diode (LED) and a photo detector. Internal Equivalence Circuit Here, we will describe how a general-purpose photocoupler with this basic structure is used.