Optocoupler

An optocoupler, or opto-isolator, is a device that transfers a signal between two isolated circuits by means of light. It contains an input light source (usually an infrared LED) and a photosensitive detector (such as a phototransistor, photodiode, or photodarlington) within a light-tight package separated by a dielectric barrier. The isolation barrier provides galvanic isolation, preventing high voltage or fast transients from passing from the input to the output while still allowing signal coupling through light.

Operation involves current through the input LED producing light, which is detected by the output element and converted into an electrical signal. In transistor-output optocouplers, the output current is proportional to the input LED current, described by the current transfer ratio (CTR). CTR varies with device type, temperature, wavelength, and aging, and is specified for a given forward current.

Types and applications include the common transistor-output configuration (phototransistor, photodarlington) and detectors such as photodiodes or MOSFETs. There are also triac- and MOS-based optocouplers for AC switching and solid-state relays. Optocouplers are used to isolate microcontrollers or digital logic from high-voltage or noisy equipment, drive relays, protect against ground loops and surges, and interface different grounds or power domains.

Specifications and mounting considerations cover the isolation voltage rating (often several kilovolts RMS), CTR, input forward current, output current or on-state voltage, and switching speed (propagation delay and rise/fall times). Packages include DIP, gull-wing, and surface-mount variants. Examples such as the PC817 illustrate common designs. Selection depends on required speed, isolation level, and permissible variability in transfer characteristics, with safety standards such as UL 1577 guiding many applications.