optokoppler

Optokoppler, or optoisolator, transfers electrical signals between two isolated circuits by using light to couple without a direct electrical connection. It contains an input light-emitting diode (LED) and an output photodetector (such as a phototransistor, photodiode, or photoSCR) separated by an insulating barrier that provides galvanic isolation.

Common output configurations include transistor-type optokopplers, MOSFET-output types, and devices with triac or SCR outputs for AC switching. Packages include DIP-4, DIP-6, and various surface-mount variants. Isolation voltages are specified as Viso and typically range from several kilovolts RMS for safety-rated parts.

Key specifications include the input forward current If, the output current or load current, and the current transfer ratio CTR, defined as Io/If and varying with temperature and LED current. Speed is characterized by propagation delay and rise/fall times; high-speed parts reach tens of nanoseconds to microseconds, while standard types are typically microseconds or longer. CTR values can span from a few percent to hundreds of percent depending on device type and operating conditions, and performance may drift with temperature and aging. Some devices include zero-cross or linear output characteristics for specialized signaling.

Applications include isolation between logic and power electronics in microcontroller interfaces, switching power supplies, motor drives, PLCs, and safety-critical or medical equipment. Optokopplers offer galvanic isolation and noise immunity but may be slower and exhibit CTR variability compared with direct electrical coupling.