optoisolation

Optoisolation, or opto-isolation, is a method of electrically isolating two parts of an electronic system by using an optocoupler. An optocoupler contains an input light-emitting diode and an output photodetector inside a light-tight package. When the input LED is driven, light crosses a barrier to the detector, allowing signal transfer without a direct electrical connection. This galvanic isolation protects the low-voltage side from high-voltage transients and reduces ground-loop noise.

The photodetector in an optocoupler can be a phototransistor, photodiode, photodarlington, or a MOSFET-based device. Depending on construction, optocouplers provide various outputs suitable for logic signals, analog signals, or AC load switching (triac optocouplers). Isolation voltage ratings vary widely, from tens of volts to several kilovolts RMS, with creepage and clearance distances specified to meet safety standards. CTR (current transfer ratio) and response speed depend on the device type, LED input current, and temperature.

Applications span microcontroller interfaces, industrial automation, medical equipment, and power electronics. Optoisolation is used to prevent hazardous voltages or fast transients from reaching control circuitry, to break ground loops, and to improve noise immunity. Limitations include slower switching speeds and variability in CTR with temperature and aging, which may require buffering or careful drive design. High-speed optocouplers and digital isolators offer faster performance, but traditional optocouplers remain common for robust galvanic isolation. Standards and safety certifications for opto-isolators commonly reference UL 1577 and relevant IEC/ISO standards for medical, industrial, and consumer equipment.