What Is an Optocoupler (Optoisolator), and How Does It Work?

An optocoupler—also known as an optoisolator or photocoupler—is an electrical component designed to transmit signals between two isolated circuits using light. It typically consists of a light-emitting diode (LED) on the input side and a photodetector (usually a phototransistor, photodiode, or photoSCR) on the output side, all enclosed in a light-tight package. When the input signal activates the LED, it emits light that is optically coupled to the output device, which then generates a corresponding electrical signal. Crucially, there is no direct electrical connection between input and output, which allows the optocoupler to provide galvanic isolation—protecting low-voltage control circuitry from high-voltage spikes, ground loops, or electrical noise. Optocouplers are essential in feedback loops, signal isolation, and safety-critical applications, particularly in switch-mode power supplies, PLC inputs, and motor drive circuits.

When and Why to Use an Optocoupler Over a Transistor or Relay

Optocouplers are ideal when electrical isolation is required between control and power stages. For example, in industrial environments where PLCs or microcontrollers interface with high-voltage devices, optocouplers help prevent damage from voltage surges or electromagnetic interference. Unlike mechanical relays, optocouplers offer solid-state switching with faster response times, longer lifespan, and no mechanical wear. The choice of optocoupler depends on factors such as current transfer ratio (CTR), switching speed (measured in Mbps), and output configuration—whether it's logic, transistor, or triac/SCR.