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Typical Optical Coupler Circuit

Fiber and communications infrastructure knowledge brief Published 03 Oct 2023

Use this reference to prepare product questions for BAGYOU FIBRE. Check interfaces, capacity, protection and installation requirements against the current product documentation.

An optical coupler circuit uses an LED and a photodetector to transfer signals between electrically isolated circuits, providing protection and signal integrity.

Overview

An optical coupler, also called an optocoupler or opto-isolator, is a semiconductor device that transmits electrical signals using light, typically infrared, between two electrically isolated circuits. The main purpose is to protect sensitive components like microcontrollers from high voltages or electrical noise while allowing signal transfer. Basic ComponentsLED (Transmitter): Converts the input electrical signal into light.

When current flows through the LED, it emits infrared light proportional to the input signal. Photodetector (Receiver): Detects the light from the LED and converts it back into an electrical signal. Common photodetectors include phototransistors, photodiodes, photo-FETs, or opto-triacs.

Working Principle

When the input circuit drives the LED, it emits light that falls on the photodetector. The photodetector then conducts, allowing current to flow in the output circuit. This optical link ensures electrical isolation, so the input and output circuits can operate at different voltage levels safely.

Optical couplers can also block noise transfer and prevent ground loops.

Circuit Design Considerations

Current Transfer Ratio (CTR): Similar to a BJT's current gain, CTR is the ratio of the photodetector's output current to the LED's input current, expressed as a percentage. It is crucial for designing the correct input and output resistor values. Inverting vs Non-Inverting Configurations: An optocoupler can be configured as an inverting circuit (like a common-emitter BJT) or non-inverting (like a common-collector), depending on whether the output should follow or invert the input signal.

Resistors: Input resistors limit LED current, while output resistors convert phototransistor current into a voltage signal.

Applications

Microcontroller Isolation: Protects low-voltage logic circuits from high-voltage spikes. Switching Power Supplies: Transfers feedback signals across isolated sections. Sensor Circuits: Prevents noise from affecting sensitive measurements.

Digital Logic Level Conversion: Interfaces circuits operating at different voltage levels.

Example Circuit

A simple optocoupler circuit includes an input switch connected to the LED through a current-limiting resistor. When the switch closes, the LED emits light, turning on the phototransistor. The phototransistor conducts, generating a voltage across an output resistor, which can then drive a load or logic input.

By carefully selecting the LED current, phototransistor load resistor, and considering CTR, designers can create reliable optical coupler circuits for both digital and analog applications.

Typical Optical Coupler Circuit - BAGYOU FIBRE NETWORKS [PDF]

Typical Optical Coupler Circuit

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Technical note

This reference is intended for preliminary fiber and communications infrastructure research. Optical interface, fiber type, port or core capacity, electrical input and output, enclosure protection, cooling, installation limits and applicable standards must be verified for the specific project.

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