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Working principle of PC817 optocoupler

Fiber and communications infrastructure knowledge brief Published 13 Apr 2025

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The PC817 optocoupler transmits electrical signals between two isolated circuits using an infrared LED and a phototransistor, providing optical isolation without direct electrical connection.

Internal Structure

The PC817 consists of two main components housed in a single 4-pin package: Infrared LED (Input Side): Emits infrared light when a current flows through it. Phototransistor (Output Side): Detects the infrared light and conducts current accordingly. These components are optically coupled but electrically isolated, allowing signal transfer without direct electrical contact.

Step-by-

Step Working PrincipleLED Emission (Input Side): When a voltage is applied to the input pins, current flows through the infrared LED.

The LE

D emits infrared light proportional to the input current. This light is invisible to the human eye but can be detected by the phototransistor. Optical Transmission: The emitted infrared light passes through a transparent barrier inside the IC.

This barrier ensures electrical isolation between the input and output circuits, protecting sensitive components from high voltages or noise. The PC817 can typically withstand up to 5,000 Vrms between input and output. Phototransistor Activation (Output Side): The phototransistor receives the infrared light and becomes conductive.

The amount of current it allows to flow from collector to emitter depends on the intensity of the received light, effectively reproducing the input signal on the output side. Signal Isolation and Transfer: Since the input and output are not electrically connected, the PC817 provides galvanic isolation, preventing voltage spikes or noise from affecting the output circuit. The output can then drive other components, such as microcontrollers, relays, or logic circuits, safely.

Key Features

Low Input Current: Operates with small currents, suitable for microcontroller interfacing. Fast Switching: Can handle digital signals efficiently. Compact Size: 4-pin DIP package for easy integration.

Wide Applications: Used in high-voltage isolation, noise-sensitive circuits, and relay driving.

Practical Example

For instance, a microcontroller outputting 5V can control a 12V relay through a PC817. When the microcontroller sends a HIGH signal, the LED inside the PC817 glows, activating the phototransistor. This triggers a transistor to switch the 12V relay, while the microcontroller remains electrically isolated from the high-voltage circuit.

This principle ensures safe signal transmission and protects low-voltage electronics from high-voltage disturbances.

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Working principle of PC817 optocoupler

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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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