Types of Optocouplers

Optocouplers (also known as optoisolators) are electronic components that transfer electrical signals between two isolated circuits using light. They are widely used for galvanic isolation, noise reduction, and safety purposes. Here are the main types of optocouplers, categorized by their structure, function, and application:

1. Transistor Output Optocouplers

The most common type, featuring an LED as the input and a phototransistor as the output. They are suitable for general-purpose signal isolation.


  • Subtypes:
    • Standard Phototransistor Optocoupler: Basic design for low-to-medium current signals (e.g., 10–50 mA).
    • Darlington Pair Optocoupler: Uses a Darlington transistor configuration for higher current gain (up to 1000x), ideal for driving relays or motors.
    • Phototransistor with Base Lead: Allows external biasing for better control over sensitivity and response time.

2. Thyristor/Triac Output Optocouplers

Designed for AC power control and switching applications.


  • Types:
    • Optoisolated Triac Driver (e.g., MOC30XX series): Controls triacs in AC circuits (e.g., light dimmers, motor speed control).
    • Optoisolated SCR (Silicon Controlled Rectifier): Used for DC switching in high-voltage applications like power supplies.

3. Logic-Level Optocouplers

Optimized for digital signals in microcontroller and logic circuit systems.


  • Features:
    • Fast switching speeds (up to 100 Mbps or higher).
    • Low input current requirements (1–5 mA), compatible with CMOS/TTL logic.
    • Examples: 6N137 (high-speed), HCPL-2630 (low-power).

4. High-Voltage Optocouplers

Designed to withstand high isolation voltages (up to 10 kV or more).


  • Applications:
    • Medical equipment (to protect patients from high-voltage circuits).
    • Industrial power supplies and renewable energy systems.
    • Examples: ACPL-078L (isolation voltage up to 5 kV).

5. Linear Optocouplers

Maintain a linear relationship between input and output signals, suitable for analog signal transmission.


  • Key Features:
    • Uses a dual phototransistor or photodiode configuration for better gain stability.
    • Reduces distortion in audio or sensor signals.
    • Examples: HCNR201, IL300.

6. PhotoDarlington Optocouplers

Combines a phototransistor with a Darlington amplifier stage for higher current amplification.


  • Advantages:
    • High current gain (up to 10,000x) allows direct driving of relays or solenoids.
    • Suitable for low-speed, high-load applications.

7. PhotoMOS (Solid-State Relay) Optocouplers

Uses an LED and a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) for solid-state switching.


  • Benefits:
    • No mechanical contacts, enabling silent operation and longer lifespan.
    • Fast switching (nanosecond range) and low power consumption.
    • Ideal for low-voltage DC applications (e.g., automotive, robotics).

8. Photodiode Optocouplers

Output stage is a photodiode, often used with an external amplifier for high-speed or sensitive signal detection.


  • Applications:
    • Fiber optic communication systems.
    • High-frequency signal isolation in telecommunications.

9. Infrared (IR) Optocouplers

Transmit signals via infrared light, the most common type in general-purpose optocouplers.


  • Variations:
    • IR LED + phototransistor (standard design).
    • IR LED + photodiode (for high-speed applications).

10. Specialized Optocouplers

  • High-Temperature Optocouplers: Rated for operation in extreme temperatures (e.g., automotive engines, industrial furnaces).
  • Surface-Mount Device (SMD) Optocouplers: Compact design for modern PCB assemblies.
  • Dual/Quad Optocouplers: Integrated packages with multiple optocouplers for space-saving in complex circuits.

Key Parameters to Consider When Choosing Optocouplers

  • Isolation Voltage: The maximum voltage the optocoupler can withstand between input and output.
  • Transfer Ratio (CTR): The ratio of output current to input current, indicating gain.
  • Switching Speed: The time required for the optocoupler to turn on/off, critical for digital applications.
  • Input Current: The current needed to drive the LED (affects power consumption).
  • Temperature Range: Operational temperature limits for the component.


Optocouplers play a vital role in electrical safety and signal integrity across industries, from consumer electronics to heavy industrial machinery. Choosing the right type depends on factors like signal type (analog/digital), voltage requirements, and switching speed.
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