How Optocouplers Work: Electrical Isolation in Circuits Explained

In the realm of electronics and electrical engineering, maintaining isolation between different parts of a circuit is often crucial for safety, noise reduction, and proper functioning. Optocouplers, also known as optoisolators, play a vital role in achieving this isolation by using light to transmit signals across a physical barrier. This article delves into the workings of optocouplers, their applications, and the underlying principles that make them effective components in modern electronic designs.

Introduction to Optocouplers

An optocoupler is a component of optoelectronics and is used to transmit a signal between two galvanically isolated circuits. It usually consists of a light-emitting diode (LED) or laser diode (LD) as an optical transmitter and a photodiode or phototransistor as an optical receiver. The transmitter and receiver components are optically coupled from each other in a housing that is opaque from the outside.

Optocouplers can transmit both digital and analog signals. A distinction must be made between optocouplers and semiconductor relays, which can contain an optocoupler as a component for galvanic isolation and are primarily used in electrical power engineering. Optocouplers differ from fork couplers and photoelectric sensors because they have an externally opaque housing and were not built for optically scanning applications. Reflex couplers and reflex sensors also have an outward-facing transmitter and receiver and are used to detect nearby reflective surfaces.

Structure and Components

The optocoupler’s LED is typically the input side of the device. When current flows through the LED, it emits light. Positioned on the output side of the optocoupler, the photodetector responds to the light emitted by the LED. Depending on the type of optocoupler, this could be a phototransistor, photodiode, or another photosensitive device.

Usually made of transparent or translucent material, this barrier separates the LED and the photodetector. It ensures that electrical signals do not directly pass between the input and output sides, preventing electrical conduction and potential hazards.

Operational Principles

An electrical signal applied to the LED causes it to emit light. The intensity of this light corresponds to the amplitude of the input signal. The emitted light crosses the isolation barrier and reaches the photodetector on the output side. The photodetector detects the incoming light and generates an electrical signal proportional to the intensity of the received light. This signal replicates the input signal on the output side of the optocoupler.

Also Read: What Are the Different Types of Light Sensors?

Benefits of Optocouplers

Optocouplers offer several advantages in circuit design. They provide complete electrical isolation between input and output circuits, protecting sensitive components and reducing noise interference.

Optical transmission is less susceptible to electromagnetic interference (EMI) compared to direct electrical connections. They can facilitate signal transmission between circuits with different voltage levels without the need for additional components like level shifters.

Applications

Optocouplers find applications in various fields. In Switching Power Supplies, they are used for isolating control circuitry from high-voltage power circuits. In Digital Communications, they ensure data integrity and noise immunity in serial communication interfaces. They safely interface microcontrollers with high-power devices such as motors and relays.

Analog signal transmission:

Power supplies that are galvanically isolated from the mains, such as switched-mode power supplies for regulating the output voltage. The output voltage on the secondary side is measured, and the deviation of the output voltage from the setpoint, for example as a result of load changes, is transmitted via an optocoupler to the primary side, where the duty cycle or control frequency is changed so that the output voltage can be maintained at the setpoint. No requirement for linearity. For high linearity requirements, optocouplers are used with a second, preferably identical photodiode, which is located in the feedback circuit of the LED driver amplifier.

Digital signal transmission:

In the case of interface cards of computers, the circuits must be electrically separated from each other, as the devices connected to each other can have different ground potentials.

Assemblies that need to be protected from transient overvoltages and common-mode interference pulses often have optocoupling of their inputs and outputs. Examples are programmable logic controllers (PLC). Musical Instrument Digital Interface (MIDI) to avoid hum loops

There are numerous optocoupler ICs available from various manufacturers, each with specific features and specifications tailored to different application requirements. Here are a few popular optocoupler ICs:

4N25: A widely used optocoupler IC with a phototransistor output. It is suitable for general-purpose isolation and switching applications.

PC817: Another commonly used optocoupler IC with a phototransistor output. It is known for its reliability and versatility in various electronic circuits.

HCNR200: A precision optocoupler IC with an analog output, designed for applications requiring high linearity and accuracy.

TLP621: Optocoupler IC with a phototransistor output, suitable for high-speed switching and digital signal isolation applications.

ILQ74: Quad optocoupler IC, capable of isolating four independent channels. It is used in applications requiring isolation of multiple signals.

Types of Optocouplers

Either the transmitter and receiver are directly opposite each other (face-to-face design) or on the same level (coplanar design). Especially in the latter case, the light beam is transmitted by reflection, similar to the optical fiber.

Light-emitting diodes or laser diodes are used as transmitters, which operate in the optimal reception range of silicon-based receivers (around 850 nm wavelength). Phototransistors or photodiodes are used as receivers.

So-called PhotoMOS relays use a series connection of photodiodes, which are operated in photovoltaic mode like a solar cell, to switch with the voltage MOSFET; This allows small and large direct and alternating currents to be switched. Optocouplers can be designed as triacs and function as optotriac or phototriac, or they can be interconnected with triacs and thyristors. This results in a solid-state relay for switching mains AC voltage.

Depending on the application requirements, optocouplers come in different configurations:

• Phototransistor Optocouplers: Commonly used for switching and amplification applications.
• Photodiode Optocouplers: Ideal for high-speed data transmission due to their fast response times.
• Linear Optocouplers: Used when linear signal transmission is necessary, such as in analog circuits.

Conclusion

Optocouplers are indispensable components in modern electronics, enabling safe and reliable signal transmission across isolated sections of circuits. Their ability to provide electrical isolation while maintaining signal integrity makes them essential in applications ranging from industrial control systems to consumer electronics. Understanding their operational principles and selecting the right type for a specific application ensures efficient and secure circuit design, contributing to the overall performance and reliability of electronic devices and systems.