Overview: This article explores optocouplers, which are important for electrically isolating circuits and enabling signal transmission. It details their working principles, types, advantages, and common applications, highlighting their significance.

Optocouplers are essential components in electronic design, offering a reliable signal isolation and transmission method.

What is an optocoupler?

Optocouplers are also referred to as optoisolators, optical isolators, or photocouplers. They are electronic components that use light to transfer electrical signals between two isolated circuits. These versatile devices play an important role in modern electronics by providing galvanic isolation while allowing signal transmission. It is a circuit or component that optically couples the signal of one circuit to the other circuit. 

The signal from the low-voltage circuit can be coupled to the high-voltage circuit using an optocoupler. By guaranteeing electrical isolation between two circuits, it can also be used to shield low-voltage circuits from other circuits that are vulnerable to high-voltage spikes and noise.

Construction of an Optocoupler

The core components of an optocoupler, as shown in Fig. 1, are:

  • An infrared LED (IR LED) on the input side. It is made up of gallium arsenide.
  • A photosensitive device on the output side, such as a phototransistor, photodiode, or photoresistor, detects the light emitted from the LEDs

The LED and photodetector are tightly enclosed in a single package to optimize the coupling between the circuits. Their spectral response and wavelength are also closely matched.

Fig. 1 Diagrammatic representation of optocoupler. Source: Rakesh kumar

Working Principle

When an electrical signal is applied to the input, the LED emits light. This light is detected by the photosensitive component on the output side, which then generates a corresponding electrical signal. The photodetector acts like a closed switch, allowing for current flow. The current flow through the external load or external device from the photodetector. The two sides are optically coupled but electrically isolated, usually separated by a transparent insulating barrier. 

Without an electrical signal on the input side, the LED will remain in the OFF condition. The photodetector acts as an open circuit; no photocurrent will be generated on the detector side. So, in this way, the optocoupler helps to couple the signal from one circuit to the other optically.

Types

Based on the photosensitive element, the optocoupler can be of different types

Phototransistor-Based Optocoupler: The input can be analog or digital signals. It can used in the saturation region for the digital input signal, while in linear or active regions for analog input signals.

Photoresistor (LDR) Optocoupler: Also known as resistive optoisolator or Vactrol. Uses a photoresistor (Light Dependent Resistor) as the detector with a slower response time but can handle AC and DC signals.

Photo Darlington Transistor Optocoupler: A Darlington pair transistor is used as the photodetector.

Photodiode Optocoupler: A photodiode is used as the detector. It is capable of high-speed switching.

Photo TRIAC Optocoupler: Uses a TRIAC as the photodetector. It is designed for AC applications and solid-state relays.

Photo SCR Optocoupler: A silicon-controlled rectifier (SCR) is used as the detector.

Logic Gate Output Optocoupler: Provides a digital interface with a built-in Schmitt trigger.

Based on the requirement and application, different photosensitive detectors can be utilized. For instance, photo triac and photo SCR are utilized in AC applications. High-speed optocouplers are used for fast-switching applications, and the optocoupler with the logic output can be used in applications where the digital output is required.

Linear Optocoupler

Generated photocurrent has a non-linear relation with LED optical power, and the phototransistor shows non-linearity in the output. Linear optocouplers are available to overcome this challenge in applications that require a high degree of linearity. This linear optocoupler is made up of two phototransistors. The photocurrent generated in the reference phototransistor acts as a feedback signal to drive the LED. Thus, the generated photocurrent becomes linearly proportional to the LED optical power.

Advantages

  • It provides electrical and high voltage isolation between input and output circuits, typically ranging from 2.5 kV to 5 kV.
  • Protects low-voltage circuits from noise and voltage spikes. By breaking the electrical path, optocouplers significantly reduce noise transfer between circuits.
  • They prevent ground loops, which can cause signal distortion and system instability.
  • Optocouplers can interface between circuits operating at different voltage levels.
  • It is extensively used in communication systems, power supplies, solid-state relays, etc.
  • It controls high-voltage circuits using digital logic or a microcontroller.

Optocouplers to Consider

PC817 and 4N25 are well-known general-purpose optocoupler ICs. In these optocoupler ICs, the photosensitive element is the phototransistor.

PC817

The PC817, as shown in Fig. 2, is a general-purpose optocoupler consisting of an infrared LED optically coupled to a phototransistor. 

Fig. 2: PC817 Source: oemsecrets

Specification:

  • 4-pin DIP package
  • Forward voltage: 1.25 V
  • Forward current: 50 mA
  • Maximum isolation voltage: 5 kV
  • Switching characteristics: Rise and fall time: 18 μs
  • Current transfer ratio: CTR: MIN. 50% at IF = 5 mA, VCE = 5 V

The PC817 is widely used in various electronic applications for its simplicity, reliability, and effective electrical isolation capabilities. Common applications include computer terminals, system appliances, measuring instruments, registers, copiers, automatic vending machines, and electric home appliances, etc.

4N25

The 4N25, as shown in Fig. 3, is a popular optocoupler device. Contains a gallium arsenide infrared LED optically coupled to a silicon NPN phototransistor.

Fig. 3: 4N25 Source: oemsecrets

Specification

  • 6-pin DIP package
  • Forward voltage: 1.5 V
  • Forward current: 60 mA
  • Isolation test voltage: 5000 VRMS
  • Switching characteristics: Rise and fall time: 2 μs
  • Current transfer ratio: CTR: Minimum 20% at IF = 10 mA, VCE = 10 V

The 4N25 is widely used in various electronic applications for providing optical isolation between circuits, especially where voltage surge protection is needed. Common applications include AC mains detection, reed relay driving, switch mode power supply feedback, telephone ring detection, logic ground isolation, etc.

To conclude, optocouplers continue to be essential components in electronic design, offering a reliable signal isolation and transmission method. As technology advances, newer variants like digital optocouplers and silicon dioxide-based isolators are emerging, promising even better performance and reliability in high-voltage applications.

Summarizing the Key Points

  • Optocouplers provide electrical isolation between circuits, protecting low-voltage components from high-voltage spikes and noise and ensuring safe operation in various electronic applications.
  • Different types of optocouplers, such as phototransistor, photodiode, and photo SCR, are available for specific applications, including fast-switching and AC signal control.
  • The construction of an optocoupler involves an infrared LED and a photosensitive device, which work together to convert electrical signals into light and back, maintaining electrical isolation.

Reference

Lakkireddy, Gopal Reddy, and Sudha Ellison Mathe. “A Strategy for Measuring Voltage, Current and Temperature of a Battery Using Linear Optocouplers.” World Electric Vehicle Journal 13, no. 12 (November 24, 2022): 225.
https://doi.org/10.3390/wevj13120225

oemsecrets. “PC817: - Compare Distributor Prices & Inventory | oemsecrets.Com.” oemsecrets.com, n.d. https://www.oemsecrets.com/compare/PC817%3A

oemsecrets. “4N25 - Compare Distributor Prices & Inventory | oemsecrets.Com.” oemsecrets.com, n.d. https://www.oemsecrets.com/compare/4N25