How Does an Optocoupler Work: Basic Concept Explained

How Does an Optocoupler Work: Basic Concept Explained

An optocoupler is an electronic device which consists of two components, an optically-coupled photosensitive transistor, or photodiode, and an optically-isolated amplifier. It allows electrical isolation between two circuits that are normally electrically coupled by using light emitting diodes (LED), infrared (IR) LED, or photo-LED/photo-SCR as the light source.

It provides the simplest way to transfer electrical signals between two circuits that are often at different ground potentials because it electrically isolates the input and output sides of the device, using no other components except an LED and a photodiode.

Moreover, an optocoupler is used for passing signals between circuit boards where the signal levels are too low for the open-collector outputs of integrated circuits (ICs) to safely interface, and where common-mode transient immunity, dv/dt is also required.

How does it work?

The optocoupler functions as a controlled switch: it transfers electrical pulses from one circuit to another with the LED and photodiode, which allows control signals or logic data to be transferred between two circuits while totally preventing passage of hazardous or noisy power-line high voltages.

First, a current is applied to the optocoupler, which causes the LED to emit light proportional to the current. When light hits the photodiode, it activates and starts conducting a current, just like any other diode.

In order to provide the highest sensitivity to LED light, the photosensitive device, by default, is typically left unconnected. Although, it can be connected to ground via an external resistor for more control over switching sensitivity.

When current ceases to flow through the LED, the photosensitive device ceases to conduct and turns off. All of this occurs through a void of glass, plastic, or air, with no electrical components between the LED or photosensitive device.

It also extends the capabilities of a device.This is mostly due to the fact that it provides galvanic isolation between two distinct circuits. In a two-circuit configuration, for example, we could add a transistor to the second circuit without affecting the first. You’d be able to control even more voltage and current this way. Furthermore, by incorporating electronic components, you may be able to automate the circuit control.

Types of Optocouplers

There are several different types of optocouplers: photo-iC, photo-transistor optocoupler, photo-Darlington optocoupler, photo-SCR optocoupler, photo-DIAC optocoupler, Photo-TRIAC optocoupler,Solid State Relay, and  gate drive optocoupler.

Additional features and functionalities can also be added to optocoupler to form,  for example smart gate drivers, that simplify gate drive design. A typical package of these devices include a comprehensive set of fail-safe IGBT and MOSFET diagnostics, protection, and fault reporting, making such gate drive optocouplers a complete cost-effective solution.

While other gate drive optocoupler packages are designed for space-constrained, high-voltage industrial applications like inverters and motor drives.

Is it similar to an isolator?

The term optocoupler is often used interchangeably with an isolator, but there is a distinction between the two. The differentiating factors are, the number layers or thickness of insulation available and the voltage difference expected between the input and the output.

The optocoupler is a device that allows transmission of digital or analog data between the circuits while maintaining electrical isolation of up to 7,500 volts. An isolator, on the other hand, is a device used to transmit analog or digital data between circuits where the potential difference can be lower than 5,000 volts.

In the event the isolation voltage is exceeded, the insulation barrier in the optocoupler will form an open circuit, which is a fail-safe condition, due to the higher number of insulation layers and thickness. Whereas, an isolator may form a short circuit, endangering the user, due to the small insulation thickness.


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