What Is An Optocoupler?

When working with electric circuits, one might find that two circuits that must remain isolated also need to pass signals to one another. In such circumstances, one should employ an optoelectronic coupler, or optocoupler. This six pin device is an electronic component that basically acts as an interface between the two separate circuits with different voltage levels. A popular device across a myriad of industries, the optocoupler is advantageous for many applications, so this blog will take a closer look at how an optocoupler works and some advantages of using one in your projects.

To understand the purpose of an optocoupler, it is first important for one to understand the meaning of the term isolated circuit. These are circuits which do not have a common conductor in between them, allowing for proper isolation to be maintained. In situations where the noise and additional distortions in the information signal from the first circuit would exceed the tolerance limits of the other circuit at the receiving end during transmission, it is required to use an optocoupler. Regardless of whether the circuits use AC voltages or DC voltages, an optocoupler can be used to maintain a steady current flow across both circuits without directly connecting the two through physical materials, such as wiring.

Instead, an optocoupler uses an infrared LED and a photosensitive device that detects the emitted infrared beam to generate communication between the two circuits. The first isolated circuit is placed at the input end of the optocoupler, that of which is attached to the LED light. Working with a resistance to limit current, high voltages pass through the input-end circuit and the current passes through the LED, causing it to emit light. The greater the current, the stronger the light emitted by the LED.

The light emitted by the LED is then received by the semiconductor photosensitive device, which can be a photodiode, phototransistor, a Darlington pair, SCR, or TRIAC. For DC circuits, photo-transistor and photo-Darlington devices are majorly used, while the rest are used more in AC circuits. This generates a directly proportional current flow through the second circuit while preventing coupling and protecting the circuits. Therefore, the low-intensity light emitted by the LED will cause a low-level current to flow through the phototransistor, while high-intensity light will cause high-level currents.

Despite their low operational speeds, optocouplers prove advantageous for many situations as they allow for easy interfacing and widened signal transmission. Furthermore, they are lightweight and work well in high power applications, those of which commonly include high power inverters and high power choppers. Optocouplers are also commonly used in AC to DC converters. A simple overview of an optocoupler highlights the six pins in order as an anode, a cathode, and a ground on the input side, as well as an emitter, a collector, and a base on the output side. The base terminal of the phototransistor is externally available. A single phototransistor is used at the output stage of a simple isolating optocoupler.

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