AS9120B, ISO 9001:2015, and FAA AC 0056B ACCREDITED

What Is the Role of Optocouplers in Circuits?

Advances in semiconductor technology have helped propel the capabilities of electronic devices across a wide variety of industries. Their small size and dual conductor-insulator action allow for accurate and creative current management solutions. One such device that has been widely adopted is the optocoupler or opt isolator. These components, which are primarily found in noise-sensitive and low voltage applications, provide circuit isolation and noise reduction. In this blog, we will discuss the operating principle behind optocouplers, as well as their design and variants.

Optocouplers convert an electrical input signal into a near-infrared light, which is then sent to another circuit or another part of the circuit using a light-emitting diode (LED). The light then travels through a closed optical channel to a photosensor, which may then generate energy or control the current of an attached circuit. Although using LEDs is ubiquitous, the photosensor may consist of a phototransistor, photoresistor, or other related components. Additionally, since most LED elements also have a light-sensing capacity, some optocouplers may be bi-directional depending on the needs of the application. Furthermore, the optical channel between the light-emitting and light-sensing components may vary between air, plastic, or glass.

In order to gain a better understanding of how optocouplers are implemented in various circuits, it is essential to review the most common configurations available.

Phototransistor Optocoupler

These devices use phototransistor components to accept the light signal. In the default state, the phototransistor is set to the OFF position but will quickly transition to the ON position when stimulated. Depending on the application and the optical isolation medium, the phototransistor may be left uncoupled from resistors in order to maintain the highest sensitivity. However, if there is a high potential for false triggering by non-light stimuli, then an external resistor may be used to increase circuit stability. This configuration is most commonly employed on DC circuits.

Photo-Darlington Transistor Optocoupler

When low-frequency applications require current isolation, the photo-Darlington transistor configuration may be chosen. This setup also uses LEDs for signal transduction and a transistor on the receiving end. The primary difference is that the Darlington pair consists of two bipolar transistors that work together to provide current amplification. While such configurations may be helpful in providing high gain, Darlington pairs are much more susceptible to phase shift and instability at higher frequencies. Similar to the standard phototransistor design, the Darlington optocouplers contain a sensitivity-controlling pin and are to facilitate DC circuit isolation.

Photo-TRIAC Optocupler

Triode for alternating current (TRIAC) components allow for bidirectional voltage and current control. These thyristors, which may be found in numerous circuits and device configurations, providing excellent AC switching capabilities. The most common application in the case of optocouplers is a circuit containing a DC-driven LED and an AC system which must be controlled by the receiver.

Photo-SCR Optocouplers

Silicon-controlled rectifiers (SCRs) are another type of thyristor commonly installed in optocoupler devices. Photo-SCR optocouplers find use in applications requiring control of high-voltage AC systems. Uniquely, the SCR output is controlled through the intensity of the LED signal, which may be modulated through the use of transistors on the pre-signal side.


Owing to the several optocoupler configurations, there are many applications in which these devices may be used. Optocouplers aid in providing accurate input and output-switching in the presence of a noisy environment, since the optical channel between the sender and receiver is isolated. Optocouplers may also be installed on PC architecture, where they facilitate coded signals through transistor-directed LED modulation. As technology continues to advance, the range of optocoupler applications is also likely to increase to encompass smart computing and automation devices.


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