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What Are Optocouplers?


An optocoupler, also known as an opto-isolator, is a type of electronic semiconductor component that is capable of transferring electrical signals between independent circuits with light as the medium. Commonly found within low voltage and highly noise sensitive circuits, the optocoupler serves to isolate circuitry in order to prevent undesirable noise and electrical collision. In this blog, we will provide a brief overview of the optocoupler, allowing you to better understand its functionality and benefits to circuitry.

How Optocoupler works?

In order for the optocoupler to function as intended, it relies on two main components which are an LED and a photosensitive transistor device. The LED is the component that creates and emits infrared light, while the photosensitive device is charged with detecting light. Placed together within black box housing, pins serve for connectivity between the internal parts. When a signal is to be transferred across optocouplers, an input circuit will receive the incoming AC or DC signal for actuating the LED.

As the LED switches on, infrared light will be produced, detected by the photosensor. The infrared light that is emitted by the LED will be proportional to the current that is traversing the device, and the light will collide with the photosensor and activate it due to a current being conducted. Once the current being supplied to the LED is cut off, the light will cease to emit. Depending upon the output circuit, the output signal from the photosensor may come in the form of AC or DC current.

Due to a current controlling the LED and the LED directly governing the photosensor, an optocoupler may be relied on for the means of controlling two independent circuits. The spaces situated between LEDs and photosensitive transistors also are non-conductive and hollow, meaning that the two circuits are electrically isolated from one another. Generally, the hollow space may be achieved through the use of glass, air, or transparent plastic. Depending on one’s needs and requirements, there are four main types of optocouplers, each of which differ in their switching capabilities and characteristics.

Phototransistor Optocoupler

The phototransistor optocoupler is quite common, often coming with a PNP or NPN transistor. In some instances, a phototransistor optocoupler will feature six pins, one of which extends from the transistor base in order to control the device’s sensitivity. With the use of a ground or negative connection, false triggering can be avoided. Generally, such optocoupler types are used within DC circuit isolation applications.

The photo-darlington transistor optocoupler is another common type, utilizing a darlington transistor which is a two transistor pair. In such assemblies, one transistor is used to control the other transistor base. This permits high gain ability, benefiting various applications and systems. Similar to many other optocoupler types, the photo-darlington transistor utilizes an LED that emits infrared light for the means of managing the base of the transistor pair. Similar to the phototransistor optocoupler, the photo-darlington transistor is often implemented for DC circuit isolation, and a sixth pin extends from the component for the control of sensitivity.

Photo-TRIAC optocoupler

The photo-TRIAC optocoupler is the third primary optocoupler type, often used for AC based control and switching. In such assemblies, the LED is managed through the use of DC voltage, and the TRIAC is then relied on for managing AC voltage. Photo-TRIAC optocouplers excel in circuit isolation, featuring low to medium speed. Generally, the current transfer ratio of such variations is very high, using the TRIAC as its sensing device.

Photo-SCR based optocoupler

The photo-SCR based optocoupler is the final common variation, utilizing a silicon-controlled rectifier as its sensor type. Similar to the other optocoupler variations, the intensity of the LED will determine the output signal, and the SCR is managed by the LED. Generally, the Photo-SCR based optocoupler is used for AC circuitry, providing low to medium speed with a current transfer ratio of 100% or more.

Whether you need optocouplers, microcontrollers, transistors, or other various electronic components, ASAP Semiconductor has everything you are searching for at competitive prices. Take the time to explore our robust part catalog at your leisure, and our team is always available for customers to request quotes for their comparisons. Due to our unwavering efforts in upholding rigorous quality assurance measures and export compliance standards, we proudly conduct business with AS9120B, ISO 9001:2015, and FAA AC 0056B certification and accreditation. Initiate the purchasing process when you are ready and see why customers steadily choose ASAP Semiconductor for all their operational requirements.


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