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Optocouplers, Photocouplers, and Optoisolators


Optocouplers, also known as photocouplers or optoisolators, are semiconductor devices that use a short optical patch or link to couple a signal from one electrical circuit to another while simultaneously providing electrical isolation. Photocouplers are typically contained in a single housing that is often similar size to an integrated circuit. Depending on the intended application, there can be a large degree of variation in size. This blog will discuss optocouplers and their functions in detail.

Optocouplers are able to carry out many functions: they can be used to link data across circuits, they can be used within optical encoders to provide a means of detecting visible edge transitions on an encoder wheel to detect position, and they can be used in many other circuits where optical links and transitions are needed. Optocouplers even form the essential element within solid state relays. In these, an optical coupling is used to isolate the input and output electrically, while allowing the output to switch based on the input state. Because of their versatility, optocouplers are used in a surprisingly high number of circuits.

There are two main components of an optocoupler: a light emitter and light detector. The light emitter, a light-emitting diode, is on the input side and takes the incoming signals and converts them into a light signal. The light detector detects the emitter’s light and converts it back into an electrical signal. The detector can be one of many different types of devices such as photodiodes, phototransistors, and others. The emitter and detector within an optocoupler are tailored such that they have matching wavelengths, allowing for maximum coupling.

The terms photocoupler, optocoupler and opto-isolator are often used interchangeably. Despite this, there are certain differences between optoisolators and optocouplers, the main one being the expected voltage between the input and output. Generally speaking, optocouplers are thought of as being used to transmit analog or digital information between circuits while maintaining electrical isolation at potentials up to 5,000 volts. Adversely, optoisolators are used in power systems and transmit analog or digital information between at potentials above 5,000 volts.

Optocouplers share many similarities with solid state relays. The main difference between the two is that optocouplers are used in low power applications, while solid state relays are used in much higher levels of power. Solid state relays are commonly used to switch voltage levels up to hundreds of volts and more, as well as switching current levels up to tens of amps and greater. Another difference is that optocouplers are contained within small IC packages either as surface mount devices or leaded semiconductor devices, while solid state relays are normally contained within much larger packages and often must be bolted to a heat sink.

The final difference is that solid state relays commonly contain additional circuitry. In fact, solid state relays are often a complete circuit block on their own. They may contain the drive circuitry for the LED in the opto-transmitter, as well as the surge and transient protection circuitry on the output. Because of their broad range of applications, optocouplers are more widely used than you might imagine. Whatever you need your optocouplers for, be sure you are getting them from a trusted source like ASAP Semiconductor.

For all types of optocoupler parts and much more, look no further than ASAP Semiconductor. We are an online distributor of aircraft parts as well as parts pertaining to the aerospace, civil aviation, defense, industrial, and electronics markets. We’re always available and ready to help you find all the parts and equipment you need, 24/7-365. For a quick and competitive quote, call us at +1(714) 705-4780 or email us at sales@asapsemi.com.


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April 5, 2021

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