There are many circumstances in a circuit or other electronic device where signals must be sent between physically isolated circuits. Although there are several solutions that may accomplish this, semiconductor technology has proven to be optimal for its size and operational lifespan. One such technology that has found use in high-speed multichannel circuits, serial interfaces, and logic analyzers, is the optocoupler. As more industries continue to adopt optocouplers, it is critical to understand the basics of their design and operating principles. In this blog, we will discuss everything you need to know about optocouplers, highlighting their importance in high-performance circuitry.
Optocouplers achieve signal isolation by converting electrical input into a near-infrared light, which is then transmitted and received by another element on the circuit. This process can be broken down into three unique steps: signal conversion, transduction, and reception. The creation of the near-infrared light is accomplished by the emitter which is commonly a LED. After the light signal is emitted, it travels through a dielectric channel, which is either transparent in the case of air or insulating when epoxy or similar material is used. A photosensor then acts as a receiver, generating electric energy or controlling another part of the circuit. Although modules are typically unidirectional, some configurations implement LEDs capable of bidirectional operation.
Optocouplers help ensure high-fidelity input and output-switching even in a noisy environment. They may also be installed on PC architecture, where they help run and control programs based on LED modulation. Below, we will now explore the various types of optocouplers that may be found in a consumer environment.
Most often, a particular optocoupler design is chosen based on the characteristics it displays when at rest. This particular optocoupler implements phototransistor components as receivers and naturally remains off until stimulated by the light signal. They may be uncoupled and not bound to any external resistors if there is a low chance of false signaling or paired with a resistor to enhance stability. Like most other models, this phototransistor type is nearly exclusively used for DC circuits.
TRIAC components are not unique to optocouplers, but when the two are paired, superior bidirectional voltage and current control are achieved. TRIAC-type optocouplers are also beneficial because they can be used to control AC systems using a DC source.
Similar to the TRIAC design, photo-SCR optocouplers make use of thyristors to control AC systems. They can take on a much higher voltage capacity, and their output can be controlled by modulating the intensity of the LED which is accomplished by adding transistors to the emitter side.
Photo-Darlington Transistor Optocouplers
Photo-Darlington optocouplers are used primarily to support current isolation in low-frequency applications. Their unique feature is a pair of bipolar transistors that provide current amplification, helping to increase gain. However, this design is more likely to experience phase shift and instability if exposed to high frequencies. Photo-Darlington models often employ a sensitivity-controlling pin and should only be used to facilitate DC circuit isolation.
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