A Look at Optocouplers

An optocoupler, also known as an opto-isolator, photocoupler, or optical isolator, is a type of electronic component that uses light to transfer electrical signals between two isolated circuits. Optocouplers prevent high voltages from harming the system receiving the signal. Commercially available optocouplers can withstand input-to-output voltages as high as 10 kV as well as voltage transients with speeds up to 25 kV per microsecond. The most common type of optocouplers feature an LED and phototransistor in the same housing. Other types include LED-photodiodes, LED-LASCR, and lamp-photoresistor packages. Opto-isolators typically transfer digital on/off signals, though some techniques allow them to be used with analog signals.

An optocoupler contains three main parts: 1.) a light source, known as an emitter, usually near an infrared LED, that converts electrical input into signal light. 2.) a closed optical channel, and 3.) a photo sensor that detects incoming light and either generates electrical energy directly or modulates electric current from an external power supply. This sensor can be a photoresistor, photodiode, phototransistor, silicon-controlled rectifier, or a triac. Because LEDs are able to sense light as well as emit it, the construction of symmetrical, bidirectional opto-isolators is possible. In an optocoupler solid-state relay, there is a photodiode opto-isolator which drives a power switch, usually a pair of MOSFETs. A slotted optical switch contains a sensor and light source, but the optical channel of these devices is open, allowing for modulation of light by external objects blocking the path of light or reflecting into the sensor.

Electronic equipment and signal transmission lines can experience surges caused by lightning, electrostatic discharge, radio frequency transmissions, switching pulses (spikes) and perturbations in power supply. For example, a lightning strike can cause a surge of up to 10 kV, which is 1000x more than the voltage limits of many electronic components. Circuits can also incorporate high voltages by design, in which case it needs safe, reliable means of interfacing its high-voltage components with low-voltage components.

The four main types of optocouplers are resistive optocouplers, photodiode optocouplers, phototransistor optocouplers, and bidirectional opto-isolators. There are also many different types of optocoupler configurations. Most commonly, optocouplers have a closed pair configuration. This refers to optocouplers enclosed in a dark container where the light source and sensor are facing each other. Another configuration, the slotted coupler/interrupter configuration, refers to optocouplers with an open slot between the light source and sensor that is able to influence incoming signals.

Additionally, some optocouplers have a reflective pair configuration. In this configuration, the optocoupler contains a light source that emits light and a sensor that only detects light that has been reflected off an object. This particular configuration is ideal for the development of tachometers, movement detectors, and reflectance monitors. These configurations are also sometimes known as opto-sensors. The main function of an optocoupler is to block high voltages and voltage transients so a surge in one area of a system does not disrupt or damage other parts of the system. Historically, this function was carried out by isolation transformers, but advancements in optocouplers have made them a more ideal means for this.

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