Optocoupler also known as opto-isolators is a system of an IR (Infrared) LED isolated together with a light sensitive switching semiconductor use to control input and output circuits without any physical connection by means of light. An optocoupler is made up of an LED which emits infrared light and a photo sensitive semiconductor components like photo transistors, and then packed into a tiny subtrate of semiconductor wafer enclosed in a tiny case with terminals for connecting to PCBs.
Optocouplers are also a type of integrated circuits, since the components are embedded into a tiny piece of silicon or germanium wafer. A photo sensitive components like photo diodes, light dependent resistors, photo TRIAC, photo SCR and photo transistors are use to detect the light emitted from the LED. Mostly, phototransistors are used, because of their sensitivity.
Optocouplers circuit symbol.
Optocouplers functions just like a relay switch, the only difference between them is that the relay switch controls a circuit by means of electromagnetism generated by current from the controlling circuit and then use it to switch of or on the controlled circuit. But an optocoupler uses light produced from a controlling circuit like an Arduino or microcontroller to switch on or off the controlled circuit without any physical connection. So an electrical isolation between the controlling circuit and the controlled circuit of 10kv can be attained.
Types of optocouplers.
There different types of optocouplers, but they have a unique characteristics which is controlling circuits with no physical connection. They differs in the in the type of photo sensors and the type of current they can control.
Optocouplers with photo transistors, photo diodes, photo resistors or photo darlington to LED pair are use to control DC circuits while photo TRIACs and photo SCR to LED pairs are use to control large AC voltage.
Uses of optocouplers in circuits.
Optocouplers are use in PCs, phone chargers, AC-DC circuits and integrated circuits for controlling. It can be use to transfer digital and analog signals and are very useful in electrically noisy environments.
•As a DC switch.
Photodiodes and phototransistors optocouplers are use to switch on and off DC circuits. As shown in the schematics above the 5VDC is connected to the circuit along with a 240ohms resistor to alter the sensitivity of the optotransistor and a diode to avoid reverse polarity connection of the battery. The sensitivity of the optocoupler depends on the applied current which then brighten up the LED. The photo transistor receives the light ray and then trigger current to flow in the output circuit. The capacitor is use to prevent unwanted spikes or transient voltage from false triggering the base of the photo transistor.
📌Note: The input voltage of the optocoupler shouldn't be more than 5v 500mA. Anything above this will end it to getting burnt.
•AC circuit control.
This type of optocoupler configuration forms the basis of a very simple solid state relay application which can be used to control any AC mains powered load such as lamps, motors and chargers. Unlike a thyristor (SCR), a triac is capable of conducting in both halves of the mains AC cycle with zero-crossing detection allowing the load to receive full power without the heavy inrush currents when switching inductive loads.
Tags:
Power Electronics