Solid State Relay/ Optocoupler Relay
Solid State Relay is a type of switch to activate Load by using without any Mechanical parts. In this there is only semiconductor devices used, no Mechanical coil relay is used to Switching the Connected Load.
Triac is used at the place of coil relay to switch ON and OFF. So this circuit is some different from the general relay circuit that is activated by transistor but here Triac is worked as Transistor as well as Relay.
Why use Optocoupler
Optocoupler is a component that is used to activate any device by using Light Glow, This glowing occurs internally in IC. In An optocoupler, an LED and a Light-activated SCR (Opto Triac) or Light-activated transistor is placed Together in the proper way to be sensed with high gain. When we give supply to optocoupler the internal Optical SCR is turned on by the light get from the LED. Internally That LED and transistor/SCR are Isolated (Not Physically Connected) from each other. This Isolation between both provides us high protection from AC supply. This protection from AC by isolation is a big reason to use Optocoupler. There are many types of the optocoupler, in which used with a fully DC circuit and some are used with an AC circuit. When a DC supply is given to the optocoupler input pins, then the internal LED glow and activates the Internal triac. The MOC3021 is Optotriac IC
Triac is a semiconductor device of silicon controlled rectifier (SCR) and works as the transistor but both are different in construction. Transistor is used to switching in DC supply but Triac is used to switching in any AC supply. Triac is widely used in AC switching applications like dimmer, voltage controller, fan regulator, etc. When A small supply is applied to the Gate terminal of Triac then it is ON and current flows through the other 2 terminals (MT1 and MT2).
This circuit is not suitable for Load More than 250W
NOTE – This circuit is dangerous for beginners so if you not fully confident to handle 230v AC, Please don’t check yourself