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 any 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 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 occur internally in IC. In optocoupler an LED and a Light activated SCR (Opto triac) or Light activated transistor is placed Together with proper way to sensed with high gain .When we give supply to optocoupler the internal Optical SCR is turn on by the light get from the LED. Internally That LED and and transistor/scr is Isolated (Not Connected) from each other . This Isolation between both provide us high protection from AC supply. This protection from AC by the isolation is the big reason to use Optocoupler. There are many types of optocoupler , in which some use with fully DC circuit and some use with AC circuit. When a DC supply given to the optocoupler input pins, then the internal LED glow and the activate the Internal triac. The MOC3021 is Optotriac IC
Triac is semiconductor device of silicon controlled rectifier (SCR) and works as the transistor but both are 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 application like dimmer,voltage controller, fan regulator etc. When A small supply is apply to Gate terminal of Triac then it is ON and current flow through other 2 terminal (MT1 and MT2) .
This circuit is not suitable for Load More than 250W