Solid State Relay (SSR) with Optocoupler and Triac

This solid state relay (SSR)  equivalent circuit is made of optocoupler and a triac. This SSR circuit is also designed with  LED protection from overvoltage and reverse polarity. The diode 1N4002 is the reverse polarity protector, as it will short the wrong polarity to null the voltage with the help of 150 Ohm series resistor.  The 2N4403 transistor will protect the LED from over-current caused by overvoltage at the input. When the over-current occurs at the LED, the sense resistor at the base of the transistor will develop a voltage which will be enough to activate the transistor, making the transistor conducting an bypassing the current through the transistor, saving the LED from over-current.  The  To handle load, triac should be chosen with proper current handling capability, you can oversize the triac twice of the maximum load specs for reliability. Here is the schematic diagram of the circuit:

[Circuit’s schematic diagram source: Motorola Application Notes]