The MOC303XM and MOC304XM devices consist of a AlGaAs infrared emitting diode optically coupled to a monolithic silicon detector performing the function of a zero voltage crossing bilateral triac driver. MOC3041. They are designed for use with a triac in the interface of logic systems to equipment powered from 115 VAC lines such as teletypewriters
Zero-Crossing-Triac-Driver-Optocoupler-MOC3041 - schematic

, CTRs, solid-state relays, industrial controls, printers, motors, solenoids and consumer appliances, etc. These were used by me to make solid state relays. Look at the circuits here Power Electronics Circuits. The were used to control Heaters from DC control signal from a Proportional Temperature Controller. The Magnetic Contactors can be avoided for lower powers. At higher powers SCR banks become a bit expensive. The SSR has no mechanical wear and tear but heat up at high currents. So if we overrate by 5 Times then it is an excellent solution for lower power controls upto 5 Kilo Watts. The cycle time of Contactors must be 20-60 Seconds to make it last long. In a SSR a one second cycle time is possible as 10-20mS is the switching speed due to zero crossing.

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