SIMPLE ZERO CROSSING DETECTOR USING OPTO COUPLER

Zero crossing detectors basically detect zero voltage points and inform the controller or controller circuit. It helps to minimize high rate change of current with respect to time (dI/dt) as result less heating and start up current in the load which improves life time of load such as motors.
Circuit 


WORKING



Fig - INPUT AC(230V RMS) , BRIDGE RECTIFIER OUTPUT( DC) AND OUTPUT OF OPTO COUPLER
  • bridge rectifier converts ac into  dc
  • Output of bridge rectifier is fed to opto-coupler
  • Led inside the optocoupler requires minimum of 1V to turn on
  • when ac wave goes near to the zero crossing line  ,ie below 1V led will turn off
  • as a result output transistor will turn of and pulled up  to 5v  
Design

Resistor design (High voltage side)
  • From above V-I characteristic of opto coupler led (from datasheet of MCT2E) requires 2mA  current at 2V.
  • INPUT AC = 230V (RMS voltage) 
  • Peak voltage of ac =√2 *  RMS voltage =325
  • Voltage across Current limiting resistor =325-2 =323V (led takes 2V)
  • R = V÷I  = 323÷ (2mA) = 161.5KΩ

take Near standard value of 180 KΩ


collector resistor
this resistor just for pull up the output.it require only small current of 0.3mA.
R=v/i
 =25/0.3mA
 =83KΩ
Take near value of 100kΩ

Advantages
  • low cost
  • transformerless
  • isolated