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.
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
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
this resistor just for pull up the output.it require only small current of 0.3mA.
Take near value of 100kΩ
- low cost