Zero crossing detector: do I really need the 7W resistor

Posted on Feb 6, 2014

Provide zero crossing detection to my controller. Using the schematic below, I managed to generate square wave signal, representing positive v/s negative half-period. The problem is that the resistor needs to be quite large : Quite the heater. Also, voltage rating above 400V is required. I realize that a transformer could be used to step down the mains voltage. However, those guys tend to be

Zero crossing detector: do I really need the 7W resistor
Click here to download the full size of the above Circuit.

bulky and rather expensive. Any recomendations I found some nice explanations on Hardware deign and noise suppression on the site. stevenvh`s answer looks especially promising on calculating the resistor value. So let`s keep my question more general. What are the approaches - resistor, transformer, others. What characteristics are desired in the optocoupler Is the chosen driving current (30mA) huge and unnecessary

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