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

zero crossing

I am trying to 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 :
CTR > 10%
Max diode current: 60ma.
I choose 30 ma working diode current. This yields 30ma * 230V = 6,9W for R1.

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 bulky and rather expensive. Any recomendations?

Edit:
I found some nice explanations onHardware 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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Best Answer

  1. Consider replacing the optocoupler chosen, with one that is designed for direct interface to a mains power line. For example, Fairchild Semiconductors MID400 is one such part.
  2. Use a much lower LED current. While your opto may specify a fairly large (20mA to 100 mA, even 1 A in some cases) drive current for the LED, many will work fine at as little as 1 to 2 mA: The part's transfer curve should provide enough information to confirm this.
  3. A capacitor can be used as an impedance element for dropping the input AC voltage, instead of the resistor. As the voltage drop is almost entirely pure reactive, there is hardly any thermal dissipation to be concerned about. You do however require "X" rating capacitors, designed for use in AC mains circuits.
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