I am building a remote monitor for a multi-zone heating system. The signals I want to monitor are standard 24VAC that feed the zone valve motors. Looking around, it seems that most people are rectifying the 24VAC and using a resistor to limit the current feeding a standard opto. My issue with this is that the resistor will drop ((24*1.4)-1.3)*0.01 = .33W (assuming 10mA current thru the opto and 1.3V forward drop for a 4N35). Having 8 of these means will dump 2.6W when all are on, plus the board space for all the parts.
Is there any reason not to use a capacitive dropper? I figure I need a reactance of ((241.4)-1.3)/.01 = 3300 ohms. C would then be 1/(2pi603300) = 0.8uF. Does this seem sound?
I will add a cap across the output of the opto to smooth out the zero crossings so I will get a good logic level when its on, and will likely use an H11AA1 opto with back-to-back LEDs to avoid needing a diode to conduct around the reverse biased one in the opto during negative cycles.
Thanks.
-PB