Triac optocoupler problem

[Larry80]

Hello,

I have been wondering about specific triac circuit schematic, with random trigger (MOC 3021 etc.) triac optocoupler. This schematic is from the generic datasheet (attached) and widely adopted it seems.

What is the purpose of the capacitor, i have not been able to find any specific information? Its somehow connected to the Igt requirement. Problem is that this type of circuit connection creates very high current pulse (1.5A) trough the 180ohm resistor when triac is turned on at the peak wave as the charged capacitor is rapidly discharged to the gate of the triac, plus it will dissipate about 1W of power vs. 0.5W that would occur otherwise (reduce footprint/BOM by wide margin) of this particular resistor. This seems to also create unnecessary EM radiation too..

Edit: I found a Fairchild AN–780A which discusses the selection. Purpose is snubbing the optotriac, there must be a better solution.

[Larry80]

I did modify the circuit as pictured. The main problem for increasing the resistance is the full duty-cycle turn on time. The increased delay is about 60uS and 15 Volts, when 1.2k resistor is used. This is also why you also cannot use the original circuit with both resistances at 1.2k (series resistance being 2.4k) as the only source for energy at 0 angle has to come from the line, not from the capacitor.

Noticable reduction of audible switching noise and power levels at the resistor went to 1/4W from the original 1W. I guess the EM noise is also reduced a lot, but have not yet measured it.