Controllic Triacs (using optocouplers)

Hi all!

I am educated within eletronics, but that is 10 years ago. I have been working on a simple controller, which should turn on/off a 12VAC/1A (10W) line. For that I use an optocoupler and and a TRIAC. (before anyone asks - the 12VAC is not my call). It has to open 5 (1-9) times per second.

I found this:

(from:

My german is rusty too, so here it goes:

I simply have a 1K resistor instead of those 2 and that is all. Could anyone explain me the idea of: a) the RC on the right? b) the 47nF condensator? Noise reduction?

Next, I have a question regarding the optocouplers: Currently I use IL420 (IL4208) from Infineon. Using that it works.

But S21ME4 from Sharp does not - it turns slightly on. Then again, the datasheets (from

says "For triggering medium/high power triac". As much as I read about it, the S21ME4 is cheaper, and may be able to drive the line alone, though I am not sure. It is also ~4 times cheaper than IL420. Maybe the ME3 (without zerocross detector may work?) I am wondering whether it does not work @ 12V.

But using a IL420 and BTA06 triac it works.

I have a solution, I just want to know why it works as it does :-)

BR Sonnich Jensen