Jose,

The problem with say just a resistor or a constant current source is that worst case it has to dissipate around 4 watts I=20mA, Vrms = 220, W=I*R=0.02*220 = 4.4W . Now, we might be able to use an opto with a led current of 5mA, this would decrease the max dissipation to 1.1W - this is getting to be a bit more manageable. If we were using a TO220 package transistor, it would be running at about 66degreesC over ambient, which is manageable, but not really too good if we want to run at a 50degreeC ambient. We could put a heatsink ,say a little one rated at 28C/w, so we'd be at 30C over ambient, so at 50C ambient the transistor would be 80C which is manageable. But I don't want to use a heatsink - it would use too much space. So it seems we have to waste the power by other means.