Hi,
Good that you have asked this question, by the way there is a mistake in the circuit, the rectifier diode should be 4x1N4003 or higher numbers of 4000 series to be able to handle 120 V AC peaks. I will see if I can reload the schematic with this correction.
Sorry for that. Now regarding the requirement of 10k,5W, you can see that when 120VAC is rectified, you get a peak voltage of about 170V (120*√2). Most of this voltage gets dropped across the collector base of the transistor and its parallel 10K resistor, which means, it has to handle a power of 170*170/10k, typically around 2.5W Watts. Even in this case, we have not considered the allowed variation of 15% on the commercial 120V supply.
Now regarding usage of 220V AC, you can find what all will have to change, with a rectified peak of 300V, you will need to use only 1N4004 and above as rectifiers, the tansistor should be changed for higher VCEO and the wattage of 10k to about 20W?
The real problem is in the range of input voltage, since the system has to work down to 12V DC. And all these, only to supply a load of around 10mW (5mA*2V), of opto power,what a colosal waste of power; the 'Greens' will have sufficient reason to revolt ! That is why I lamented in the previous post about my inability to locate a suitable switcher with such wide ranging inputs to supply this trivial load and to save the power.
Regards,
Laktronics