Wawa:
Both correct.Common 5volt sugarcube relays draw about 75-80mA.
From memory, the 12volt types are about 35mA.
There is a datasheet for those blue relays online.
Leo..
You are awsome Leo. Thank you so much. So I will go with Opto board anyway as I will be powering AC main 220V and will isolate arduino from the relay board.
BTW, I have just thought of a case where someone will connect a power supply +v to JD-VCC and arduino 5V to VCC and BY MISTAKE, will also connect both JD-VCC and VCC pins (basically tying together arduino's 5V with a power supply's +V (it can be 5V or 12V or 24V). What happens in such case?
If I generalize the question: What happens in a case where you connect +5V or arduino to +V of a power supply? (let's assume grounds of both are tied together)