Thanks for the explanation. I think I'm starting to get it.

Then I get this question: do I really need a relay and connecting the heatbed to the power supply if I don't care about speeding up the heating process?
Is there any reason not to connect directly to the motherboard as the current silicone heatpad is if that is not an issue?
I don't know what overload scenario you mentioned there can be.

here's the heatbed: http://www.geeetech.com/mk2b-heated-...del-p-802.html
here's the motherboard: http://reprap.org/wiki/Printrboard

Again I'm not very experienced with electronics so sorry if this is a very simple question.