Separated powers and high power switches? Alternatives to relais?
But the relays which I use use too heavy coils for the arduino to pull off.


If you are connecting relays directly to Arduino output pins you might destroy your Arduino! You can use a power MOS-FET to directly drive the relay coil like this:


Schematic by Jeff Sampson.

Instead of motor you connect the relay coil.

To separate Arduino from power line, use an optoisolator like this:


Schematic by Lewis Loflin.

Usually optoisolators are used to separate low from high voltages. They have nothing to do with the current. The MOS-FET handles the current.

There is no need for relays as long as your MOS-FET supports the 40 A current. Just connect the load in series with its drain-source pins.