3,5 ide drive powered via floppy connector on mobo(a1200)

[_yak_]

Yes, there were some kind of Amiga 1200 HD kits that included cables to draw the power from floppy power connector (they allowed to connect the power to the floppy at the same time of course). If you don't plan to have some BPPC there then a standard A1200 PSU (which is weaker than A500 PSU btw.) should be enough.

[_yak_]

@_ThEcRoW

Small molex (floppy) power connector (CABLE SIDE):
http://pinouts.ru/data/SmallPower_pinout.shtml

Big molex (hdd) power connector (CABLE SIDE):
http://pinouts.ru/data/BigPower_pinout.shtml

If you have a multimeter, be sure to double-check everything before power up.
Good luck!