Salokin thinks that the 30 watt power supply will over-power his little 3-watt PLC. He is afraid that the PLC will not be able to withstand so much power! He would be happier with a 2 watt power supply. That way there could be no danger, right?
Don't worry. Your PLC will draw only as much watts as it needs (as long as you don't have a short-circuit). You probably should put in a 1/3 to 1/2 Amp fuse between your Power Supply + and your PLC + terminal, just in case you do create a short circuit somehow.