Basically working on a project, still in the conceptualizing stage, where I need to output power to 3 devices, 12v 2A, 12v 2A and 5v 2A, ideally off one ac adapter. I was thinking of using a dc to dc step down converter for the 5v 2A device, that way I could power everything off of one ac adaptor. What size voltage/amp ac adaptor would I need to pull this off? I'm not sure exactly what it should be taking into account the input of the dc to dc step down converter. Thanks
Your two 12V @ 2A loads will add together to 4A total.
For the 5V @ 2A load that is (5V * 2A) = 10W of power. If we assume that your DC-DC converter is on the order 70% efficient at converting energy then the input energy to the DC-DC converter would be (10W / 0.70) = 14.2W. Converting that back to a current delivered at 12V yields (14.2W / 12V) = 1.19A.
Based upon this then the total current requirement at 12V is (2A + 2A + 1.19A) = 5.19A.
It is always advisable to rate the source power supply at a bit higher rating than needed so that it is not working at full load. This can extend its life and reliability due to not having to operate at the limit of its design margins. An often recommended margin is to operate at about 80% of rating. Using this factor the rating of the power supply you want is going to be (5.19A / 0.80) = 6.48A. Rounding up to even numbers you would want an AC adapter that can supply 12V @ 6.5A or greater.