schematics and voltage drop

I wasn't sure where ask these questions since they are both projects and homework help. So I thought I would start here.

First I am looking for some good schematics all schematics used from electrical to hydraulic. I have found some but they are here and there and I was wondering if I could find them all in one place that would be real simple for me.

Next I was reading and thinking about my project and since it is mostly going to be powered by DC. I was trying to remember and figure out if I need more than one power supply or one be good enough. Lets say that I am running couple of servo control valves, PLC, relay and a temperature switch. If I have one DC power supply that is is 12 volts and 3 amps. I think i get the specs from one and calculate how much the voltage drops from one to the next right?
thanks for any advise and further help.

One power supply should be good enough. However, you need not worry about voltage drop. You will essentially be connecting the devices in parallel. You need to find out the current rating for each device and ensure you have a power supply that can supply enough current. As far as your first question, try google. Doing an image search turns up quite a few schematics. It helps to know exactly what you want a schematic of.