There are different ways to look at it. Recently someone posted his view here, but I can't find it.
My personal view is to compare electricity with water flowing through a pipe:
The water represents the electrons and thus the current. The more electrons move the higher is the current.
The voltage is the driving force which exerts a pressure on the electrons to move. The higher the pressure, the more electrons move, thus the current rises.
The pipe is the conductor. The smaller the diameter of the pipe, the less water flows. translated: small pipe = thin wire = high resistance
Now if you keep the pipe (resistance) constant, but increase the pressure (voltage), the water flow (current) increases.
OR
If you keep the pressure (voltage) constant, but increase the pipe's diameter (lower resistance), more water (current) flows.
The power is the product of water flow and pressure: if a lot of water flows without much pressure (thick pipe), you need little power at the pump (battery) to drive the flow.
If you increase the resistance (small pipe) you need more power at the pump to drive the same flow.
Now, as we''ve had before her many times: the power rating of the supply says nothing about the power that is actually used by the load. Come back to the pump and water example: A pump can deliver a certain amount of water (current) while keeping a certain pressure (voltage). But that doesn't necessarily mean that it outputs all that power at any time. Imagine your household tap:
If the tap is closed, there is pressure in the pipes, but no water flow. Thus the power delivered is actually zero.
If you open the tap, water starts to flow and powre is delivered to you (you can use that power e.g. to operate a lawn sprinkler. But the more water you draw, the lower the pressure becomes because the pump can only deliver a limited amount of power. Have you ever noticed the drop in waterflow if someone in the house e.g. flushes the toilet while you were standing under a shower?
But: it is not like the pump spitting out water constantly at full power and if you don't use it, the surplus water is drained. What a waste!
It is the same with an electric power supply. The rating tells you the limits. But how much power (amps) you actually draw from the supply depends on the load. If you put a light load (high resistance) to the supply, only little curent will flow. If you put a high load to the supply, a high current will flow.
All as long as you stay within the ratings of the supply. If you start drawing more power from the supply, different things can happen depending on the design of the supply.
Comming back to the pump examle: The power supply doesn't pump all available current into the output and discharges everything you don't need. Waste, too. Instead it pumps only so many electrons (current) into the output as is required to keep the voltage constant.
When you look at the ratings of a power source and a load, make sure the source can deliver as much or more power than the load requires. They don't have to match 1:1. Just avoid having a higher load rating than source rating.
I hope that helps.
Harald
