I started to learn some basic idea on circuit building last semester. I got some chance to build up some simple circuit to construct some cooling system (with fan and some other IC). I thought I understand the concept of voltage, current and power but when in practical application I think I still not fully understand them. For example, I am building a circuit to drive the computer fan of 20w. There is a label on the fan stating that it should be driven as 12V. I know that power is voltage*current, so if we need 20w and drive it with 12V, I think we need about 1.6A. I made the circuit and it works. But today I am reading some books for beginner in circuit construction, in one project, they said you only have to provide 20w to drive one specific model of fan. So does it mean we can choose different combination of voltage and current if only we have the power is 20w?
Yesterday, I was working something with the peltier pad for cooling some stuff. It is said in the manual that we need to provide 96w (need 20V and about 4.8A) to drive the peltier and it comes with a power supplier, which provides 20V and about 5A output. It works pretty good when I connect the peltier to the power supplier. But I am pretty curious why in the manual state the specific working voltage and current if it needs 96W. I tried to use a DC power supplier with variable voltage and current output. I tried to make the voltage 5V and make the current about 1.5A, the peltier cooler still works (though not as efficient as with the typical input). Then I try to make the voltage pretty higher and with very low current so that the power is still about 96W. It is still working pretty well. So it is confusing me why most devices need to label the suggested voltage and/or current as well as the input power? But for some other devices, they only states the input power and voltage, they different mentioned any thing about the current!