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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!

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    \$\begingroup\$ You can crush something only so hard before it breaks. \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 5 '13 at 3:00
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Those input power and voltages are rated input power and voltages.

For example you can drive electric motors over rated power but they will get too hot and eventually break. There are also electric motor duty classifications for industry. See this for short introduction: http://www.electricalengineering-book.com/duties-of-induction-motors.html

In practice this means that one same machine could be used for two different applications. For constant non-stop usage rated power can be e.g 100 W but then for cyclic usage where motor stops e.g for 5 minutes and then drives for 2 minutes the rated power can be e.g. 130 W.

This was just an illustrative example from industry machines but I have not checked how big difference in power output there actually is between these two types.

Back to this case: Peltier element's rated input power in this case is that around 96 W. You can also use it with lower power. For example you could attach a sensor system that measures temperature of cooled object and then the input power is adjusted by control circuit to adjust for example voltage given to the element. Since peltier is a semiconductor device it is likely more prone to break with over-rated power even for short times. I do not recommend trying that.

The rated numbers for that element can be based on theory and then it is tested properly to be sure that it works under that load for long time enough to be sold for consumers.

Also shortly about fundamental theory:

  • Voltage (potential difference between two planes, nodes... etc.) produces electrical current -> resistance limits current -> power is consumed to that resistance to get over it. Refer to Kirchhoff 1st and 2nd and Joule's law.

In practice you can buy cheap multimeter, small battery and a small resistive lamp, couple of resistors and see with measurements when you change resistance in that circuit and see in practice in brightness of the lamp. This is brilliant way of getting started in practice.

Remember to stay safe while measuring and do not measure any high power device voltages or currents if YOU are not familiar with the theory of electric laws! Small 9 V alkaline batteries are safe enough but things get much more dangerous even with 12 V car batteries if you don't know what you are doing!

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In general, when you are talking about components which performs work (motors, heaters, servos, etc..) you will use the "power notation". This is useful because these components dissipate a lot of power, which should be supplied to them from power supplies of various kinds (otherwise they won't work). It is also useful because this power is (usually) dissipated as heat, therefore knowing the power rating you can estimate the amount of heat produced.

However, each electric device (even those that characterized by their power rating) has a range of proper operational conditions. Proper operational conditions will be stated in terms of voltage/current/temperature etc... Thus, for example, if your fan is specified at 20W power consumption, but its minimal operating voltage is 10V - it won't work properly if you try to drive it with a supply that is capable of supplying a current of, say, 10A, but does not go above 5V (50W supply).

The reasons why the above example (driving 20W fan with 50W supply) won't work may be numerous, but you may think in terms of transistors - the transistor (MOSFET for example) requires some voltage level in order to conduct current. If this voltage constraint is not satisfied, the transisor won't open and the current won't flow into your fan. Therefore your 50W capable supply won't force any current into the fan and no power will be dissipated.

NOTE: some devices have very wide ranges of operating conditions (spiral heaters, which are simple resistors, are good example of such devices). Others have very sever restrictions and may be permanently damaged by even slightest exception over their maximal/minimal ratings.

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It is not essential to understand the theoretical properties of how materials work in thermo-electric effects to use them. The same applies to PN junctions, electric motors and thermistors. But each of these examples has limits to prevent stress, damage, and early failures by stress near certain limits for each parameter such as voltage, current, power, temperature or even rate of change of these parameters or nmber of cycles.

However unless you study the physics of the properties of these materials to understand how they work to appreciate how they can fail and why there are limits, just respect that there are limits to use when applying these units to an application.

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