Ohm’s Law, that is… Periodically questions appear on the Forum regarding the wiring of Peltier units, fans as well as other basic electrical subjects. Because of this I felt that a short explanation of one of the most basic laws of electricity is called for.
An analogy to clarify a few terms before we get down to business:
Electricity and wiring can be loosely compared to water and plumbing.
The wire is similar to the pipe; the bigger the pipe, the more water it can carry without rupturing (melting). As the pipe gets smaller, it’s resistance to flow increases. Resistance (measured in Ohms) increases in a smaller wire as well.
Voltage (measured in volts) is similar to pressure at the faucet – without pressure, it is hard to get much flow.
The water itself is related to Current, measured in Amperes (amps for short), or in mA – milliamps, which are equivalent to 1/1000th of an Amp)
It gets a little more complicated when we get to the subject of Power. Power is measured in watts. Small applications may draw small amounts and use Milliwatts or even Microwatts (1/1000th and 1/1000000th of a watt). Other applications may use large amounts and measure them in Kilowatts and Megawatts (1KiloWatt=1000 Watts, and a Megawatt=1000000 Watts).
However you label it, it’s still just Watts. The complications come about because the term Watt is used to describe 2 different circumstances, and understanding the difference can be confusing since they are actually interrelated at a higher level than we will attempt here.
The Watt is used as a measure of capacity, in the sense of power distribution. Your computer has a 250 Watt power supply. That means that it has the ability to provide POWER to a LOAD of up to 250 Watts.
In the water analogy, the power supply is the faucet. When it is opened up all the way you are at maximum flow capacity.
If we split the pipe into two pipes with faucets (loads), we obviously cannot open both and get full flow.
The other instance where the unit Watts is used is in discussing dissipation. As a load utilizes power, it dissipates it in a number of forms – the most common being heat. A light bulb uses electricity to heat a wire to incandescence where it gives off light; energy that can not be converted into light is given off as heat. Every time a current passes through a resistance it loses a little power in the form of heat.
This applies in all electronic devices, including your CPU. The losses incurred within your CPU are the reason for your oversized heatsink and fan. Your CPU consists of many millions of transistors which are switching on and off at very high rates. All of these tiny “switches” represent a load to the power supply, and draw current from it. In spite of each device drawing a miniscule amount of power, with millions operating at once, the current demand is high.
As we just learned, the relationship of Current and Power is directly proportional – when Current goes up, so does Power.
Now – to the fun part!
Ohm’s Law in its simplest form states that Voltage (in Volts) equals Current (in Amps) times Resistance (in Ohms). This shows the mathematical relationship of these entities and makes it possible to determine the value of any one, when we know the value of the other two. The traditional method of remembering this law is through a visual aid:
I = Current (in Amps)
R = Resistance (in Ohms)
By covering up the value that we want to calculate, this diagram shows the necessary math to calculate its value using the remaining 2. If we know the Voltage and the Resistance, we know to divide the Voltage by the Resistance to determine the Current. If we know the Current and the Resistance, we must multiply them together to find the Voltage.
If we have a 12 Volt fan that draws .175 Amps, to determine the fan’s resistance we must divide 12 by .175 or 68 Ohms.
I realize that the application of this information is not going to be a common occurrence for the average reader, so I won’t belabor the point. However, it is required in any dissertation of Ohm’s Law, and besides I HAD TO DO IT when I learned all this stuff so many years ago. Besides, there will be a quiz on this material later.
Now – a variant that may prove a little handier to the average Overclocker.
We return to our old friend Power. Since Power is mathematically related to Voltage and Current, Mr. Ohm carried his basic law another step.
I = Current (in Amps)
E = Voltage (in Volts)
The same relationship applies for our trusty visual aid; cover the one we seek to see the relationship of the other two.
We know that our 68 Ohm fan draws .175 Amp at 12 Volts, because we did the math a while ago. How much of an impact is that fan going to have on our 250-Watt power supply when we improve our case ventilation?
12 Volts times .175 Amp equals 2.1 Watts – in other words, not much affect at all unless we are running many other Loads (HDD’s, CD-ROMs, lights, bells, whistles, gongs, etc).
In the lamp above the computer on your desk, you have a 60 Watt light bulb. Since in America we use 120 Volt AC power for most applications, we can calculate the amount of current we are drawing from the wall socket in the same way.
60 Watts divided by 120 Volts equals .5 Amp
I hope that this was helpful to at least a small degree.