For 5 fans you may need quite a large rheostat. Following is a cut from Cliff's 'fanbus.com'. For more info go to fanbus.com, click on 'The Lab' and at the bottom of the page ' rheostat tutorial'.
First off, rheostats (or rheostat-like devices) allow you to infinitely vary the speed of a fan between it's maximum speed and some arbitrary minimum speed determined by the fan's specifications. In order to accomplish this, a rheostat acts as a variable resistor. In other words, the rheostat is essentially a valve for electrical current, impeding the electrons from reaching the fan or device. The excess voltage is usually dissipated in the form of heat. The terms rheostat and potentiometer refer to the same type of device, differing mostly in terms of arrangement of the terminals.
Regardless of the name you give them, a variable resistor is what you get. Since this business of reducing a fan's spin rate is what's pertinent, we care mostly about two factors: Capacity and Ohm rating.
The capacity of a rheostat is given in Watts. This number tells you how much juice the device will take before some type of failure is imminent. Since most rheostats or pots are constructed using a wiper that contacts a semiconducting trace of some sort, the interface between the wiper and substrate becomes the weakest link. This is an inherent reason why most rheostats and pots are relatively low wattage devices. In order to find out how much wattage your 12 volt fan draws or produces, multiply the voltage (Volts) times the current draw (Amps).
Watts = Volts x Amps
Example: Watts = 12 Volts x 500mA x (1Amp/1000mA)
Watts = 6
In this example, a 12 volt fan draws 500 milliamps. Since the formula requires Amps, we make the conversion to Amps before multiplying. I point this out since most fans are labeled in milliamps (mA) rather than Amps. To simplify further, just hang a decimal point in front of mA to get the corresponding Amps for the calculation.
Don't try to confuse yourself with overcomplicating things. Yes, a fan that is hooked up to a rheostat won't be receiving the full 12 volts, so the wattage at lower voltages will be less, but this is putting the cart before the horse. Use the full 12 volts when determining the wattage of your fan.
Once you have determined the wattage or power of the fan, you are halfway there. The next step is determining just how much resistance to add to the circuit to give you the range of reduction you require. This is not quite so straightforward, but it can be accomplished.
In general, each fan will have three voltages associated with it: A starting voltage, a sustain voltage and a full voltage (or rated voltage). Each fan differs somewhat in these values, and can usually be ascertained from the manufacturer. The starting voltage is the minimum voltage required to get the fan starting initially from a dead stop. The sustain voltage is the minimum voltage required to keep the fan spinning at it's slowest rate. Note that the sustain voltage will always be less than the starting voltage , since the starting voltage must overcome the inertia and static friction of the motor's bearings. The full voltage or rated voltage is the full 12 volts in virtually every computer case application. Most fans will run at higher voltages, at the cost of longevity.
Ideally, you would like the rheostat to control the fan between its maximum voltage (high) and its sustain voltage (low). In other words, you would like to be able to use the full range of the rheostat to control the fan. It is not imperative that the fan be able to start itself when the rheostat is turned fully down, since we can turn the fan up to start it and then turn it down all the way after it is spinning. If we limit the rheostat to the starting voltage at minimum turn, we are not getting the full range of RPM's out of the fan and rheostat.
A rheostat, as discussed earlier, is simply a variable resistor. It will deliver a full amount of voltage (almost no resistance) at maximum turn, and some smaller voltage at minimum turn depending on the Ohm Rating of the rheostat. The voltage delivered across a resistor is based upon Ohm's Law.
Ohm's Law: Volts = Amps x Ohms
In other words, the amount of voltage delivered across a circuit is proportional to the current draw (in Amps) of the device and the resistance encountered (in Ohms).
If we rearrange this equation using some simple algebra, we can say that
Ohms = Volts / Amps
What we really care about is not the starting voltage and ending voltage, but the Voltage Drop encountered between the two. In order to figure out what kind of resistance value we need, we need to consider the amount the voltage drops, and what kind of resistance is required to make this happen. So again rewriting the equation, we get:
Ohms = (VoltageMax - VoltageMin) / Amps
This is the "magic formula" that so many of you have asked me for. You may recognize this as the same equation used for figuring the resistor needed for an LED. Same concept, really. Here we go:
Let's say that the manufacturer of the fan I used as an example earlier states that his fan draws 500mA, and the sustain voltage is 7 volts. So plugging into the equation above we get:
Ohms = (12Volts - 7Volts)/.5Amps
Ohms = 10
There you have it. The voltage drop is what we are concerned with, and a resistance value of 10 Ohms should give us that voltage drop. In this example, with this fictitious fan, a rheostat that is rated for 10 Ohms and 6 Watts would give us the range we need at the power requirement for reliability.
Logic goes away if it is not the first or seemingly obvious answer to a problem.
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