Yeh its possible to run em off a rheostat. I will be runing two 92mm fans off rheostats myself. Here is what I used as a guide when I first got started.
I hope you don't mind, but I was just searching for this same infor myself, except that I want to run three off the same rheostat. I think you need to post the fan info to get a better answer. For instance: My fans are all matching and are rated as 12v(8-16v) 3,5W 2800U/min according to the print on the hub. I don't know if watts are added, or volts, or whatever. A real electronic newb. The fans will be run parallel off of the 12v line (17a). 6 fans total, three per rheostat. What rating of rheostat am I looking at? I would really like to run them all off of the same rheostat, but am not sure if one rheostat can handle the load? Again, not trying to hijack, but figure why make two threads when a simple explanation can probably help us both.
To figure the amount of power the rheostat has to handle you can use the formula P=I x V or power equals the current multiplied by the voltage that will be dropped by the rheostat. If your motor requires 3.5 watts of power @ 12 volts then the current draw is about 0.3 amps per motor. I assume you do not want to make the fans turn off all the way but just run at a slower speed. Let's say a speed that will draw .1 amps less current out of the .3 amps that each motor draws. You have 3 motors so adding the current that will be needed for the 3 motors while they are cut back is .6 amps, instead of the full 0.9 amps with no restriction. Since you are dropping the power of each motor by a third you will need a resistor or pot that will handle about 3.5 watts. Dividing the power by the current, 3.5/.6 = 5.8 volts that will be dropped across the pot. Since R=E/I or R=5.8/.6 your resistance needed for the pot will be about 10 ohms. Using a 5 watt pot at 10 ohms should make things work ok.
The fan resistance is, using Cougar4's formula, about 41 ohms, so 3 in parallel will be 13.7 ohm, and a 10 ohm rheostat will do nicely to turn them down to 6.9V. Rheostats are rated in watts, but it's amps that burn them out, usually when they're turned up 99% so taking near full fan current on a single turn of the wire winding. Most specs will show the maximum current for each model, which depends on the resistance value and wattage rating. Maximum current will be about 870mA when they're at virtually full speed, and the bit of rheostat wire in use has to stand this without overheating, so W = I2R = 7.6W for a 10R rheostat @ 870mA, even if only 0.1 ohms of wire are in use actually producing 0.076W of heat - the heat is highly concentrated so higher temperature. With wire-wound rheostats you can overload about 25% as the cold unused wire takes some heat from the used turns, but with 5W never leave them "almost full on". Turning down so the fans get 9V reduces the current to about 570mA and a 5W rheostat will cope easily.
Oh, God, my head. I was almost grasping what Cougar4 posted then Cpemma slapped me around with the amps and the possibility that Im going to burn out the Rheostat leaving it at full. Maybe it would help if I just explained what I was doing. This case will have a total of 6 120mm fans on it...three in front, three in rear. I would like the ability to turn them from 12v to 7v with a single dial. I thought I could do so with a Rheostat but now it appears that I'm going to burn out the Rheostat if left on at 12v to long? Ok, I've got my dunce hat on, lets work on this. Lets say all 6 fans are going to be on one rheostat. Working with the numbers cougar4 put forth, I come up with a 4.84 ohm resistor being needed. This doesn't make sense to me since it took a 10 ohm resistor with only 3 fans. More fans means it takes less resistance to achieve a greater affect? Or am I just totally off? EDIT: ReKarp? You there? Are you learning anything from this? What are the figures for your fans?
Your figure's not far off. The more fans in parallel, the lower the overall resistance, and the rough aim is for the rheostat resistance to equal the fannage load resistance, which gives a turndown range of 6-12V. You said the fans were 3.5W, 3.5/12 = 0.29A per fan, 6 fans=1.75A, load resistance = 12/1.75 = 6.86 ohms. A 5 ohm rheostat will give a turndown range 6.9-12V, which should be OK. The rheostat has to carry 1.75A max, so a 5 ohm model should be (1.75)2 x 5 = 15.3W, or 12W without any safety margin. Rheostats capable of running several fans are expensive (and may be too big to fit a bay panel), a semiconductor rheobus would be much cheaper.
i decided to build a rheobus using the guide on the site. It ended up alot cheaper than a huge rheostat and it works great. This thread has taught me alot about current, volt, ect.
Ok, thanks for all the info. My ears are still smoking, but I'll survive. I think I may go with the Digibus guide for this project...its pretty well layed out.
Yeah that kinda confused me a little bit... <- minor understatement radioshack sells a 25ohm 3watt rheostat, should that be fine for most single 120mm fans? I have quite a few el cheapo fanbusses (but surprisingly nice, rated like 17w or something and they were like $2.50 each) lying around but I really only have one fan to control and.. umm... I don't want the extra knobs lying around That or I'll need to find other 12v stuff to control... maybe lighting.
Using a transistor to control the speed is a much better way to go I think. This will be a good project to learn some electronics. Remember...Ohms Law must prevail! E=I x R P=I x E
