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Fans and resistors; a short how to...

post #1 of 2
Thread Starter 
Are your case or radiator fans too fast or too noisy?

Do you not like or not wish to run fan profiles for some reason?

Are you thinking of getting a fan controller but not sure if things will all work out?

Well, most of us know that you can run fans off of 5v or 7v derived from a molex plug but how about if you want to run at 9 or 10volts?
Here is a solution for just such an instance…Add a resistor to the fan circuit.
By calculating the correct size of resistor you can try out various voltages and see the effect it has on your rig and if you add a switch in parallel to the resistor you can still run full speed whenever you want.

How to calculate…

Original fan voltage: 12V

Original fan current: 0.2A (if you can, use a meter to read fan current with the fan mounted in it’s normal place in your rig, failing that use the figure on the fan sticker)

V = I * R (Ohm's Law), so 12 = 0.2 * R therefore R = 12 / 0.2 = 60

Fan resistance: 60 Ohms

Target voltage: 9V

V = I * R (Ohm's Law), so 9 = 60 * I therefore I = 9 / 60 = 0.15

Target current: 0.15Amps

Voltage drop across resistor: 12V - 9V = 3V

V = I * R (Ohm's Law), so 3 = 0.15 * R therefore R = 3/ .0.15 = 20

Target resistance: 20 Ohms

Power dissipated by resistor: 3V * 0.15A = 0.45W

Cheap or what ? in this instance just pop in to your local electronics supplier and get a 20ohms 0.5w (or bigger, say, 1watt) resistor ....job done.
post #2 of 2
cool, i´ve been looking around how to do something like this, i am really noob at this, i just started studying electronics, i am on my 2nd semester, just one question, how do you wire a resistor to a ac adapter? i am trying to connect an ac adapter to a dc fan,
-the ac adapter outputs dc at 9v at 210ma and
-the fan works at 12v at 1.6w..about at 0.134ma

so i did some calculations and i should use a 30ohms resistor, i only have 33ohms resistor, we just started the electronics labs and clases about a week ago, and i just can´t wait for the next class to use my solder iron, so my question is, how do you wire the ac adapter to the resistor? and then how do you wire that to the fan?, any help would be much appreciated, thx in advance
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