I needed a way of controlling fans. I don't like the noise, and when the computer is idling the fans don't really need to be working at full pelt. Lately more fans come with a "4-pin" setup, allowing PWM speed control, but so far I've only seen it for CPU fans, plus my fans (and motherboard) don't support it. As such I set out to build some fan controllers, starting from the simplest single fan controller onwards to a (currently theoretical concept) a fan controller capable of controlling 32 fans independently.
Pulse width modulation (PWM) is good, but it would not have worked for computer fans. Computer fans do not appear as induction motors, because they are brushless. The brushless controller expects a clean stable voltage source to function properly. Sending it pulse width modulated signals will just cause problems (This is why they had to add another pin for speed control, those fans have a variable speed brushless controller). So for this to work, we need to use the resistive method, which is less efficient, but does the job well and is simple to implement.
Here are my projects, from simplest to most complex:
This is a simple single fan controller, it has three voltage steps 5V, 9V and 12V and makes use of three mosfets and three resistors. Diagram below.
As you can see, very simple! I didn't specify the values for the resistances because they vary based on the current draw of the fan. You will need to calculate it yourself!
Luckily for you, I wrote a program that will calculate this for you, you can find it here
This is my fan:
As you can see, my fan needs 0.32A of current at 12V. In our circuit the fan acts like one side of the potential divider. Now we need to decide how much voltage we want over the fan.
If we want 5V over the fan, we need to get rid of 12V-5V = 7V
V = IR
V / I = R
7 / 0.32 = 21.875Ohms
So in order to get 5V over the fan, we need a 22ohm E24 resistor capable of dissipating 2W of power (I2R)
For 9V over the fan, we need 12-9V= 3V
3 / 0.32 = 9.375
For this we will need a 10ohm E24 resistor. again capable of dissipating 1W of power.
This is all well and good, but I don't have any resistors capable of dumping 1W/2W of power, so I am going to think of another way of doing this, using what I already have of parts.