# Best way to drop 12V fan to 7-9V? [closed]

So my Xbox one PSU died and microsoft doesn't sell official ones yet, so I bought a third party one and the fan that is in it is a 12V blower fan with 2 wires running right to the board (it says 12V right on the fan). So obviously one of the wires is ground and the other is 12V. So I need help determining what would be the best method to lower the voltage to around 7-9V.

(The reason I'm trying to drop it is because this thing is extremely loud and pretty much unbearable, it runs cold all the time so I do not think heat will be an issue here, if it is I'll buy a new PSU)

So here is a photo of the set up: Album with pictures in it

How do you think I should go about doing this (resistors, diodes, I have no problem installing or trying anything, I was thinking a 25 ohms resistor with a rating of at least 5 watts).

• Then fan is loud because the xbox likely drives the fan with PWM. Driving the device with its max operating voltage ensures it goes as fast as possible 100% of the time - this will greatly reduce the life of the fan. By running the fan at a lower voltage you will effectively be accomplishing a similar feat. – sherrellbc Jun 13 '14 at 18:27
• Can't add an answer as it's been closed, but you could simply try connecting the fan between the +12v and +5V rails, giving it a 7V supply. – John Jun 13 '14 at 21:47

You could just put a bunch of cheap 1N4001 diodes in series until you get what you want in terms of speed. About 5 should do it, since each would drop about 800mV when warmed up and conducting 150mA-ish. More like 1W total dissipation than 5W. Heating will make the fan go a bit faster, but not much. Less tendency to stall than with a resistor.

Note: You're obviously asking for trouble- high voltage stuff in there that can be hazardous- if you compromise isolation the game could be dangerous to users, fire hazard, reducing the cooling could cause a very early failure, and a lot of the aftermarket stuff doesn't have proper overvoltage protection, so you could kill your game box. So I don't recommend you do it.

• Why I chose this answer - It gave helpful insight and heeded a warning along with it, thank you – Cup of Java Jun 13 '14 at 18:43

At 12V 0.2 amps, you have 60 ohms equivalent motor resistance at speed (Ohm's law).

Using voltage division:

$$V_{out} = V_{in} \frac{R_1}{R_1+R_2}$$ $$\frac{V_{out}}{V_{in} \cdot R_1} = \frac{1}{R_1+R_2}$$ $$R_1 + R_2 = \frac{V_{in} \cdot R_1}{V_{out}}$$ $$R_2 = \frac{V_{in} \cdot R_1}{V_{out}} - R_1$$ $$R_2 = R_1 \cdot (\frac{V_{in}}{V_{out}}-1)$$ $$R_2 = 60(\frac{12}{8} - 1) = 60 \cdot 0.5 = 30 \Omega$$

$$30 \Omega \cdot (0.2 A)^2 = 1.2 W$$

A 1.5 or 2 watt resistor will suffice if it is 30 ohms. You can go bigger if you like, but it won't do much more.

Make sure your fan still spins well though. If it doesn't you could be drawing more current than you want. In that case, up the voltage some. I would find a high power Potentiometer to test this out with. Or perhaps get a couple different resistance values to try out what's the optimal amount of fan cooling vs sound.

• So would 10 - 100 ohm resistors do the trick if they are all rated at 2 watts if I tested each individually – Cup of Java Jun 13 '14 at 2:50
• @user3010773 Yes, that would work. – horta Jun 13 '14 at 3:38

Another option would be to place a rheostat. It's like a potentiometer, but simpler in that it's a variable resistor. I have done exactly what you are doing with some pc fans that were a bit too loud. At least this way, you can always adjust the fan speed if your xbox gets too hot.