Question Which wire should I connect resistor to?

Jul 18, 2019
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Hello. My laptop fan is running full speed at 2000rpm all the time because some sensor is broken or something. I want to solder a 0.25W resistor, but I'm not really sure which wire should I solder to. The red or the blue one? And how much quieter will the fan be? This is the fan (HP DV7-6000):
 
Are you sure this works? You may be right but my thinking (not tested) is you will still get the same rpm but draw more current from the motherboard.
Even PWM mode works on 12v, if you lower the voltage it just will not be able to produce enough power to turn full speed. Besides, if header and/or controller doesn't work it will not be able to even try to adjust speed.
 
Reactions: TJ Hooker
Are you sure this works? You may be right but my thinking (not tested) is you will still get the same rpm but draw more current from the motherboard.
There's no way you're increasing current by adding resistance. By putting the resistor in series it'll lower the effective operating voltage of the fan, so it'd be like running the fan off a 9 V (or whatever) rail instead of 12V. Speed and current will be reduced.
 
Reactions: CountMike
Watt is the measure of work done in this case fan, so W(att) = A(mper) times V(olts), Resistor lowers the Voltage so Watts are lowered too. Less Watts is produced, less power will motor have and air resistance will slow it down. Simple laws of physics learned in elementary school.
 
Reactions: TJ Hooker
Lol, my primary school was not so advanced! I feel like we only got into basic physics, and maybe some introductory electrical stuff like ohm's law, in junior high at the earliest :p
Just like Newton's law of gravity applies to everything from an apple falling of a tree to space rockets so Ohm's law applies to everything from simple light-bulb to super computers, worth knowing because it explains many things.
 
Watt is the measure of work done in this case fan, so W(att) = A(mper) times V(olts), Resistor lowers the Voltage so Watts are lowered too. Less Watts is produced, less power will motor have and air resistance will slow it down. Simple laws of physics learned in elementary school.
Yes this is basic but this is missing the point I was making which maybe I explained poorly. From what I read these motors are not DC but are multi phase and their rpm is controlled by switching frequencies and not directly correlated to voltage. Now a question I have is if this is correct, I’m not sure. However if the switching frequency determines the rpm and we operate within the voltage limits of the motor then if the rpm/work/watts does stay constant, say 1000 rpm regardless of 9v or 12v using the same formula you have increased the current.

Now this theory is all based on that idea that the rpm is not directly controlled by voltage, this may be wrong.
 
Motor itself is a brushless, polyphase permanent magnet (ring in fan hub), AC motor with DC-AC inverter built in. Up to 12vDC is supplied thru red wire and negative for it is the black one next to it.
Usually blue wire, sends tachometer signal to MB so you can see RPM. PWM fans/motors also have a simple transistor switch inside which turns off power to motor in various lengths and frequency from controller on the MB. Those pulses are adjusted according to tachometer reading to keep fan working at desired speed. For such motor/fan it's enough to just supply 12vDC power to red and black wire and it will work at it's full rated speed which is also determined by inverter's frequency. If you lower the voltage (by resistor for instance) it will loose some power and spin slower.
 
Yes this is basic but this is missing the point I was making which maybe I explained poorly. From what I read these motors are not DC but are multi phase and their rpm is controlled by switching frequencies and not directly correlated to voltage. Now a question I have is if this is correct, I’m not sure. However if the switching frequency determines the rpm and we operate within the voltage limits of the motor then if the rpm/work/watts does stay constant, say 1000 rpm regardless of 9v or 12v using the same formula you have increased the current.

Now this theory is all based on that idea that the rpm is not directly controlled by voltage, this may be wrong.
The fan speed is a function of both the supply voltage (i.e. 12V) and the PWM signal duty cycle. A 4 pin fan would usually have all speed control done via PWM. But given that the PWM control appears to be broken in the OP's case, the fan essentially becomes a 3 pin fan. So speed is controlled via voltage.
 

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