# 4-pin PWM fans and speed modification



## pwnsweet (Jun 30, 2011)

Hi,

I have a laptop with a fan that has 3 speed levels. The speed level that the fan runs at is dependant entirely on the temperature of the CPU. When I turn the computer on the fan is completely off. As the temperature of the CPU rises, the 1st speed level kicks in. At this speed the fan is barely audible. If the computer is placed under load, the CPU temperature continues to rise and the 2nd and then eventually the 3rd speed level initiate. Both of these speed levels are loud and annoying. 

I have confirmed that the CPU will operate below the maximum temperature threshold under full load for a long period of time on just the 1st fan speed level. Therefore my goal is to modify the fan such that it is off by default and then spins up to the 1st speed level when the CPU begins to heat up under full load. I do not ever want the fan to initiate the 2nd or 3rd speed level.

I have done some research into accomplishing this goal, but my lack of knowledge of electronics is hindering my progress. Here's what I know so far:

*1.* The fan is a Pulse Width Modulation (PWM) fan and connects to the mainboard with 4 wires - red, black, green, blue. According to allpinouts.org these wires are for 12v (5v for this particular laptop fan model number), ground, sense and control respectively. 

*2.* The fan speed cannot be manipulated in any way in the laptop BIOS or in software. I have researched this extensively and it absolutely cannot be done.

In my attempt to try to understand what the Green and Blue wires are for, I have tested the behavior of the fan when one of these wires is disconnected and then again when both wires are disconnected. This is what I observed:

*1.* The fan runs at full speed (3rd level) when only the Red, Green and Black wires are connected.
*
2.* The fan runs at full speed when only the Red and Black wires are connected.

*3.* The fan alternates on/off continuously at a rate of once cycle per second when the Red, Blue and Black wires are connected.

I then did some more research and came upon some information that suggested that I may be able to achieve my goal by soldering a resistor in line with the sense (Green) wire. I suppose the theory is that by soldering a resistor in line with the sense wire, you effectively make the computer think it's cooler than it actually is, where the amount of resistance on the sense signal determines when the 1st speed level kicks in. In the link above the person has used a 17k Ohm resistor, however the closest I had was a 10k Ohm so I used that instead. The result was the same as above where the fan alternates on/off continuously at a rate of once cycle per second when only the Red, Blue and Black wires are connected. 

I then proceeded to do the same thing except this time soldering the resistor in line with the tach (Blue) wire. Unlike my experimentation with the Green wire above, I tried this with a 75ohm, 1k ohm and 10k ohm resistor. In all cases the temperature which the 1st speed level kicks in is the same as when no modification is present. Similarly, the temperature which the 2nd and 3rd speed levels kick in is also the same as when no modification is present.

So now I have given up. I have accepted that I really have no idea what the blue and green wires actually do and which one is responsible for controlling the speed of the fan. Therefore I am reaching out for further knowledge and recommendations from you. If you can please answer these questions and tell me what I need to do exactly to accomplish my goal I would be eternally greatful.


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## streetfighter 2 (Jun 30, 2011)

Welcome to TPU!

Here's the 4-pin PWM fan specification (for reference).

The green wire is "sense", which provides the fan's RPM with two pulses per revolution.

The blue wire is the "PWM signal", which is a rectangular wave oscillating at around 25kHz (25,000 pulses per second).  By varying this signal's duty cycle, the fan's speed is adjusted.

Unfortunately you've made the standard new poster error, you forgot to tell us the model of the laptop.  

Most PWM fans can be controlled using the BIOS or through software which can access the SMBus, such as SpeedFan.  You've stated that neither of these are possibilities so let's go to the hardware approach . . .

You have a few options here depending on the fan's specs.  The first thing that comes to mind is adding a resistor in series with the fan [red wire] to get the voltage across the fan down to about 7V (assuming it's a 12V fan).  In this case you'd just cut the blue PWM control wire because you're setting the fan speed using the resistor.  Here's an example of how some of that Ohm's law voodoo goes down:


> Originally Posted by *Alastair M. Robinson*
> 
> 
> _Original fan voltage: 12V
> ...


I've never tried this with a 5V fan.  If you give me the fans specs I'll do the math for you and you can test it yourself .  This also has the unfortunate side-effect of making the fan run all the time, albeit at a low speed.  You can try keeping the blue wire connected which in theory, would allow the mobo to regulate the fan but the maximum speed would be set by the voltage on the red wire.

Another option is to replace the fan with another fan that spins slower.  This is a bit of a pain in the back because it's a laptop fan and I'm willing to bet it's proprietary.

Yet another option is to build/buy your own PWM or linear regulator, but it might be tough stuffing it under the hood.


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## pwnsweet (Jul 17, 2011)

Thanks for the welcome. My apologies for not replying sooner but I have been very busy, even on weekends. I've lurked these forums for a while in the past (regarding videocard BIOS' and overclocking) but only just recently signed up to get some help with this problem. 



streetfighter 2 said:


> Here's the 4-pin PWM fan specification (for reference).
> 
> The green wire is "sense", which provides the fan's RPM with two pulses per revolution.
> 
> ...



Sorry,my laptop is actually a tablet PC and is the HP 2730p.



streetfighter 2 said:


> You have a few options here depending on the fan's specs.  The first thing that comes to mind is adding a resistor in series with the fan [red wire] to get the voltage across the fan down to about 7V (assuming it's a 12V fan).  In this case you'd just cut the blue PWM control wire because you're setting the fan speed using the resistor.  Here's an example of how some of that Ohm's law voodoo goes down:
> 
> I've never tried this with a 5V fan.  If you give me the fans specs I'll do the math for you and you can test it yourself .  This also has the unfortunate side-effect of making the fan run all the time, albeit at a low speed.  You can try keeping the blue wire connected which in theory, would allow the mobo to regulate the fan but the maximum speed would be set by the voltage on the red wire.



OK fan specs are:
Sunon Maglev GC054509VH-A, 5v, 1.6W. 

According to the equation P=VI, this fan runs at 0.32A. I would consider going with 'fan-always-on' option _IF_ I couldn't achieve my initial goal. 


streetfighter 2 said:


> Another option is to replace the fan with another fan that spins slower.  This is a bit of a pain in the back because it's a laptop fan and I'm willing to bet it's proprietary.
> 
> Yet another option is to build/buy your own PWM or linear regulator, but it might be tough stuffing it under the hood.



I'd prefer to avoid going down either of these two routes if possible.



streetfighter 2 said:


> If you give me the fans specs I'll do the math for you and you can test it yourself



Assuming I _did_ go ahead with the 'always-on' option, I would be very interested in learning how to figure out what voltage I need to give the fan such that it has enough power to _only just_ start spinning (or can that only be achieved by trial and error?)


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## streetfighter 2 (Jul 18, 2011)

pwnsweet said:


> My apologies for not replying sooner but I have been very busy, even on weekends.


I'm not in a rush. 

Furthermore I just finished my coffee, which puts me in a particularly good mood. 


pwnsweet said:


> Sorry,my laptop is actually a tablet PC and is the HP 2730p.


Well here's some fun stuff for you!  I had a peek on the interwebs and here's what I found.

There are aftermarket BIOSes and BIOS editing tools for your laptop.
There is something called a ACPI (Advanced Configuration and Power Interface) DSDT (Differentiated System Description Table) which can be used to control the speed of the CPU fan in Windows with a laptop similar to yours (2510p).
Also got the repair manual for your laptop, and I'm not impressed with its design 
If I were you I would hop on the notebookreview.com forum and ask those cats what they think about whether to edit your BIOS (to add quiet_fan) or to try modifying the DSDT in Windows.  I wish I could offer more help here, but I'm reluctant to gamble with hardware that isn't mine .


pwnsweet said:


> OK fan specs are:
> Sunon Maglev GC054509VH-A, 5v, 1.6W.
> 
> According to the equation P=VI, this fan runs at 0.32A. I would consider going with 'fan-always-on' option IF I couldn't achieve my initial goal.
> ...


I believe you'd need a lot more mechanical details on the fan to computationally determine the lowest voltage it will run at.  On the other hand you could use a voltage regulator and a multimeter and experiment with it.  I've built many linear regulators but mine are usually bulky and designed for several amps.  I took a quick peek on google and found one that claims to be easier to build, much tinier, and almost as capable.  Alternatively you could swap resistors, use a resistor decade box (redundant to swapping), or use a potentiometer (pot).


pwnsweet said:


> I'd prefer to avoid going down either of these two routes if possible.


I figured you'd say that, but I thought it was worth mentioning.


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## pwnsweet (Jul 19, 2011)

It looks like I was wrong after all. There does seem to be a possibility of a software solution to my problem after all. I'll contact this 'nando4' guy and see if he can make a modified BIOS for me. Thanks for pointing me in his direction streetfighter 2!


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