What's the difference between 3-pin and 4-pin fans?

floepie · Post by **floepie** » Wed Mar 05, 2008 8:33 am

OK, I've done a bit reading on this topic, and I'm becoming more confused the more I get into this. Perhaps someone can fill in the gaps for me and others reading this thread?

3-PIN FANS

1. A 3-pin fan can of course be controlled by a physical voltage adjuster, such as those provided by Zalman.
2. 3-pin fans can be used with either 3-pin or 4-pin headers - an adapter should not be necessary.
3. 3-pin fans can be controlled either by the mobo/BIOS, if the BIOS allows it, or by SpeedFan (usually, only if the BIOS can control the fan in question).
4. The BIOS or SpeedFan controls a 3-pin fan by varying voltage or by adjusting a low-frequency PWM signal. This is motherboard dependent.
5. 3-pin fans may not necessarily be regulated if the motherboard does not allow it. On some older boards, SpeedFan can control fans that the BIOS can not, however, there must be circuitry on the board to allow that to happen.

4-PIN FANS

1. Can be used with a 4-pin header of course or a 3-pin header (again, no adapter should be necessary).
2. 4-pin fans can and should be controlled either by the mobo/BIOS OR SpeedFan.
3. Most often, 4-pin fans are controlled by varying high-frequency PWM signals.
4. The presence of a 4-pin header ALMOST (>99%) ensures that some control will be had over fan speed - either with the BIOS or SpeedFan.

If there is anything wrong above, please correct. If there is anything that could be added, please do so. Thanks!

blackworx · Post by **blackworx** » Thu Mar 13, 2008 6:10 am

Hi Floepie,

I can confirm point 3 about 3-pin fans. Point 5 - yes you are correct and no there isn't - sort of... As I understand it, Speedfan (or any other fan control software) is only an OS interface for the mobo hardware which allows system monitoring/fan speed control in the first place (e.g. the Winbond uGuru chip on Abit mobos) so if you cannot control or monitor from the BIOS then you probably won't be able to do so from the OS.

But... I should probably clarify and say that only if the controlling hardware is not present will you definitely not be able to use Speedfan etc. This is because I know of at least one (older) mobo where the BIOS did not allow fan adjustment (although it did report speeds) but Speedfan still worked because the controller chip was present and functional. I can't speak for more recent boards in that respect.

Unless your fan or mobo has a non-standard pin arrangement (unlikely) then there should be no need for an adapter to run a 4-pin fan off a 3-pin header (or vice versa) - see http://www.intel.com/sup...2074.htm

The rest of your statements about 4-pin fans seem correct to me, but I've only had experience with a small number of 4-pin fans and have yet to find any proper specifications/documentation. I'm sure the information we need is out there, it's just lost in the noise of so much nonsense being spouted on forums and by people who should know better (i.e. review sites etc.).

Finally, if anyone knows I'm talking crap, please tell me as the last thing I want to do is add to that noise!

BillyBuerger · Post by **BillyBuerger** » Thu Mar 13, 2008 9:26 am

The wires on a 3-pin fan are ground, power and tach. They can be controlled only by adjusting the power line. Either by reducing the voltage applied to it or by using PWM. PWM in this case acts by switching the fan on then off very quickly. Whether or not they can be controlled by any means is completely dependent on the motherboard.

The wires on a 4-pin fan are the same except the additional PWM wire. They too can be controlled in the same way as a 3-pin fan by adjusting the power wire. But they are designed specifically to receive the full 12V on the power wire and use the additional PWM wire to control the speed. As this additional wire is specifically for controlling the speed, I would agree with your point 4 that any motherboard that has a 4-pin header should be able to control the speed of the fan. But again, this is completely dependent on the motherboard.

They did a pretty good job of making the change to 4-pin fans as really, 3-pin and 4-pin fans are interchangeable. You can plug either type of fan into either type of header without any adapters needed. And 4-pin fans can act just like 3-pin fans. But, you only get the benefit of 4-pin fans if you use a 4-pin fan on a 4-pin header.

To answer your questions...

1. Yes
2. Yes
3. Depends on mobo
4. Depends on mobo
5. Depends on mobo

1. Yes (no adapter needed)
2. Yes
3. Yes (Don't know why they would do it any other way then that)
4. I would assume that as well

floepie · Post by **floepie** » Thu Mar 13, 2008 9:43 am

Thanks for the replies. I've edited the list to reflect any changes or confirmations. Hopefully, this will help others untangle any confusion between these 2 types of fans and how they are controlled.

XIONG · Post by **XIONG** » Sun Jun 01, 2008 6:23 pm

Cool, thanks for the heads up!

Mr Reorg · Post by **Mr Reorg** » Sun Jun 01, 2008 8:55 pm

I appreciate the clarification. I'm begining to look at parts for my first build and I've been a bit confused on this subject. Thanks for having that cleared up.