3 to 4 pin fans

[CrayZE]

All my case fans only have 3 pins, and my motherboard only has 4 pin fan connectors. Can I plug in the 3 pin fans into the 4 pin connectors somehow? I want to be able to control my fan speed. As of now theyre all plugged into my PSU running on full

 

[giantbucket]

sure can. your mobo manual should tell you which headers have control and which are always-on-full

 

[Shaneoh]

Yes you can, you should be able to just plug them in (if im understanding your question right). And I do the same, and I am able to control speeds through BIOS.

 

[Paperdoc]

Probably not, but maybe. Obviously, such a mixed reply needs explanation!

A true 4-pin fan port supplies a constant 12 VDC power supply to the fan on port Pin #2 at all times. It achieves control of fan speed by supplying also (on Pin #4) a PWM signal. This is used within the fan case by a little chip to modify the flow of current from this constant DC supply through the fan so it runs slower.

A 3-pin fan port works differently. The voltage it supplies on Pin #2 varies from 12 VDC down to very few volts, and this is how it controls fan speed. It does not supply a PWM signal, and a 3-pin fan cannot use such a signal, anyway.

Now, the physical layout of fan connectors and fan ports and their electrical details are designed for backwards compatibility of a sort. Ideally, one uses 4-pin fans only on 4-pin ports, and 3-pin with 3-pin. If you mix, you get these results: (a) a 4-pin fan connected to a 3-pin port WILL work and the fan's speed will be controlled by the mobo because the voltage supplied to the fan is changed by the port and there is no PWM signal supplied to the fan to alter that; (b) a 3-pin fan plugged into a 4-pin port (your situation, OP) will always run at full speed, because its voltage supplied on Pin #2 is always the full 12 VDC.

So, OP, if you plug your 3-pin fans into mobo 4-pin ports, they will run, but always at full speed. There will be no speed control. In such a situation, you could achieve the same result by using adapters to connect those 3-pin fans to a fixed +12 VDC supply from a 4-pin Molex power output connector from the PSU itself. The one advantage to plugging into the mobo, though, is that then each fan's actual speed signal will be fed to the mobo and it can be measured and displayed if you like.

There are three exceptions to this general rule.
1. For the CPU_FAN port only, it is common on many mobos to allow you to change this behavior. Check your mobo manual and go into BIOS Setup to examine the options for monitoring and control of the CPU cooling fan. Some mobos allow you to set that port to behave as a true 3-pin port, rather than 4-pin. (Some even are so clever they detect the fan type connected and make this change automatically.) In this 3-pin mode the port DOES vary the voltage on Pin #2 and accomplish fan speed control. For this and other reasons I advise you always should connect your CPU cooling fan to the mobo's CPU_FAN port. It will control the fan's speed (and hence CPU cooling) automatically according to the actual temperature measured by a sensor inside the CPU chip. PLUS the port will keep an eye on the CPU fan's speed and send out an alarm if it stops.
2. For case ventilation fans connected to the SYS_FAN or CHA_FAN mobo ports, you MAY be able to set the port(s) manually in BIOS Setup to become 3-pin ports. But many mobos do not include this feature for these ports.
3. I understand that a few mobo makers have begun making the 4-pin mobo SYS_FAN ports as "fake" 4-pin ports. That is, they have 4 pins and look like standard 4-pin ports, but in fact they operate exactly as 3-pin ports (with one useless pin). Such a port CAN control the speed of both 3-pin fans (because they really are 3-pin ports) and 4-pin fans (by using the designed backwards compatibility feature that a 4-pin fan works OK on a 3-pin port). So if your mobo happens to be this design, your fans might work just fine.

There is one other possibility to look for, OP. Some mobos have BOTH 3-pin and 4-pin ports for the SYS_FAN or CHA_FAN functions. You get to use whichever port type matches your fan. So look at the mobo and its manual closely.