Understanding 3-pin and 4-pin case/CPU fans

The world of case/CPU fans is new to me, and I want to make sure I understand what I've read about 3-pin and 4-pin fans. I'm in the process of upgrading to an Ultra-120 Extreme, which requires the purchase of the fan separately. I'd appreciate it if anyone can confirm/correct the following.

3-pin fans - the fan is controlled by voltage. I think that means fan speed is adjusted by some physical means, such as a knob to turn. It's not possible to adjust the fan speed through the motherboard (i.e. Speedfan, bios, other software won't have any affect on the speed fan).

4-pin fan - the fan is controlled by pulse-width modulation (pwm), which seems to mean you can adjust fan speed through the motherboard using software/bios.

One of the reviews for the Silverstone FM122, which I'm leaning towards, states the fan is powered directly from the system PSU via the Molex connector. Should I understand what that means?

Also, it appears 3-pin fans are more common. I would have thought 4-pin fans would be the default choice due to the extra control, but perhaps thats a cost issue.

your pretty much correct on all of them..

the silverstone is powered by the 4 pin molex which is voltage controlled.. so it pretty much runs full speed all the time.. 3 pin you can not adjust the speed of the fan and would have to use a fan controller to do that.. with the 4 pin if you have a intel system speedstep can automaticly control the fan speed when idle and at full load or you can control it manually through software or the bios.. and if its an amd system cool 'n quiet can automaticly control the fan speed..

generally you see 4 pin fans for cpu's for the automatic control and 3 pin for case fans.. so that you can control them your self with a fan controller or just leave them on full speed all the time.. or they come with a little fan controller on them like the crappy Tri-cool fans that antec uses..

3-pin fans - the fan is controlled by voltage. I think that means fan speed is adjusted by some physical means, such as a knob to turn. It's not possible to adjust the fan speed through the motherboard (i.e. Speedfan, bios, other software won't have any affect on the speed fan).

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True most of the time. However it is possible for the motherboard/bios to control the voltage of the motherboard fan headers and thus the fan speed on a few motherboards. Server boards in particular, Abit boards had (they are in process of being out of business) excellent fan speed control through the bios. 95% of the time your statement is correct but it can be done and is done on some products. Also in the bios of some boards for the CPU fan - there is a "Legacy" fan speed control setting that will adjust the CPU fan speed by voltage control in case you do not use a CPU heatsink with a pwm 4 pin fan. Say a 3rd party heatsink in which you supply your own fan and it is 3 pin. Motherboard header is 4 pin. You an disable CPU fan control and it will run 100% speed for a plain fan or go "legacy" in which typically the motherboard will use the "CPU temp" thermal sensor located somewhere near the CPU to automatically adjust fan speed based on old speed step or cool n quiet temp vs fan speed curves.

There is also a class of 2 and 3 pin fans that have a built in temp sensor that reads the temp of the air blowing through the fan and adjusts (hotter = faster) the fan speed. These are useful for rear exhaust fans. Some work really well, and some are crap as they dont rev up until it is IMO too damn hot. These dont work well for intake fans as the room air is usually always so cool, even in summer, the fans run at their lowest speed.

Other thoughts, you can plug a 3 pin fan into a 4 pin motherboard header. It will typically just run at 100%. see above for CPU motherboard header operation.
You cannot plug a 4 pin pwm fan into a 3 pin fan header - will not run.

So you basically have the right idea but I just wanted to let you know about some of the less common options out there. I love the thermostatically controlled fans on high performance builds with a budget. No fan controller cost, no issue with customer forgetting to turn up the fans and calling me about BSOD when gaming, just plug it into any 3 or 4 pin motherboard header and forget it. Only issue is to make sure you find the fan speed vs temp curve and make sure it is suited for your needs. My builds use 120mm fans front and back as a minimum and thus the case airflow is very good, so I need a fan that will rapidly start to ramp up fan speed with only minor increase in case temps as opposed to ones that have a slow ramp up. The case will never get hot enough inside for the fan to work hard and most of the heat will go out the bottom fan of the power supply increasing its working temp - something I like to avoid.

One of the reviews for the Silverstone FM122, which I'm leaning towards, states the fan is powered directly from the system PSU via the Molex connector. Should I understand what that means?

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Yes, what it means is the fan comes with the bigger 4 pin connector like used for CD/DVD drives older Hard Drives, etc. It does not plug into the small motherboard pin headers, Instead that fan will plug into one of the large 4 pin "molex" large female nylon connectors in the string of connectors from your power supply. The fan gets its 12v directly from the power supply. What that is telling you is that the fan does not come with the smaller connector used for plugging fans into the motherboard.


Some fans come with one or the other type, some have both, and adapters can be bought for $2 (with $8 shipping LOL) to convert one type to the other.

One other thing that will either simplify this for you or make it even more confusing...

On both 3 and 4 pin fans, one of the wires is used for speed sensing. So even 2 pin fans can be compared.
2 pins: Voltage and ground (the same as many other electrical devices)
3 pins: Voltage, ground, and RPM sensing
4 pins: Voltage, ground, RPM sensing, and PWM control

With 2 and 3 pin fans, you change the voltage to alter the output. As stated, this can be done with a knob (similar to dimmer lights in your house) or sometimes through software/BIOS (SpeedFan, etc.).

4 wire PWM fans always get full voltage, but the PWM signal tells them how much of that power to use. Because the fan always gets full voltage, you won't run into the problem of using a low voltage setting which is enough to keep the fan spinning, but not enough to start the fan up from a stop (can be an issue on 2 and 3 wire fans which are run very slowly).

4 wire PWM fans are probably the best, but they're also the newest. A lot of motherboards have one 4 pin header for the CPU, and 3 pin headers for any other fans. I wouldn't be surprised if everything moved to 4 pin eventually, but we're still in the early stages. If your motherboard has a 4 pin header for the CPU fan, I'd look for a 4 pin fan for the best control. Any option can be made to work though.

InvisiBill said:

4 wire PWM fans are probably the best, but they're also the newest. A lot of motherboards have one 4 pin header for the CPU, and 3 pin headers for any other fans. I wouldn't be surprised if everything moved to 4 pin eventually, but we're still in the early stages. If your motherboard has a 4 pin header for the CPU fan, I'd look for a 4 pin fan for the best control. Any option can be made to work though.

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All things being equal, I was wondering why I shouldn't go with a 4-pin fan. Thanks for addressing that point.

And thanks to all for the responding to a budding enthusiast. I've been reading this forum for a year, signed up 5 months ago, and this was my first post. The quality of this community continues to impress me.