First, you are wrong: a fan hub CAN control fan speeds IF the hub AND the fans are of the right design. And you also need the right type of signal feed from a mobo fan port.
But before that, you need to understand some fan background.
1. Right now there are three main types of computer fan designs:
(a) Fans to be plugged into Power Supply Outputs only. Typically these have only two wires coming from the fan motor to its connector, and the connector is what is called a male 4-pin Molex. It is about ¾" wide with four pins in a straight line inside a protective shroud. The female mating connector on wires from your PSU was used more commonly to power IDE and optical drives, but can be used for many other things. SOME such fans also come with additional connectors on their wires: some have another Molex, but female, so you can plug another fan into the same power source; some have a smaller female connector with three holes and three wires (see item (b) below) so that as an ALTERNATIVE (not to be used at the same time) you can connect to a mobo 3-pin fan port; some use a different connector so they can be plugged into a PSU's SATA device power output, instead of an 4-pin Molex output.
(b) 3-pin fans. These have three wires (Black, Red and Yellow) from motor to connector, and the connector is female (with 3 holes) with a couple of bumps on one side. This mates with a 3-pin male fan port on a motherboard; because of the bumps, it only will fit on one way. This type of fan can have its speed controlled by the mobo port it is plugged into, but ONLY in Voltage Control Mode. That is, the mobo connector can supply a voltage from 0 to 12 VDC on the Red wire (Black is Ground) to change the fan's speed. Any mobo fan port with ONLY 3 pins will provide this type of signal. The third wire (Yellow) takes a pulse train signal generated inside the fan back to the mobo for counting so the mobo can display the fan's speed. Note that this signal is only to give info to the mobo - it is NOT actually used to control fan speed.
(c) 4-pin (PWM Mode) fans. These also can be speed controlled by a different type of mobo port. The four wires typically are Black (Ground), Yellow (+12 VDC supply), Green (Pulse Speed signal back) and Blue (PWM Signal to fan). For this fan design, the power supplied on the wires is always +12 VDC. Inside the fan case is a small circuit and chip that takes the PWM signal from the 4th pin and uses it to control how much of the time that 12VDC supply is actually fed to the motor. The PWM signal is a fast one, so the motor gets pulses of 12VDC separated by no power; the more time it is "on" the faster it runs. So this fan design also can be controlled by a mobo port, but only if that port has 4 pins and is operating in PWM Mode.
When 4-pin fans were introduced, they included backwards compatibility features. The wiring and connectors for them is the same as for a 3-pin fan, just with a fourth pin added on one end. Mechanically the connectors are designed so you CAN plug any 3-pin OR 4-pin fan into any 3-pin OR 4-pin mobo port. The results are not always exactly what you want. If you match fan pins to mobo pins they work as designed. Plug a 4-pin fan into a 3-pin port and it will appear to work just fine, too. The 4-pin fan won't get a PWM signal to alter the power supplied, but the 3-pin port supplies a Voltage that is lowered to slow down the fan, and this works. It is not quite ideal for such a fan, but it does work. Plug a 3-pin fan into a 4-pin port and it will always run at full speed - it won't get the PWM signal it could not use anyway, and it will always receive the full 12 VDC supply. But it WILL cool your system - just not under any reduced speed control.
Mobo fan ports can be confusing. In the original designs, 3-pin ports always operated by varying the Voltage on Pin #2, and 4-pin ports kept that voltage at 12 VDC and then sent out the PWM signal on Pin #4. Now we are seeing some flexibility. It is VERY common for a mobo's CPU_FAN port to have 4 pins and default to operate in PWM Mode. BUT you also have the option of configuring that port in BIOS Setup to change it to operate in Voltage Control Mode (like a normal 3-pin port) just in case your CPU cooling system is designed only for the 3-pin system. Similarly, SOME mobo SYS_FAN ports are flexible in this way, but not a lot of them. Many mobo SYS_FAN ports are just one type or the other. There are even some that are "fake": they have 4 pins, but are truly 3-pin ports operating only in Voltage Control Mode, and their ability to control a 4-pin fan relies on the backwards compatibility design of that newer fan type. The "problem" with this port type is that there is no PWM signal available on its useless Pin #4.
Automatic control of cooling fan speeds by mobo ports operates this way: It really is a TEMPERATURE control system, not a fan speed control system, but it does use changes of fan speed to meet its temperature targets. Each control "loop" has a sensor that measures an actual temperature at some important spot and a target for what that temp should be. It constantly compares measured temperature to target and adjusts fan speed to maintain temperature on target. The two different fan designs just need slightly different ways of adjusting them.
On most computer motherboards there are TWO different temperature control loops. The first is the one for the CPU, and it uses a temperature sensor built into the CPU chip itself by its manufacturer, with its signal available on one pin of the CPU case. This control system also does a couple of special checks on the CPU cooling fan to be SURE it does not fail; if it does, the system will shut down very rapidly to protect the CPU from damage. So it is IMPORTANT to connect your CPU cooling system, and not something else, to this mobo port. The second temperature control loop is based on a different sensor built into the mobo by its maker, and it controls case ventilation fans connected to the mobo's SYS_FAN port(s).
There is a significant limit on what mobo fan ports can do. Each typically can power up to 2 fans per port (you need a fan splitter adapter to do this) but not more. People who want to use more fans have some work to do to achieve that, and one answer is third-part fan controllers. They draw their power directly from a PSU output which can supply power for MANY fans, but most have no way to do automatic control based on temperatures. Using such a controller normally means that YOU are the brains of the control system: you must check temperatures somehow and decide how to change fan speeds as your workload changes, then set the fan speeds manually.
Now there's another way using 4-pin fans. The limit of 2 fans per port is because of the 12VDC power supply at the port. However, fans to not draw much power at all from the PWM signal line, so it is possible to feed one port's PWM signal to many 4-pin fans. Thus, you can buy and use successfully 4-pin fan hubs, but they MUST be 4-pin hubs, they MUST power only 4-pin fans, and you MUST connect the hub to a 4-pin mobo port. Such a hub also connects to one power output from the PSU and supplies power to all its fans from this source; it does not use a mobo fan port's limited power for its fans. The hub gets a PWM signal from Pin #4 of the one SYS_FAN port it is plugged into and simply shares that out to all its fans. In fact, some of these 4-pin fan hubs do not even look like boxes with wires - they can be as simple as wires and connectors only.
You can see how that will work for a system that is all of the PWM (4-pin system) design. Such a hub system is not possible for the 3-pin system.
If you were trying to use a 4-pin hub system and plug a 3-pin fan into a hub port, that fan would only operate at full speed all the time. This is the same as plugging such a 3-pin fan into any mobo 4-pin port. That is probably where you got your initial misunderstanding.
I know of one exception to all this that will be of interest to people who have 4-pin mobo ports and want to use several 3-pin fans for whatever reason, or even to mix 3- and 4-pin fans in their case ventilation system. It is called the Phateks PWM Hub, and it looks a lot like other 4-pin hubs. The key difference in its design is that it does not merely share the PWM signal it has picked up from a mobo SYS_FAN 4-pin port. Instead internally it uses that signal to create six actual 3-pin fan ports operating in Voltage Control Mode, each with that common limit of no more than 2 fans per port. So it CAN control many 3-pin fans from a 4-pin PWM mobo port. It even can control any 4-pin fan attached to it by the backwards compatibility feature of the 4-pin fan.
Of course, there are situations where you can run several fans under mobo speed control without a hub. You can use splitters to connect 2 fans to each SYS_FAN port. To make that work well you must make sure the fan type (3-pin or 4-pin) matches the SYS_FAN port type, and use the correct type of splitter. But if you don't have too many fans and your mobo has 2 or 3 SYS_FAN ports, splitters may be all you need.
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