Help understanding 3 pin fan speed control

[BrettonF]

I understand how a four pin fan with PWM is controlled, but I'm confused on how 3 pin fans are controlled in practice.

So lets say you have an AeroCool DS120 (http://www.aerocool.us/accessory/images/dsfan_12blue.html) that runs at 12V normally, or 7V with the inline adapter. If the fan is set to 12V, and a fan controller wants to turn it down, does it drop to 7V immediately or can it go to 11V, 9V, 8.5V etc? How does the controller know to raise the power if the fan stops spinning and how low can the controller take the fan (Down to 7V? 5V?). What happens if the 7V adapter is then used?

I know this sounds like a textbook question, but I'm just confused and want to understand

 

[Gam3r01]

The fan will run at 12V, or max speed by default. If you use the in line cable provided, it will run at 7V.
The fan will not fluctuate speeds or voltages from there.

 

[giantbucket]

3 pins = ground, 12V (variable), and rpm signal from fan to mobo/controller/etc

the controller simply modulates (increases / decreases) the 12V line, reasonably smoothly. it uses the RPM signal from the fan to determine how fast the fan is spinning and/or if it's completely stopped.

those "low noise" or "7V" adapters are basically just a single resistor, of the correct value to drop the 12V coming in to 7V at the fan - since the manufacturer knows how much current the fan takes, the resistor choice is easy! you couldn't use a single one of those adapters to run TWO fans, though, since they would pull double the current and the resistor would decrease the voltage too much, choking the fans completely. UNLESS instead of a resistor they use a zener diode, in which case you COULD run multiples. but those are more expensive. not by a lot, but fans are cheap to begin with. you can easily check if it's a resistor or a zener diode - just connect TWO fans through it, and if they still spin it's a zener and if they don't it's a resistor.

 

[BrettonF]

The difference between these two answers is my confusion. giantbucket's answer sounds right to me based on experience because I have three fans connected to my motherboard, two are PWM and one is a 3 pin. I can adjust the speed of any of these three fans though my motherboard BIOS (the 3 pin included) and I can clearly tell they spin up/down.

So a few follow up questions
If a 7V resistor is used, is there a risk of the fan not spinning if the motherboard thinks it is 12V and lowers the voltage too far so that after the resistor, the voltage is too low to spin the fan?
And then on the note of running multiple fans, if you wanted to run two fans (each using a 7V resistors) from one fan header, you would just need to have the splitter first, and then each line has a resistor (so one splitter and two resistors total) correct?

 

[giantbucket]

a) kinda, since the resistor only reduces the 12V signal. the rpm signal going from fan to mobo is untouched, so the mobo/controller simply thinks it's a slow fan to begin with. but yeah, if it tries to slow it down a bit, it'll be fine, but when it tries to slow it down some more it'll choke and stop spinning so the mobo will panic and drive the voltage back up. on the flip side, the mobo can't drive the voltage higher than 12V so if the cooling isn't enough, the mobo will be stuck and you could cook your goose. moose. venison. with blueberry sauce.

b) yes, split before the "fancy" adapters



PWM fans have 4 pins - the first 3 are the same as above, except that the 12V does NOT get reduced by a PWM controller. however, a PWM controller sends pulses along the 4th line which causes the fan to see 12V / 0V / 12V / 0V, and inertia ends up causing the fan to spin slower. the 12V pulse makes sure that the fan gets enough kick to start spinning, and the 0V "pulse" allows it to slow down unpowered. repeat! the ratio of 12V time to 0V time determines how fast the fan ends up spinning.


look up PWM duty cycle.

 

[RedJaron]

Bucket and Gamer are both correct, but they're talking about different scenarios. Gamer's answer is the simple one of what would happen if you had the mboard BIOS set to max fan speed ( or if you had the fan connected to a fixed 12V source like a PSU Molex adpater ) and had the resistor wire attached.

Bucket is talking about what happens if you combine the resistor wire with a separate fan control. Generally it's not necessary, and in some cases is a bad idea. The built-in fan control in your mboard's BIOS automatically adjusts the outgoing voltage to your fans to spin them up or down. The resistor wire is trying to do the same thing. Those resistor wires were popular when not all mboards had fan control abilities, either through PWM or variable voltage. Most simply output 12V all the time so your fans were always at max. The resistor basically made it so only 7V of the 12V reached the fan, slowing it down if you didn't need the airflow or noise.

However, most mboards nowadays have both PWM control and variable voltage output, giving them much finer control over fans than a simple resistor wire can. You also have third party fan controls with external dials that you can adjust manually on the fly. These dials are variable resistors, and if you use one you don't need to use the mboards fan control or the resistor wire.