# Why is a 12V 3-wire CPU fan very slow at 12V input?

I play around with an old 3-wire CPU fan from a dell server PC. It is this thing: 3612KL-04W-B66 https://www.coolingsurplus.com/index.php?option=com_content&view=article&id=86:3612kl-04w-b66&catid=6:92mm-fans&Itemid=7

It was mounted at the end of heat pipe over the CPU. It was pulling the air from the CPU to the outside of the case.

I found a datasheet here (https://www.electronicsdatasheets.com/manufacturers/nmb-technologies/parts/3612kl04wb66) but this particular model B66 is not in it, only B50 and B60 and other round numbers and they are all 2-wire fans.

According to some info I found about dell fan wiring, black wire should be GND, red wire is +12V and white wire is TACH feedback.

As I understand the basics of 3-wire brushless fans, I can control the speed by varying the voltage (10.5 - 13.2 V according to datasheet) or low frequency PWM the 12 V input.

As a less experienced fan user I would expect the PWM method to cause the fan to slow down while running at full speed at 12 V. But the fan acts in the opposite way, at least for me.

It spins up to speed for a short period after connecting the positive wire to 12V PSU (made from ATX PSU), then it slows down to very low speed.

When I connect and disconnect the positive wire manually in short succession (0.5 Hz by my fingers judgment) the fan speeds up significantly.

I ask you guys for some advise how to actually control the speed of this fan, roughly and manually.

Edit: I managed to resolve the mistery behind the low RPM at 12 V continous. There is some speed controll built into the fans board. I can see a small part coming out from the side of the stator case. Might be a temperature sensor. I pointet a heat gun at it from behind the air flow. The fan revived to significant speed :-) I am stil curious, what is the white wire for (seems not to be TACH, only bias on the analog scope) and how to get rid of the internal speed control

Edit: Shorting the leads of the termistor (youtube.com/watch?v=AVovy7UWDQ0, is exactly the same fan by the way) lets the fan run at full speed and I can control RPM now by lowering the voltage. The question about the function of the white wire stays in the room :-)

• @DKNguyen You can't PWM the power input on a fan because it goes through the electronic controller that is driving the brushless motor itself. Actually you can. Today's PCs control the speed of fans by driving them with low-frequency (or even sometimes high-frequency - >20kHz) PWM. Measuring the speed can be problematic though, but I'm sure they drive with PWM. Jun 8 '20 at 19:43
• @RohatKılıç Oh really? I did not know that. Are you sure they aren't using a buck of some kind? How does the integrated driver handle constantly losing power like that? Jun 8 '20 at 19:44
• All of the server PCs I have looked at use fans with a direct attached voltage and ground connection. The fan speed is controlled by a third wire that receives a 0 to 5V PWM signal which is normally modulated at over 20kHz. For failure detection a fourth wire may be present on the fan to be able to monitor a 0-12V signal that shows a TACH signal that follows actual fan speed by some factor of 1X, 2X or maybe even 4X. Jun 8 '20 at 19:55
• @DKNguyen I had read an article about that but couldn't find it now. I'll drop the link here once I found. I don't know the details about the internal circuitry but I'm sure they can be driven via a low-speed PWM. IIRC, there are a few problems with LF PWM like audible noise and reading the tacho signal (it's present if and only if the power is present). Jun 8 '20 at 20:04
• @RohatKılıç Was it this article? analog.com/en/analog-dialogue/articles/… Jun 8 '20 at 20:11