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My computer's motherboard uses all four pin fan connectors with no support for speed controlling the older three pin fan connector. The problem is, all the PC fans that I have use a three pin connector. As fan as I know there are no premade solutions available, so I was thinking of building an adapter that sits in an unused drive bay.

I can't seem to find much information on how the four pin fan connecter works. Apparently, it looks something like this:

  1. 12 V
  2. 0V
  3. Tachometer
  4. PWM

The three pin header is numbered similarly:

  1. Vcc

  2. 0V

  3. Tachometer

Could I simply use a logic level MOSFET to do the following?
enter image description here

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  • \$\begingroup\$ 3 pin \$\endgroup\$ – jippie Jan 17 '16 at 7:37
  • \$\begingroup\$ 4 pin \$\endgroup\$ – jippie Jan 17 '16 at 7:38
  • \$\begingroup\$ The sources themselves don't tell me more than I already know but they link me to the Intel references which are very handy. Would still like to know if my circuit is okay. \$\endgroup\$ – ZeroKelvinKeyboard Jan 17 '16 at 8:00
  • \$\begingroup\$ I think the idea is OK, but I don't know if it will work in practice due to PWM voltage levels and transistor parameters (will it switch fast enough?). Another unknown parameter is PWM frequency, it may be audible and the fan may start to whistle. \$\endgroup\$ – jippie Jan 17 '16 at 8:12
  • \$\begingroup\$ Probably not audible as Intel's reference says that you have to use a 25 KHz signal. \$\endgroup\$ – ZeroKelvinKeyboard Jan 18 '16 at 2:24
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You can just plug it in, the connectors are designed to be compatible. the fan will only run at full speed, that may or may not be a problem.

if you want variable speed that's obviously not a suitable solution.

using the PWM signal (which is is probably only 0-3.3V) to switch a mosfet probably won't work well, and if it does work the fan will tend to stick on some harmonic of the PWM frequency instead of following the PWM ratio.

a better approach is is to use the PWM signal to control the set-point of a voltage regulator and use the regulator output to power the fan, thus controlling it speed.

schematic

simulate this circuit – Schematic created using CircuitLab

here I show the potentiometer on a typical adjustable lm2958 LM2956 buck converter module and then the same module with the potentiometer replaced with a PWM control circuit. it works by the PWM controlling the discharge speed of C2 and thus the bias given to feedback signal.

C1-R5 compensate for the low-pass formed by the presence of C2

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  • \$\begingroup\$ I can't seem to find a datasheet on the LM2958. Is this the correct part number? I would like to ensure this circuit can handle at least 2A of current, to account for multiple fans and higher starting currents. Also, is there an easy way to add more than one channel? \$\endgroup\$ – ZeroKelvinKeyboard Jan 17 '16 at 19:34
  • \$\begingroup\$ indeed wrong number LM2956, is the main chip you can get an assembled module with the chip and inductor, two capacitors and a diode for about $1: search buy lm2956 DC-DC buck \$\endgroup\$ – Jasen Jan 18 '16 at 9:17
  • \$\begingroup\$ I found the datasheet for the LM2596 IC, which seems to be what you are describing. There are premade modules but they are for voltage conversion rather than accepting a PWM input signal, so I'll make the circuit you described myself. \$\endgroup\$ – ZeroKelvinKeyboard Jan 18 '16 at 11:42
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This solved my problem:

Cited from 3pin to 4pin fan adapter(Chinese)

4pin adapter for 3pin fan

Explanation:

  • two fast-switch triodes (2N-2907 and 13003) utilize PWM control over 12v power.
  • diode protects the fan from voltage higher than 12v
  • 1K R gives a 12/1000=12mA ceiling to the current from 12v to ground, just in case 2N-2907 is damaged.
  • 0.5K R does the same thing for 13003, limiting the max current approximately to 2.5/500=5mA

All these stuffs are easy and cheap to buy, enjoy..

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  • \$\begingroup\$ Hello. Do you have ready to buy boards available on aliexpress? \$\endgroup\$ – puchu Apr 5 '19 at 9:21
  • \$\begingroup\$ Sorry, I don't produce it. I'm only sharing my solution. \$\endgroup\$ – keos Apr 13 '19 at 2:44
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Most 3-pin fan controllers control the fan by varying a DC voltage across the fan. This basically means that there is a PWM circuit (most likely) inside that controls a (discrete) buck regulator that brings the voltage down. The fan stays on permanently and has a functional tacho output.

You CAN control a 3-pin fan with PWM on the power pins as well (but not as you have drawn - this is not how a P-MOSFET will work), but be prepared to sacrifice the tachometer function. After all, if you cut power to the fan there is no way for the fan to pull down (it's an open collector/drain output) the tachometer pin. Therefore you get the tacho signal interleaved with the PWM signal, resulting in a garbaged tacho output.

4-pin fan controllers hand off the PWM signal to the fan itself, and these fan controllers supply an uncontrolled a 12VDC. This is probably why they are not able to speed control your fan. 3-pin fans will power up at 4-pin connectors, but only at full speed.

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This should work correctly :

enter image description here

source: https://www.techpowerup.com/forums/threads/so-you-want-pwm-control-of-your-3-pin-fan.115752/

12V is permanently connected, ergo, you will have correct RPM readings. You need a 5.1V Zener Diode, different value for resistor and positioning and uses a n-Mosfet.

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This is rather old question, so I wanted to give a brief update for the folks stumbling upon it now. Note, that this is not an answer per se, since it does not work for everybody, unlike other answers here.

While answering similar question, I've stumbled upon the notion of motherboard support for both 3-pin and 4-pin fans. So, if you have one of the newest boards, before you start soldering any adapters go check BIOS settings - it just might have everything you need already.

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  • \$\begingroup\$ For me a newer motherboard was the problem. My old motherboard had a setting to switch between voltage and PWM controls; but when I updated to a newer motherboard of the same price point just before my original question, the setting wasn't there anymore. I just kept my old 3-pin fans expecting a setting to support them, but the newer motherboard just expected people to be using 4-pin fans by now. \$\endgroup\$ – ZeroKelvinKeyboard Jan 24 at 23:49
  • \$\begingroup\$ Interesting... then it looks like there was a transition period when motherboard manufacturers supported both, then decided to scratch old interface as most people buying new boards would buy 4-pin fans anyway. As my own PC is about 6 years old I missed that period entirely. \$\endgroup\$ – Maple Jan 25 at 0:34

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