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Hardware

PWM fan control

With PWM, a signal on the fourth wire sets the speed while the 12 V stay. See how to run fans quietly and precisely, on the board or under Linux.

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Content date
25 Oct 2026
Status
Source-based, no review record yet · What does this mean?
Sources
7

A 4-pin fan does not need a smaller voltage to slow down. It needs a fourth wire and a controller that knows when to be quiet.

The short answer

PWM (pulse-width modulation) fan control changes a fan's speed with a control signal on a fourth pin, while the fan keeps its 12 V supply [1][2][3]. The signal is a fast on-off pattern, and the share of "on" time (the duty cycle) tells the fan how fast to run: at 100 % duty the fan runs at its maximum speed [1]. Compared with a 3-pin fan, which can only be slowed down by lowering its voltage, a 4-pin fan allows lower speeds and more precise control [3]. Our assessment: for a quiet PC or homelab box, buy 4-pin fans, plug them into 4-pin headers, and let the board or a small controller set the curve.

How it works

  1. Four wires. The 4-wire fan specification from Intel (revision 1.3, September 2005) adds a PWM control line on pin 4 to the usual ground, 12 V and tachometer wires. The target PWM frequency is 25 kHz, within an acceptable range of 21 to 28 kHz [1].
  2. Duty cycle sets speed. The signal is not inverted, so 100 % duty means maximum speed [1]. The fan has a pull-up on the control line of at most 5.25 V, so a controller only needs to pull the line down [1]. Our reading of the specification.
  3. No signal means full speed. Noctua's white paper says a fan without a PWM signal runs at full speed. Behaviour below 20 % duty is not defined, and most fans stop at 0 % while a few keep running [2]. Real minimum speeds differ per model: the white paper uses a fan with 2000 rpm maximum that still turns at 450 rpm at 20 % as an example [2].
  4. Never feed 12 V to the control pin. Noctua warns against connecting 12 V to pin 4 [2].
  5. Mixing 3-pin and 4-pin. According to Noctua, a 4-pin fan on a 3-pin header runs at full speed unless the header can lower the voltage. A 3-pin fan on a 4-pin header works, but is then controlled by voltage if the header supports it [4].
  6. PWM or DC mode. Many boards can drive a header in either mode. Noctua's tip is to set 4-pin fans to PWM mode and let the BIOS tune the fan range [3].
  7. Reading the speed. The tachometer wire gives pulses per revolution, so a controller can measure rpm and not only set a duty cycle [2][5].
  8. Linux. The kernel's hardware monitoring interface describes fan[1-*]_input for the measured value, pwm[1-*] for PWM fan control, pwm[1-*]_enable for the control method and pwm[1-*]_mode for direct current or PWM. Writable attributes are writable only for privileged users [6]. The pwm-fan driver documents pwm1 as a relative speed from 0 to 255, where 255 is the maximum [7].

A practical example

You build a small server with a case fan on a 4-pin header. In the BIOS you set the header to PWM mode and choose a curve that stays low at idle and rises with the CPU temperature. Then you check the result under load: the tachometer reading should rise, and the case should stay quiet at idle. If you drive a fan from a microcontroller instead, Noctua's guide covers PWM setup and rpm monitoring [5]. We have not documented any fan controllers in our inventory (14 to 16 Oct 2026), so this is a generic walk-through.

Checklist

  • Buy 4-pin fans and use 4-pin headers for anything you want to run slowly [3][4].
  • Set the header to PWM mode for a 4-pin fan [3].
  • Test the lowest duty cycle your fan accepts, because some stop and some do not [2].
  • Never wire 12 V to the control pin [2].
  • Check the rpm reading after every change [5][6].
  • On Linux, write to pwm attributes only as a privileged user and only after you have read the driver's documentation [6][7].

Primary sources

These sources are the basis of this page. That does not mean every claim was checked individually; whether a review record exists is shown above under “Status”. The interpretation and checklist are our own work.

There is no documented review record for this version yet. The text relies on the sources listed.

How this page was made: Bytearium creates these pages with the help of AI and is responsible for publishing them. AI can be wrong, so we show our sources and review status openly.

  1. Intel4-Wire Pulse Width Modulation (PWM) Controlled Fans
  2. NoctuaNoctua PWM specifications white paper
  3. NoctuaHow do I optimise my fan settings?
  4. NoctuaCan I run Noctua 3-pin fans on 4-pin PWM connectors or 4-pin fans on 3-pin connectors?
  5. NoctuaMicrocontroller guide: PWM setup and RPM monitoring
  6. The Linux kernelNaming and data format standards for sysfs files
  7. The Linux kernelKernel driver pwm-fan

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