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A "standard" computer fan has two pins; one for power, one for ground. A three-pin fan adds an additional pin used for the signal from the hall-effect sensor. A speed-controlled fan generally uses a four-pin connector, and is commonly referred to as a "PWM" fan.

It is my understanding that any DC motor can be driven with PWM instead of a steady signal, so I am confused why there is a need for an additional pin for the PWM signal. Is the PWM signal fed into a MOSFET or some similar switching component on the fan itself in order to control the speed of the fan?

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  • \$\begingroup\$ A lot of words in the answers below that don't seem to answer your question. I don't know, but my guess would be 1:power, 2:ground, 3:pwm and 4:some pulse index or encoder. \$\endgroup\$ – kenny Dec 3 '11 at 2:33
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    \$\begingroup\$ As far as I know, standard PC fans use 3 pins (+5V, GND, rpm sense), so that motherboard can see the actual speed at which the fan is running. PWM regulated fans add 4th wire - the PWM signal which says to the fan on how much % it should run. \$\endgroup\$ – Marki555 Jan 18 '12 at 7:56
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    \$\begingroup\$ @Marki555 this question is about WHY you need the 4th wire. Why can't +5v just be switched on/off at a particular duty cycle? \$\endgroup\$ – Mark Jan 20 '12 at 15:28
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    \$\begingroup\$ I suppose that without proper +5V signal, the fan electronics would be unable to determine its RPM. In fact, this is what you get when you attach some 3rd party fan regulators - they just switch the +5v at the desired duty cycle, but because the fan doesn't have stable +5v, the reported RPM is often bogus (too high values, jumps all the time, ...) \$\endgroup\$ – Marki555 Jan 20 '12 at 20:03
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    \$\begingroup\$ By the way, Marki555: Standard fan connector power is 12v, not 5v. Only PWM signal is 5v. Some motherboards do supply 5-12v to fan(s) depending on temperature. \$\endgroup\$ – user28735 Sep 9 '13 at 15:25
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While the statement that "any DC motor can be driven with PWM" is broadly correct* if the actual motor is PWM controlled, in a given implementation the motor proper may be hidden behind an internal controller, and this the case for the very large majority of devices that use small BLDCMs (Brushless DC motors).

Most small modern fans use BLDCM's.
In a BLDCM the motor speed is notionally independant of applied voltage. A range of voltages will be requied to operate correctly but within that range the voltage will have either essentially no effect on motor speed or a second order one.

If a system uses PWM to control an external motor's speed, special attention will be required to translate the speed control signal into actual control of speed. A BLDCM usually uses magnetic sensors ** (usually Hall sensors) to determine rotor position and to switch voltages appropriately. The electronics may be as simple as the sensors but more usually there is a control IC. If voltage is changed the controller will usually attempt to oppose any change and maintain constant speed. A PWM signal or a DC level could be used as a signal to a controller re appropriate speed.


  • Some DC motors are not overly keen on being PWM'd due to interesting arrangements of field coils. While small brushed DC motors in consumer equipment may use permanent magnets, larger motors tend to have wound rotors and may have fields in series ("Universal motor" as in vacuum cleaners - revs unto death if unloaded), parallel or some compound arrangement. Consultation of dry and dusty tomes and manufacturers' spec sheets recommended if ever considering PWMing "real" motors.

  • ** Some controllers determine rotor position from back-emf on windings and other esoteric schemes may exist. Hall sensors seem to be a common solution.

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The PWM input on a typical speed-controlled computer fan expects a logic-level PWM signal, usually around 20 kHz. These sorts of fans have an on-board embedded microcontroller (or similar hardware) which controls the speed by controlling the motor sequence timing (most PC fans are brushless DC motor driven).

In my experience, a BLDC motor won't regulate its speed smoothly if the input DC is chopped. If the average DC voltage is too low, they may not even start up at all, or the speed becomes erratic. Sometimes the fan will abruptly change speed, other times it will smoothly ramp up and down (it depends on the fan manufacturer and their fan control logic).

(Some Delta fans also don't like excessive jitter on the PWM signal, by refusing to change speed or taking a long time to transition.)

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  • \$\begingroup\$ You need to clarify your answer some. A BLDC motor is generally NOT a stepper motor. They rotate by a proper sequence of excitation to the motor windings similar to stepper motors but they do not operate in a cogged step by step manner as steppers do. \$\endgroup\$ – Michael Karas Apr 27 '16 at 8:03
  • \$\begingroup\$ You could also post your own answer with these details. Considering that this question is more than 4 years old though, I doubt that it will get any attention from OP. \$\endgroup\$ – Adam Lawrence Apr 27 '16 at 13:36
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    \$\begingroup\$ Removing the downvote since you updated the answer. This will make the answer more valuable to folks that come here to read this in the future. \$\endgroup\$ – Michael Karas Apr 28 '16 at 21:38
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The PWM input pin is a 5 volt pwm signal that drives a mosfet gate inside the fan.

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  • \$\begingroup\$ Right, that requires one wire + GND (already provided to the fan, right?) Why are there two? \$\endgroup\$ – Kevin Vermeer Dec 2 '11 at 22:42
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Backward compatibility.The 4 pin controllable fan standard is built on the 3 pin feedback standard which was built on the 2 pin power standard. It is fairly common to be able to plug in a 2 or 3 pin fan in a 4 pin fan connector, or vis-versa.

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