I was reading an articles about brushless DC motor, used in CPU fans, from this link. First I thought that the Hall sensors are used only for measuring the speed, but that is not correct, it has something to do with the current flow in the coils. Because, even a two wire CPU fan has a hall sensor inside. Can anybody explain the exact use/uses of this sensor in a BLDC?

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3 Answers 3


Your question seems to center around the use of the Hall Effect sensor in a 2-wire fan. I believe what you are asking is, "If the Hall Effect sensor isn't feeding a tachometer signal back to the motherboard, what is it doing?"

In a two wire system, the only signals fed in a voltage and ground. There is no control. So what is it that is alternating the phases and causing the rotation? If it were just a DC signal, the permanent magnet on the rotor would spin until it's magnetic field aligned with that of the coils in the stator and stay in that position.

Instead what happens is, a DC is fed in and it finds the path of least resistance to ground. Assuming the transistor on the left is open, it would flow through the coils and transistor on the right. The rotor spins and the magnetic field passes the Hall Effect sensor. The Hall Effect sensor sees this rotation and turns on the transistor on the left. This side now has a lower resistance so the current flows through it, reversing the field in the stator which pushes the rotor around again. The Hall Effect sensor senses a change in the magnetic field from the continued rotation of the rotor and then turns off the left transistor. Current then flows through the right.

This alternation continues as long as power is supplied and is the source of the alternating magnetic field that causes the rotation of the fan.

  • \$\begingroup\$ What you got from my question is exactly the same as I intended. You can edit the question if you wish to do so. \$\endgroup\$
    – noufal
    Jan 3, 2014 at 14:33
  • \$\begingroup\$ @noufal Your question was clear to me so I see no reason to edit. But the other answers seemed to answer a more general version of your question. I figured I'd just clarify the specifics in my answer. \$\endgroup\$ Jan 3, 2014 at 14:36

The hall sensor tells the BLDC controller when the rotor is in the right position to switch over to the next coil. Using a hall sensor is much easier than measuring the back EMF, another method of determining the time for switching. It's also mentioned on the website you linked in the second last paragraph.


Hall sensors measure magnetic fields so they can have 2 functions..

  1. as a Tacho output by measureing the pulses as the magnetic field changes in the motor due to it rotating. So you can determine if the fan has stopped.
  2. as a sensor for measuring the magnetic field and hense current in a DC circuit. If the current gets to high the motor has stalled (motors are similar to a short circuit when they are not rotating) or they have failed in some way (current to low or to high)

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