John, My earlier message was wrong about the signal direction of the white or yellow wire, but the signal wire is not only for whether the fan is rotating or not (to check if it is stalled to avoid too much compensation current) but to check the current speed of the fan to determine whether to increase the pulse-width or not, and to determine a synchronizing signal to time pulse to the fan (generally not used in this fashion for a PC fan, but in big motors like those cooling the braking resistors of a railroad locomotive.
If you didn't already know, PWM, or pulse width modulation, changes the time on versus the time off of a square wave without changing the frequency (called the Duty Cycle). (courtesy of Circuit Digest "What is PWM?" https://circuitdigest.com/tutorial/what-is-pwm-pulse-width-modulation) The full power during the on portion overcomes the angular torque problem of turning the roters instead of a gradual increase in voltage like that of a sine wave (building up the magnetic field more quickly). The inertia of the rotating blades acts to smooth and average the pulses into an even movement - especially at higher frequencies of the pulse stream -- not as important for fans motors but really useful for things like model trains. Electronically, the average amount of power delivered to a motor is proportional to the duty cycle: What is PWM: Pulse Width Modulation (ref: Circuit Digest) This works for dimming LEDs and other devices or components that require full voltage to operate. This averaging effect can be done electronically as well, passing it through a low-pass filter to extract out the fundamental frequency components (a square wave is an {practically} infinite sum of sine waves of the odd-harmonic frequencies), and by rectifying the pulse stream and then using a capacitor to smooth the ripple. This will result in a voltage proportional to the pulse width. See also: Arrow Electronics: What is PWM? Pulse Width Modulation Explained" https://www.arrow.com/en/research-and-events/articles/what-is-pwm-pulse-width-modulation-explained See these videos: "Electronics Basics 23: Pulse Width Modulation" https://www.youtube.com/watch?v=GQLED3gmONg "What is PWM" https://www.youtube.com/watch?v=B_Ysdv1xRbA - sgh On 2/2/21 1:43 PM, John Vaughters via TriEmbed wrote: > Computer fans are almost always brush-less, actually never seen a brushed > one. Because the electrical noise is too high. She 3 pin motors are simply > two power pins plus one tachometer pin, which you generally only want to > determine if the fan is failing. I could be wrong about this part, but I > expect that they are single voltage and reducing the voltage may or may not > affect the speed much and possibly could damage the fan over long use. > However, I seem to remember some schemes to make 3 wire fans speed > controlled. it's worth a look online I suppose. > > John Vaughters > -- Scott G. Hall Raleigh, NC, USA [email protected] _Although kindness is rarely a job, no matter what you do it's always an option._
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