[TriEmbed] laptop fan motor control question

[Scott Hall]

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
ScottGHall1 at GMail.Com

_Although kindness is rarely a job, no matter what you do it's always an option._

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.triembed.org/pipermail/triembed_triembed.org/attachments/20210202/ccd08e59/attachment.htm>