0

I am using the 3 wire PWM fan. By measuring the signal on the 3rd wire (yellow color) on the oscilloscope, the signal is square wave output. Can anyone tell me how this tach signal indicate the speed of the fan?

user2943160
  • 2,878
  • 1
  • 17
  • 32
Dush
  • 21
  • 4
  • RPM = Hz * 3600 – Tony Stewart EE75 Dec 02 '19 at 01:30
  • thanks for the comment. but somewhere i read this tach signal shows the feedback from the pwm signal. can you give some insights of the tach signal? – Dush Dec 02 '19 at 01:35
  • Look at the data sheet for the fan, the number of pulses per revolution can vary. Commonly it is 2 pulses per revolution. – Kevin White Dec 02 '19 at 01:35
  • i didn't have the data sheet of the fan that's why i am confused. i read on the internet about the tach signal. some says it;s the feedback and some says that indicate the speed of the fan, so wanted to know more about it – Dush Dec 02 '19 at 01:39
  • My understanding is the Hall sensor detects a tiny magnet 1/rev and the CPU uses the BIOS-enabled port to detect the speed. – Tony Stewart EE75 Dec 02 '19 at 01:42
  • in the bios setting i can change the fan controller to auto or full or stop mode fan. in full mode, i can see the square wave pwm signal, in stop mode there is no pwm signal. so just wanted to know the supply voltage can helps to indicate the speed of the fan? or the tach signal availability of the feedback from the fan? – Dush Dec 02 '19 at 01:47
  • I remember from a while ago that if you want variable speed and speed detection, you would need at least 4 wires. Some early motherboards modulates the PWM into the +12V supply and turned out to disrupt the tach signal, making it useless. So now the mainstream design is either 3 wire with either tach or speed control but not both, or 4 wires with both. I suggest you try to ground the PWM pin and see what happens, if the fan stops then it's the PWM control pin, not tach pin, so it's impossible to detect speed from it. Maybe you can make a table and look up speed from duty cycle. – user3528438 Dec 02 '19 at 02:32
  • Have you looked into this post? https://electronics.stackexchange.com/questions/8295/how-to-interpret-the-output-of-a-3-pin-computer-fan-speed-sensor/52877 Looks like it has your answer – eeintech Dec 02 '19 at 02:37
  • so the tach signal which is showing the PWM square wave, the duty cycle changes or the frequency changes? when full speed mode it has sqaure waves and when stop mode it has 3.3Vdc startight line. what affects duty cycle or frequenyc? – Dush Dec 02 '19 at 03:03
  • It's a tach signal. Why is this so difficult for you? Do you know what a tach is, or are you just repeating the word? Have you googled any of this? "Don't have the data sheet" can't find, or didn't look? Coz it sounds like you have not, and just want us to handhold you. On StackExchange you are expected to meet us halfway and start by doing your own research. – Harper - Reinstate Monica Dec 02 '19 at 14:10

1 Answers1

1

The tach wire emits some integer number of pulses per rotation, measure the pulse frequency to determine fan speed.

The tach pin is connected to the sensor that detects the rotor magnets as part of the driver circuit for the stator. it's just a tap into the internal working of the motor.

grounding the the tach pin will prevent the motor from running. and driving it with a particular frequency can control the speed of the rotor, and sending PWM in there could modulate the stength of the motor drive and also control the speed.

Jasen Слава Україні
  • 31,874
  • 1
  • 31
  • 65
  • tach signal depends on frequency or duty cycle? because when i'm changing the duty cycle from 10% to 30% by signal generator, upon reaching 80% then fan starts to spin. at 80 % it's frequency is around 5KHz and then when reaches to 86% it's frequency is 157hz and at the end 100% cycle it stays to around 172hz. so wanted to ask does tach signal depends on the frequency or duty cycle? – Dush Dec 02 '19 at 10:42
  • tach signal depends on the position of the rotor. frequency of the signal thus depends on the speed of the rotor. – Jasen Слава Україні Dec 03 '19 at 00:12
  • pwm affects the speed of the fan and thus the tach signal – Jasen Слава Україні Dec 03 '19 at 00:14
  • and how do calculate the rpm of fan? i used the above formula, rpm = hz*3600. it coming out to be 619200 since frequency is 172 hz. how do we get to know each rotation sends two pulse? since i used the formula rpm =(172/20)*60=5160. this is correct rpm speed. so wanted to know why frequency divides by 2? – Dush Dec 03 '19 at 03:33
  • 1
    Are you sure you can drive the tacho pin to control the fan? it should not be the case – FarO Mar 02 '22 at 09:56
  • that should work for one pole fan motors – Jasen Слава Україні Mar 03 '22 at 01:36