# 3 wire PWM fan tach signal [duplicate]

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?

• RPM = Hz * 3600 Dec 2 '19 at 1:30
• 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 2 '19 at 1:35
• Look at the data sheet for the fan, the number of pulses per revolution can vary. Commonly it is 2 pulses per revolution. Dec 2 '19 at 1:35
• 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 2 '19 at 1:39
• My understanding is the Hall sensor detects a tiny magnet 1/rev and the CPU uses the BIOS-enabled port to detect the speed. Dec 2 '19 at 1:42

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.

• 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 2 '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. Dec 3 '19 at 0:12
• pwm affects the speed of the fan and thus the tach signal Dec 3 '19 at 0:14
• 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 3 '19 at 3:33