How to PWM 3-Wire PC fan, and have clean tach output

Discussion in 'General Electronics Chat' started by Mad Professor, Feb 9, 2010.

  1. Mad Professor

    Thread Starter Active Member

    Apr 15, 2009
    Good Day All.

    I tought I would start a new topic as I am starting from a fresh sheet of paper.

    I know the best route to go down, would be to buy new 4-wire PWM Fan.
    But I am yet to find any 40x40x28mm 4-Wire PWM Fans here in the UK.

    I have been playing around with the idea of using PWM to control the fan speeds for my U1 Cobalt RaQ 550 Servers.

    I used low-side siwtch at 20kHz and the fans run fine, but the tach signal is a mess and the servers can not read the right fan speed, and shuts down after 5 sec.

    The server fan monitoring can not be disabled.

    Here is the fan running at 20kHz PWM @ 95% Duty Cycle.

    And again at 100% Duty Cycle or full power, how ever you want to read it.

    Blue trace is the PWM output from a PIC.
    Red tracie is the Fan Tach output.

    I am now looking at some way of getting PWM to work and have a clean tach output.
    Or some other way of driving the fan with adjustable power, like an adjustable Voltage Regulator, but if this is possible I don't know how I would interface it with a PIC.

    Can you please advise.

    Thanks for your time.

    Best Regards.
    Last edited: Feb 24, 2010
  2. boriz

    New Member

    Jul 16, 2009
    Simple RC low pass filter on the TACH line should clear that up. Something like this:


    Fingers crossed it goes into a Schmidt type input on the motherboard. If not, a comparator/buffer may be required to ‘square’ up the pulse edges (Not very likely though).

    Your messy TACH signal should look something like this after it’s been through the filter:
  3. Mad Professor

    Thread Starter Active Member

    Apr 15, 2009