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4-pin pwm fan.

Discussion in 'Electronic Repair' started by [email protected], Jan 31, 2008.

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  1. Guest

    I work as a coputer tech (fixing pcs) in a major university as part of
    a work study program. Im also a junior in computer engineering, so I
    know my way around a breadboard.

    One of the machines is a newish dell btx, that's having major cooling
    problems (random page faults, hd temp goes up to about 130 F, etc). I
    narrowed the problem down to the main case fan, which is a four pin
    PWM (pulse-width modulation). Apparently, the bios (happens without
    hd/os) forces the fan to spin at a ridiculously low speed,
    approximately 10% of capacity. After tinkering a bit, I found that
    cutting the blue (PWM) wire was adequate enough to disable pwm, and
    forced it to 100%. But now it sound's like one of those handheld
    vacuum cleaners, and is likely to have a very low MFT (mean failure
    time). I've tinkered a bit more, and found that by grounding the pwm
    with a suitable resistor, it slows it down to an acceptible level.
    I've run a few tests, and I've come up with the following data:

    Fan Power draw at 100%: ~375 mA
    Ideal Power draw (flow vs sound): ~210 mA

    Pwm voltage (fan to ground): ~3.266 V
    Pwm Current (directly grounded) ~0.52 mA
    Ideal Pwm>Resistor>Ground: ~3.2 kOhm

    I was wondering if anybody had any additional input about this before
    I screw something up royally. With these Ideal values, everything
    seems to work fine, and I don't notice any risky voltages or
    currents. I'm on a tight schedule, so I'm likely to begin soldering
    everything into place soon.

  2. James Sweet

    James Sweet Guest

    The fan should run just fine at full power without significantly shortened

    You can get small fan speed controllers that are standalone, they're cheap.
  3. mng

    mng Guest

    It's only 0.5 mA so you'll probably be okay.

    Non-hardware options: Call Dell and complain. Use SpeedFan (software
  4. TonyR

    TonyR Guest

    I had a noisey fan like that once and put a resistor in series with it to
    make it run at 9V which was much quieter. It ran for years and is still
  5. Rich Grise

    Rich Grise Guest

    If this box belongs to a customer, don't install any hacks. If you're
    stumped, ask the supervisor to help or give it to one of the other
    techs. It sounds like you need either a new fan or a new MB. But I
    would not trust a fan that's got a resistor to the pwm lead, and I
    would not trust a repairman who hacks on my equipment trying to fix
    some problem he doesn't understand.

    Good Luck!
  6. Ecnerwal

    Ecnerwal Guest

    If it's newish, call Dell and have them send you a new motherboard, or a
    new fan, or both, under warrantee.
  7. Guest

    I've considered the speed fan route, but the machine is most likely
    going to spend most of it's time with a logon prompt, which very few
    aplications will continue to run on. It'll mainly be used for data
    entry and research. To machine itself is pseudo-departmentally owned,
    and this seems to be the most reliable technique. I considered using
    a resistor on the 12 volt line, but that would require either a three
    watt resistor, or 12 quarter watts in parallel (I can get .25W's for
    free, but ppl will ask questions if I take that many. It'd be a mess
    too). Putting a 2.6k resistor between pwm and ground is the most
    sensible (and fastest) way to go, especially since I need this up and
    running tommorow; plus at 1/588 watts, it is well within the
    tolereance of the available resistor(s). I have tested this approach
    thourougly enough to make it final, and all of the variables are well
    within range.
  8. Baron

    Baron Guest

    I agree with Rich. You don't have the knowledge or skills to repair
    this. Speak to your supervisor!
  9. Guest

    I agree with Rich.  You don't have the knowledge or skills to repair
    But mom! Already did. I did the hack (with the proverbial nod), and
    it works fine. Still a little bit on the loud side, but not nearly as
    bad as it was before.
  10. Guest

    I dunno about that. He did after all narrow down the HDD faults to
    improper cooling. That's pretty good.

    That's always a good idea. If s/he is around, that is. Besides, I
    don't think having a noisy fan is that big of a problem. Better to
    have more cooling.

  11. Ian Malcolm

    Ian Malcolm Guest

    If you experiment with adding a diode in the line you cut with the
    cathode towards the motherboard, Your resistor to ground (on the fan
    side of the diode) should set the minimum fan speed but the motherboard
    could still increase it as required. If you've got a fan extension
    cable spare you could butcher, I'd splice neatly and heatshrink sleeve
    the cut wire and make up the kludge as a plug in unit, (avoid hassle if
    it ever needs to go to Dell).
  12. Franc Zabkar

    Franc Zabkar Guest

    The PWM input is a logic level pulse train, either on or off.
    Therefore it makes no sense to add a resistor between the PWM pin and


    - Franc Zabkar
  13. Ian Malcolm

    Ian Malcolm Guest

  14. Franc Zabkar

    Franc Zabkar Guest

    IMHO that's not a good idea because the fan's electronics expects a
    stable DC supply for the Hall sensor and possibly for the PWM circuit.

    - Franc Zabkar
  15. James Sweet

    James Sweet Guest

    These fans are almost always controlled by an external PWM circuit, the
    electronics in the motor are very simple and work fine with this
    arrangement. Internally temperature controlled fans are available, but
    relatively uncommon.
  16. Jamie

    Jamie Guest

    Not to stir up the pot or anything..
    But I've seen a couple of CPU 2 wire fans, designed to operate
    from 5 Volts (Low speed) up to 12 volts (full speed).
    the internal electronics is regulated for 5 volts always
    and uses the incoming voltage to govern speed for the
    controller internally.
    The last MB I played with used a PWM circuit into a local
    LC circuit that generated a rather smooth variable DC voltage
    for the fan supply.
    On this MB, a jumper had to be set to indicate you having one of
    these fans.
  17. Franc Zabkar

    Franc Zabkar Guest

    OK, I stand corrected. However, here is an interesting thread that
    shows that strange things can happen when you reduce the supply
    voltage of an internally temperature controlled fan:

    Specifically, the fan RPM may *increase* when you *reduce* the supply
    voltage. Of course the OP's fan is not one of these.

    - Franc Zabkar
  18. Baron

    Baron Guest

    He is talking about the case fan in a Dell machine which draws 0.2 amp @
    full power. Dropping the whole 12 volts at that current would be 2.4W

    If we assume that we use a resistor to loose 3v, that would only be 0.6W

    This guy needs supervision and guidence !
    Maybe he did ! But he his changing the manufacturers design to disguise
    a fault ! I would not accept a repair done in this manner. In simple
    words "its a bodge" find and cure the fault.
    More cooling is always better.
    If the machine is your own property you can do what you like with it !
    In this case its not. If his supervisor takes that decision, then
    fine, it becomes the supervisors responsibility.
  19. cpemma

    cpemma Guest

    No it wouldn't, if you dropped 3V off the supply the fan would be drawing
    around 280mA and the series resistor dissipating 0.85W

    A better way with case fans is a 1.3W zener in series with the 12V line.
    Plenty of values in the 3V-5V range to pick from.

    But first check the BIOS. There should be an option, possibly on the ""PC
    Health" page, to give either PWM control or voltage control to the fan
    headers, "for fans that don't meet the Intel 4-wire fan spec". Or swap the
    fan for a quiet 3-pin, they still fit and work on most boards -- they've got
    to, 4-wire fans are still rare.
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