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200 vs. 1200 ohms in laptop fan

Discussion in 'Electronic Basics' started by Nick, Jun 27, 2007.

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  1. Nick

    Nick Guest

    Is a small fan for a laptop computer basically in working order if its
    resistance measures about 1200 ohms +/- 4% when it should be 200 ohms?
    I don't know the proper design tolerance.

    The fan is labeled for 12VDC and 0.06A, which is how I arrive at the
    200-ohm calculation. I measured it using a cheap multimeter. The
    measurement tolerance is from the literature published with the
    multimeter. The multimeter shows virtually no resistance when the
    probes touch each other or when I measure a new incandescent 60W
    120VAC light bulb.

    The fan is from a Compaq Armada 7800 laptop, and it appears to not be
    turning anymore, so the laptop refuses to run (preventing
    overheating). However, I can turn the blades with my fingers,
    apparently normally. I haven't decided whether to replace the fan or
    if the problem is elsewhere.

  2. Lord Garth

    Lord Garth Guest

    As you surmised, the problem isn't always a defective fan. In my Vaio, the
    temperature sensor was not functioning. I wired the fan directly to a
    of 5 volts so that the fan runs as long as the laptop is on.

    A further problem is that the interface between the CPU and what ever other
    IC are using the heat sink needed tending. In my case, the video controller
    and the CPU has a pad that conducted the heat away. That pad had
    with age and required removal. I used thermal grease in its place. My
    result is
    that the laptop no longer shuts down at inappropriate moments.
  3. Phil Allison

    Phil Allison Guest

    ** Computer fans contain an electronic drive circuit - they are not

    ** Forget the multimeter.

    Connect the fan to a 9 volt radio battery - see if it spins.

    ........ Phil
  4. Eeyore

    Eeyore Guest

    It's electronically commutated and you can't measure any meaningful

  5. Nick

    Nick Guest

    Thanks. A 9V battery seemed to spin the fan just fine. Also, if I
    reversed the leads from the battery it did not spin, which seems to
    support comments above. I may be wrong on the resistance amount
    anyway. I assume the problem is somewhere else, that somehow power is
    not getting to the fan. Using an external power supply just for the
    fan probably means adding an external fan and a funnel, not good where
    I use the machine. In reference to LG's heat sink losing contact with
    what's hot, my machine is probably not overheating elsewhere other
    than as a result of the fan not turning. I could probably buy a 2d-
    hand system board but it's probably not worth it economically, given
    that the problem might be the power supply getting weaker or some
    other circuitry being bad, given other failures the machine's begun
    suffering, and compared to buying a working laptop. Thanx again.
  6. Lord Garth

    Lord Garth Guest


    My first shot at problem was to wire power directly to the fan.
    When my system is on, the fan is on. I came to this conclusion
    because the system was obviously hot and at power on, the fan would
    spin for about 5 seconds then shut down. The computer soon
    shut down as well. I cooled it with some freeze spray and the
    system ran until it again over heated.

    Do not ever reverse the polarity to the fan! It has no brushes and
    will not spin backwards. The fan uses electronic commutation and
    while many designs have a shunt diode in to protect from this fault,
    some do not. You risk breaking the fan. It's probably hard to find
    a suitable replacement. The fan assembly in this Sony was about
    $80 because it is an integral part of the heat sink assembly.

    By-passing the temperature sensor and connecting the fan to
    switched power, which I took from the hard drive connector, was
    the answer. Well, that and thermal grease.

    How are your soldering skills?
  7. neon


    Oct 21, 2006
    god why you cannot think try the fan to 9v was one solution the other is put an external fan to see if it is heat or bad components failing. my bid is a bad capin the power supply with age they begin to leak and causes problems. put the thing into an ice box if you have too but find your trouble there is no short cuts just elimination
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