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Thread: connecting case 12V fan to to Mobo or, LED driver 12vdc auxiliary output

  1. #1
    Registered User
    Join Date
    Sep 2013

    connecting case 12V fan to to Mobo or, LED driver 12vdc auxiliary output

    Newbie questions

    Trying to understand basics of how rated current for fan is not exceeded when powereing from a Mobo, or an
    auxiliaary 12vdc power supply output

    !. Mobo. I am going to use 2 Noctua A14Flx fans - 12vdc, 80Ma each (160Ma total)

    Q - how can the Mobo's current be limited to that specifically needed by fan?
    Example, One Noctua fan, needing 80Ma. I plug in the molex and everything works.
    If I have 2 fans connected via Noctua's Y-splitter needing 160Ma I can plug the connector from the
    Y-splitter into the Mobo and everything works OK.

    How is the current from the Mobo being regulated to match that needed in both situations requiring
    differnt max currents?

    2. LED driver auxiliary 12vdc output. Since the rated current exceeds 350Ma, how can I control, downregulate the current to 160Ma and safely power both Flx fans. I read about using a potentiator control but I understad that the potentialtor only modulates voltage. Do I use a resistor.


  2. #2
    A fan or other power consuming device only draws the power it needs to run. The voltage is controlled but not the current. The mobo or a power supply doesn't regulate the current supplied to the fans unless the power draw exceeds what the mobo or PSU is capable of providing, then the regulation is just a result of the power source having insufficient capability.

  3. #3
    Registered User
    Join Date
    Sep 2013
    I am a bit new to Mobo and PSU. But I can undertand now that a Mobo/PSU act like a power station and any device, such as a fan, that is connected will "request/demand" the current it needs from the Mobo or PSU (i assume voltage is already built-into the fan headers,
    generally @ 12vdc )

    What cinfused me is that I am more familiar with LED drivers and a high power LED that will be connected to the driver is a slave device and doesn't "request/demand power but instead is forced fed what the driver will putput at its rated VDC range. So unless I can control the power flow, first in volts (that is alredy fixed by VDC range) followed by the inherent current of the output at the rated volatge the
    LED will be forced fed that power. That's why high power LED users will generally order a constant current driver so that they may fix the current at the proper amp profile of the LED.

    That"s a long way to say that once I connect the fan to the auxiliary output of the driver I will be force feeding the fan at the inherent
    current at the 12VDC range of the wattage driver model (inherent current pushed out by a 12vdc output from a single output 200w
    driver will be ~twice then that from a 12vdc 100w driver.

    For the reasons discussed above, I believe there is still a problem with overpowering the Noctua
    case fans with unregulated current from a 12vdc auxiiary output of a LED power supply. Even a 12w driver @ 12vdc would put out ~1A.
    The amperage problem can be solved if the auxilairy line is the second channel of a 2-output constant current driver and one can fix the current manually on the auxiliary output. But I must use power supplies without a mechanism to adjust constant current or VDC (I can
    live with the range of VDCs of certain wattage power supplies (a 150w @ 36v (vdc range 32-39v) but I can't lock-in the current setting but again, can live with its inherent amp property. So I still have to deal with a 12 vdc auxilairy output current exceeding 500Ma.

    I think that is right and thought a Mobo/PSU "put" out amps in a similar manner.
    Re-formulated Question - Is there a device like a potentiometer fan controller, but instead of modulating voltage would regulate

    Thanks much for clearing up my Mobo/PSU mental block.

  4. #4
    An LED current driver is essentially a current limiter. It allows you to supply a fixed maximum quantity of current to an LED to control it's brightness and prevent damage or a shortening of the LED's life from excessive power consumption in an unregulated state.

    A normal DC power consuming device (such as a fan, incandescent light bulb, motor, etc.), works on internal resistance to regulate/determine it's power consumption. Thus it should not matter what the LED driver allows for maximum current when used with a fan or similar device as long as it supplies sufficient current to meet the power consuming device's needs. You can't force a fan or other device to consume more current than what it wants to use based on it's electrical resistance. The LED driver in this case would just be acting as a small PSU as far as the fan or other DC device is concerned.

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