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Thread: help with fans

  1. #1
    Constant Bitrate tibimakai's Avatar
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    help with fans

    I have installed a 5v 25x25mm fan and it's very noisy. I would like to drop the voltage to make it spin slower. how would I do that?
    second issue is how to filter fans to reduce interference? the more fans I'm installing the more interference I have on the lcd.

  2. #2
    MySQL Error soundman98's Avatar
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    your building a all-indash setup right?

    you should be able to use resistors to drop how much voltage gets to the fan. just connect in series with the red wire, or in series with the black wire. if you have a radioshack nearby, they have a assorted 1/4w, or 1/2w resistor kit that would be perfect for this.


    with the interferance issue, i assume your using the pc power supply to power the fans. i wonder if you connect the fans to their own buck- voltage regulator to separate them from the pc components. the second idea is to use RF chokes, or different vga cabling for your screen..

    but if you have a all-indash setup, i wonder if the interference is due to the close proximity of the fan/screen. in this case, you might need to look into shielding the pc components.. an easy way to test for this would be to connect the fans to a external power supply, and see if you still have the issues..

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    Interference from the fans cannot be RF, either it's magnetic interference from the coils and magnets, but again, i never seen an circuit sensitive to this at all (except from an old CRT monitor tube) but more likely yout supply voltage is jumping up and down a bit, so you need to add some capacitors on the 12 volt line rail you use for powering your LCD (and fans) and that should help with your problem.

    Start out with something like 4700 to 10000 uF/25V and see how it goes.

    // Per.

  4. #4
    MySQL Error soundman98's Avatar
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    ^ in line with the thinking that he is using a super-compact in-dash pc, and is powering the fans, pc, and screen off the same 12v rail of his dc-dc psu, wouldn't he also need to add a diode to the + line before the capacitor to prevent the capacitor from back feeding into the pc components?

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    Quote Originally Posted by soundman98 View Post
    ^ in line with the thinking that he is using a super-compact in-dash pc, and is powering the fans, pc, and screen off the same 12v rail of his dc-dc psu, wouldn't he also need to add a diode to the + line before the capacitor to prevent the capacitor from back feeding into the pc components?
    Eh, No.

    Think of the power supply, it has capacitors on all the rails, it isn't back feeding anything. The Mobo has capacitors on the supply lines too.

    Adding a diode is just asking for trouble and getting an unwanted 0.4-0.6 volts drop in the voltage.

    // Per.

  6. #6
    MySQL Error soundman98's Avatar
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    doh... completely forgot about those caps, and should've remembered the diode voltage drop.

  7. #7
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    Quote Originally Posted by zapro View Post
    Interference from the fans cannot be RF, either it's magnetic interference from the coils and magnets...
    Er, that's called RFI - eg, EMI, EMC etc.

    It is electromagnetic interference. (Can you separate them?)

    Note too - small DC fans (as used in PCs) are usually AC....

  8. #8
    Constant Bitrate tibimakai's Avatar
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    the problem it was at one of the fan power plugs on the mobo causing the interference(on the Intel Atom 230 mobo, the side one). when I have unplugged that fan the intereference stopped. so I played with speedfan, dropped from 50% to 20% and the interference it's gone.
    now I have to deal with the voltage drop from 5V to something adjustable to see at which rpm would work better? any ideas? couple of diodes in series maybe? pot would be a much better solution.
    the fan is a 5V fan.

  9. #9
    Maximum Bitrate Mickz's Avatar
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    As soundman98 said, the usual way is with a resistor, depending on the fan and required speed you may need a couple in parallel or a 1watt resistor if they get too hot.

    Depending on what the fan cools, where the PC is mounted and the environment itís in it may not be in your best interest to slow the fan too much.

    The fan should not be causing interference and by slowing it you may be masking a future problem.
    Have you tried another fan to see if the interference stops?
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  10. #10
    Constant Bitrate tibimakai's Avatar
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    the interference it's gone by switching two fans(gpu with cpu) and by lowering the % in speedfan.
    this fan it's cooling the hdd only.
    I have another 25mm fan, I can try that too.
    my problem is that I'm afraid that when all mounted it will be to noisy.

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