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Low Pass filter doesnt work ?

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Discussion in 'General Electronics Discussion' started by Ultrasonic2, Jul 14, 2012.

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  1. Jul 14, 2012 #1
    Ultrasonic2

    Ultrasonic2

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    Jul 14, 2012
    i'd like to make a high powered fan controller for my PC based on the Arduino

    Now this works perfectly however the noise the fans make from the PWM frequency is terrible.

    I thought to my self no big deal i'll just add a low pass filter on and it will be converted to anology and everything will be sweet but adding a cap across anything either gives max volts or does nothing ?

    im lost .

    please help

    [​IMG]


    [​IMG]
     

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    Last edited: Jul 14, 2012
  2. Jul 14, 2012 #2
    CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Not surprising. A big cap like that swamps out your PWM. You should be using bypass/decoupling techniques on your power rail to the uC. I'm assuming that you mean electrical noise and not audible. On the other hand, if it's audible hum that's offensive, a small cap like .1uF across the motor might help. Most of the noise is generated on the rising and possibly falling edge.
     
    Last edited: Jul 14, 2012
  3. Jul 14, 2012 #3
    (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Agree on the capacitor. And also note that a reverse biased diode across the fan is also a very good thing.

    If the noise is audible, the simple answer is to adjust your PWM frequency. You may find a frequency where the motor doesn't make a noise, or you can simply increase the frequency to above the audible range.
     
  4. Jul 14, 2012 #4
    Ultrasonic2

    Ultrasonic2

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    Jul 14, 2012
    i was referring to audible noise. i can set the arduino to 122hz to 32k which i can hear the noise at all those points and between.

    i turns out my design does work.

    im running 2000uf across the load. While the fans stopped making a noise i could hear a noise coming from the PSU powering everything So i've added caps across the incoming power too
     
  5. Jul 15, 2012 #5
    (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    A capacitor across your power supply is good.

    A capacitor across your load is bad.

    A diode (reverse biased) across your load is good.

    Be as good as you can be,

    What sound do you hear is 32 KHz? Is it a very high pitched whistle?

    I would also consider using a mosfet to switch the load (but this depends on the current and your actual drive circuitry). If your load is under 500mA, an arduino driving a transistor might be OK. Above that you need something better and a darlington will cause you to drop a significant voltage.
     
  6. Jul 15, 2012 #6
    Ultrasonic2

    Ultrasonic2

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    Jul 14, 2012
    im not sure you actually looked at the pics i posted, there is a mosfet in there because im switching more than the 5v.

    The cap across the load is fine that part of the lowpass filter which converts the signal to anology
    http://sim.okawa-denshi.jp/en/CRlowkeisan.htm
     
  7. Jul 15, 2012 #7
    davenn

    davenn Moderator

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    you dont show a MOSFET where is it ? I see a NPN bipolar transistor
    converts the signal to anology ?? huh what's that ? :)

    Dave
     
    Last edited: Jul 15, 2012
  8. Jul 15, 2012 #8
    Ultrasonic2

    Ultrasonic2

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    Jul 14, 2012

    NPN bipolar is actually a hex mosfet . wrong graphic. at the time of writing the pic of the mosfet didn't mater as the question was about the lowpass filter
    and the low pass filter converts the digital signal to a analogy one
     
    Last edited: Jul 15, 2012
  9. Jul 15, 2012 #9
    Ultrasonic2

    Ultrasonic2

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    Jul 14, 2012
    [​IMG]
     

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