reducing voltage noise

Discussion in 'General Electronics Chat' started by Mark3722, May 23, 2015.

  1. Mark3722

    Thread Starter New Member

    Dec 26, 2014
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    Hi All,

    I have a project where I need to drive a 12v 0.15A computer fan with a mosfet in addition to a raspberry pi which will control the mosfet with a DAC board.

    My initial thought was to get a 12v supply and a buck power converter. Before going to far, I wanted to get an idea of what would happen with respect to noise. Suprisingly, I am seeing massive voltage spikes on my scope when measuring both the 12v and 5v from the buck converter with the fan where I do not see this with an incendecent bulb.

    Clearly the fan is pulling back a lot of noise. Is there a simple circuit I can implement to reduce this noise level?

    Thanks,

    Mark
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Most PC fans now have internal circuitry and BLDC commutation, the DC brushed are rare now in PC's.
    Max.
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Try connecting a low ESR fat capacitor (tens of uF) across the fan.
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    While the repetition rate of BLDC fan noise is very low (3600 rpm is only 240 Hz pulse rate), the noise bursts have high frequency noise. A pi filter works best, 100 uF in parallel with a 0.1 uF ceramic, the biggest inductor you can fit in your application, and another pair of caps. A separate ground return from the fan to the ps will help, but not as much as the caps.

    ak
     
  5. ian field

    Distinguished Member

    Oct 27, 2012
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    There never seems to be much in the way of electrolytics inside those things, whether its commutated mechanically or electronically, there's going to be spikes that end up somewhere.
     
  6. Mark3722

    Thread Starter New Member

    Dec 26, 2014
    13
    0
    Hi All,

    I am back with some more data. Attached are 2 images of interest.

    One of the images shows the actual fan specs. It is a 12v brushless fan that pulls 0.15A.

    The other image is of the scope. As you can see, there are some spikes that occur every 5ms (200hz) of around 4V. From what I can tell, there are 3-4 harmonics that all seem to fit into this profile at 50hz, 100hz and 200hz. If I were to build a Pi filter, what size of caps and inductor would I use where I might be able to salvage the inductor from some consumer electronics PCBs? I did some of the math and came up with LC = 0.634.

    Any guidance on this greatly appreciated.

    Mark
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The Intention of my previous post was to point out the possible futility of trying to control it with a PWM signal when the fan most likely has a BLDC controller internally to control rpm.
    If control of the rpm was the intention.
    Possibly has one of these already internally http://www.microchip.com/wwwproducts/Devices.aspx?product=TC652
    Max.
     
    Last edited: May 24, 2015
  8. Mark3722

    Thread Starter New Member

    Dec 26, 2014
    13
    0
    Quick update after some scrounging for an inductor and some breadboard work.

    I found an inductor on an old wifi router and placed it in a Pi configuration with 2 100uF capacitors. I have no idea what the L value in Henry's of the inductor is.

    Just the mere presence of the inductor seemed to have created some level of impedence such that the voltage swings dropped from 4.5 to 1.5 volts. On the other side of the Pi circuit, the noise is still there but must be less than 50mV. Niiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiice. Now to figure out why..... :)

    The inductor seems to have around 30 winds. Would anybody care to guess the L value? I am looking at a youtube video to calculate L in Henry's but will have to do this another time....

    Mark
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Depends on the core material, I just wound one almost exactly on large ferrite core rings and they measure +600μH.
    Max.
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
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    From time to time there's an occasional magazine project for controlling PC fans, but I don't really take a lot of interest in those.

    Plenty of PC PSU boxes had very simple fan speed control, just a thermistor and a few transistors for enough gain and current capability to drive the fan.

    I just assumed the TS would have the usual LC filter on the output rather than just feeding raw PWM into the fan.
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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  12. ian field

    Distinguished Member

    Oct 27, 2012
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    Sometimes when Elektor publish one of my projects - they send me a little gift as well as monetary reward.

    Found a couple of them while clearing out an old cabinet yesterday - there's some sort of Micro by Maxim and a little Philips LPC board.

    When they arrived I looked them up online and downloaded a load of documentation/development tools - just never had time to fire them up and see what they do.
     
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