EMI on a USB Extender

Discussion in 'The Projects Forum' started by seascan, Aug 28, 2010.

  1. seascan

    Thread Starter New Member

    Aug 28, 2010
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    I am working on a project that has 6 12V motors (each about 20W). The motors are controlled by a relay board (12 relays total). These relays are activated by a USB DAC that is plugged into a USB Hub.

    Here's where it gets tricky: All this stuff is about 100' from the controller computer. A USB over CAT5 extender is being used to handle this. All the cables for the motors and the CAT5 must be bundled tightly together at the far end. Also, all the devices at the far end are being powered by the same 12V battery (some voltage regulators are being used to get 5V for the USB Hub and relay boards).

    The CAT5 and the USB extender devices are shielded but, the motors are causing the USB connection to drop out routinely. Does anyone have any recommendations on what I can do to the CAT5 or the motor cables to remove this interference?

    When I plug the USB Hub directly to the computer everything works perfectly which makes me wonder if there is something I can do with the CAT5.
     
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
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    201
    First thing I'd try is some 0.01 uF caps across the CAT5 lines at each end.
     
  3. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Do not use the 12V power at the far end to power your relay board and USB HUB.

    Get 12V power directly from the source, via either adding a new cable or use some spare cores in the CAT5 cable.
     
  4. windoze killa

    AAC Fanatic!

    Feb 23, 2006
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    I would add some extra filtering on the regulators. Also some small ceramic caps on the motor terminals to ground. Maybe instead of using CAT5 UTP try CAT5 STP (shielded). Only ground the source end (PC) of the cable otherwise you may get earth loops.
     
  5. seascan

    Thread Starter New Member

    Aug 28, 2010
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    0
    Would installing a chokes on either end of the CAT5 work?

    Also, for the those that recommend small caps on the motors -- can these be installed where the motor cables attach to the relay boards?
     
  6. marshallf3

    Well-Known Member

    Jul 26, 2010
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    You'll have to experiment with capacitor placement, as a rule the closer to where the noise is coming from is best.

    Chokes could be used but they cost more than caps and unless the motors draw virtually no current you'll need some that won't insert too much resistance in the circuit.
     
  7. windoze killa

    AAC Fanatic!

    Feb 23, 2006
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    I would install the caps as close to the motor as possible otherwise the wires running from the motor to the relay board will act like an antenna and radiate what you are trying to disspate.
     
  8. seascan

    Thread Starter New Member

    Aug 28, 2010
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    0
    Would the recommendatioin for the cap size on the ethernet cable still hold for the motors (0.01 uF)?
     
  9. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    605
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    It would be a good start. Grab a bunch of different sizes and see what works best. I wouldn't go above about 0.1μF.
     
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