Cable protection from noise and distortion

Discussion in 'General Electronics Chat' started by Dritech, Apr 27, 2014.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Hi all,

    For a project that I am doing, I will have two power cables (5V and 12V) passing from the same cable carrier and trunking (total length of approx. 4m). The 12V will be used for powering the motors while the 5V will be used for powering the MCU and other components. Since the 12V cable with carry high current, I am concerned about noise in the 5V cable.
    For eliminating noise I was considering two options:

    1) Instead of 5V, I pass 8V from the cable and then use a regulator with filtering at the end of the cable. This way any noise in the cable will be filtered out.

    2) Replace the 5V standard cable with a shielded cable.

    Which is the best option? Will these methods eliminated any noise or distortion ?
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,791
    1,103
    IMO either solution would help. You also need to prevent motors and the MCU from sharing a ground return path. Use a star ground system.
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    Generally noise in such a system is caused by ground currents, not radiation. You should use two twisted-wire pairs and keep the motor ground current away from the electronics ground as much as possible. Using a star ground at the power supply source is the preferred way if possible as Alec mentioned. Shielded cable should not be needed. You might use a ferrite bead at the end of the cable to improve the MCU power filtering if there are any high frequencies involved in the motor control (e.g. PWM).

    Will the MCU be driving the motor? Is PWM used to control the motor?
     
  4. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Thanks for the replies. So passing signal wires in the same trunking used for the motors power will not cause any problems if the ground is not common? Is shielded cable totally useless?

    Yes the MCU will be driving the motor using PWM, but I used a high speed opto-isolator to eliminate using a common ground.
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    Shielded cable is useful to protect high impedance signal lines from radiated EMI. It's generally not needed for low impedance power cables, but using shielded twisted-pair for the MCU power certainly wouldn't hurt. But the shield should be connected to chassis/earth ground, not circuit common.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,526
    2,369
    The systems I install I try and keep everything connected to earth ground at a central star point.
    Including power commons.
    There is a practice now called Equipotential bonding to prevent ground loops etc.
    Also the thinking has changed on grounding shielded cables just at one end, when using the E.P. bonding, now it is suggested that both ends should be connected to ground point, (see Ch6).
    http://www.automation.siemens.com/doconweb/pdf/840C_1101_E/emv_r.pdf?p=1
    Max.
     
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