AC shield to DC ground?

Discussion in 'General Electronics Chat' started by qweety, Nov 25, 2012.

  1. qweety

    Thread Starter New Member

    Jan 13, 2011
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    0
    hello , i have a relay inside a chassis. The relay is controlled by a switch and 5v dc regulated supply. And the relay is used to turn on and off balanced AC audio.

    i would like the chassis project box to be connected to the shield of the audio, so to protect the audio signal whist inside the chassis from noise.

    but I wonder should i connect the dc ground that controls the relay to ground plane of the pcb so that all grounds/shields are connected together or should i keep them separate?

    thanks for your help.

    qweety : )
     
  2. ifixit

    Distinguished Member

    Nov 20, 2008
    638
    108
    Hi qweety,

    • What is the impedance of the balance audio signal? 600Ω?
    • Is it a twisted pair with no shield?
    • Is it a twisted pair with a shield?
    • What is the signal level minimum and maximum?
    • What kind of relay? Model #?
    • If the signal is shielded, is it grounded to something at the other ends?
    Avoid ground loops,
    Ifixit
     
  3. MrChips

    Moderator

    Oct 2, 2009
    12,414
    3,353
    Keep the grounds separate.
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
    5,979
    3,690
    If the project is AC with a transformer, you should use a there pronged grounded cord and ground the chassis to the metal project box for safety. Make sure to use fuses too. Then Do Not connect the DC ground or any other wires from the secondary side of the transformer to the box.

    If it is battery powered, yes, you can connect the DC ground but, like was said before, watch for ground loops. More details would be nice but the safety statement of the first paragraph stands.
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,414
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    The question the OP is asking is what to do with the audio ground?

    Personally, I would do as GopherT says. Connect the relay ground to the metal chassis.
    But I would not connect the audio ground to the chassis.
     
  6. qweety

    Thread Starter New Member

    Jan 13, 2011
    14
    0
    hello thank you for your replies.

    the audio comes from a mixing desk (point A) that has no earth connection on the wall socket just live and neutral, the audio travels out from this unit, balanced , along a shielded microphone cable ( not twisted pair, but has + - and shield.)

    it arrives at a project box, this could be 50 meters away. To a jack socket on the rear panel (the jack socket could have its shield connected to the chassis) it will then connect to a PCB where there is a relay,
    the relay is powered by a 5v regulator and a switch on the front panel and an arduino.
    the power for the regulator on the project box also at the moment has no earth. just live and neutral.

    but i can change, i haven’t started the schematic yet as i am unsure how to go about with the ground.

    MRchips you could be right , because after it leaves the project box, it travels to another input of another mixer (balanced in). so there will be a sheild connection eventually. ( The reason for the relay or mute in this box is so that it can be controlled by an arduino micro controller. that too will have a usb ground on the chassis connector coming from a PC) so yes i think i shall keep the shields separate. maybe the 5v reg and usb together on the chassis. and the audio shield will just pass through not connected.

    when i use my multimeter and touch the chassis of a mixing desk and the shield of an audio cable it does goes "beep". they are connected usually.
     
    Last edited: Nov 26, 2012
  7. MrChips

    Moderator

    Oct 2, 2009
    12,414
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    That is the preferred way to connect audio cable shield - no current must be allowed to flow through the shield. I would connect the shield of the input cable to the shield of the output cable, but not to the chassis.
     
  8. qweety

    Thread Starter New Member

    Jan 13, 2011
    14
    0
    Thank you Mr. Chips : )
     
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