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shielding/grounding question

Discussion in 'Electronic Basics' started by tempus fugit, Jan 31, 2009.

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  1. tempus fugit

    tempus fugit Guest

    Hey all;

    When a circuit is built inside a metal box to prevent hum pickup (in audio),
    does the metal box need to be connected to ground to provide the shielding,
    or is it the metal itself that blocks the unwanted interference?

    Thanks
     
  2. Both. A metal container acts as a shield because no static electric field
    can exist inside of it. The electrons distribute themselfs on the surface to
    maximize the distance between them(since they repel).

    But dynamically fields can exist and by grounding the circuit to it, any
    dynamic change on it will cause the circuit to follow it and hence the
    potential difference will be 0.

    So even if you somehow gave the chasis a potential it wouldn't effect the
    circuit.... which it could otherwise.

    e.g. suppose you ran a van de graff generator to put a large voltage on the
    chasis.... The circuit inside could be effected as the electrons move
    through the chasis to attempt to get to reduce the electric field.

    http://en.wikipedia.org/wiki/Faraday_cage

    Try using google next time? You'll get more out of it in the long run.
     
  3. tempus fugit

    tempus fugit Guest

    Thanks Jon

    So it's better to have the chassis grounded from a shielding perspective.
    Now, does it matter where it's grounded to? For example, I'm trying to avoid
    ground loop problems, so I was going to keep my circuit ground separate from
    earth ground. Could the chassis be grounded to earth ground with my circuit
    ground (or, more accurately, reference) floating from it, or should the
    chassis be grounded to the circuit ground?

    Thanks again
     
  4. Well, if you ground in multiple places you reduce the ground impedence.
    Remember that the connection to ground isn't 0 resistance. Hence you end up
    increasing the potential difference between the two. I doubt it would be a
    problem though.

    What I suggest is that you give yourself the ability to make several
    connections to the chasis then do tests. One central wire might be good
    enough. (if you have a ground plane for example) I've rarely seen more than
    1 or 2 connections to the chasis in audio equipment though so it's probably
    not a big deal.

    I imagine you'll have more issues with the circuit itself rather than the
    chasis and a well defined circuit doesn't need one except in rare occasions.
     
  5. tempus fugit

    tempus fugit Guest

    Thanks again Jon. Although I will naturally try to lay things out with no or
    few ground loops in the circuitry itself, I'm concerned about ground loops
    when using the circuit with other equipment. This particular piece of gear
    will have 5 guitar effects pedals mounted to it, and hopefully I'll be able
    to keep the ground that they will use floating from actual earth ground to
    avoid the problem with ground loops (and thus ground loop hum) when it gets
    connected to other equipment.

    Thanks
     
  6. Phil Allison

    Phil Allison Guest

    "tempus fugit"

    ** The metal box and audio signal ground must be at the same potential to
    provide effective shielding.

    Direct connection is not essential, it may be via a low value resistance.

    Connection of metalwork to AC supply ground is purely a safety measure - it
    prevents the metal acquiring a voltage potential above that of ground.




    ...... Phil
     
  7. tempus fugit

    tempus fugit Guest

    Thanks Phil

    Ugghhh... So if I use a metal enclosure, both the signal ground and the
    earth ground must be connected to the chassis - the 1st for shielding, and
    the other for safety, which introduces the possibility of ground loops with
    other equipment. When you say 'low vaue resistance' what type fo values are
    you thinking about? A few ohms or a few hundred ohms? Also, do you have any
    suggestions for combating the ground loop issue?
    Thanks again
     
  8. Phil Allison

    Phil Allison Guest

    "tempus fugit"
    "Phil Allison"

    ** A few hundred ohms is usually OK.

    Maybe add a 47 nF cap across it for RFI suppression.


    ** Power the device with a wall wart.

    No need for any safety ground then.




    ...... Phil
     
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