Actions
Forum Thread Details
  • Replies 2 replies
  • Subscribers 178 subscribers
  • Views 461 views
  • Users 0 members are here
Related

0V, GND, EARTH

mrmarple

Probably not the correct forum to ask this question but the standard of the

contributors here suggests I'll get a good response, or at least provoke

some discussion.

 

Been around electronics a few years (think 12AX7/ECC83) and I never found a

satisfactory (to me) answer on what to do with 0 volts, chassis ground and

protective earth. The equipment I've worked on over the years have used

different strategies, some tie everything together, others attempt to keep

things separate, through in two and three pin plugs and it looks a bit of

an untidy mess.

 

I'm thinking horsepower BLDC motors, H-bridge drivers, controllers full of

CMOS and computer power supplies. I've had zaps and seen sparks when

plugging pieces of gear together which continue to function but I do wonder

what currents are flowing in a bit of 30AWG ribbon cable, and what's

happening to noise margins.

 

Keeping the +V sides of power supplies isolated with optocouplers is

straightforward but still puzzled about the 0V side. Tie the lot together

on the board, at the power supply, keep the protective earth separate?

 

Your thoughts.

--

Web access to CadSoft support forums at www.eaglecentral.ca.  Where the CadSoft EAGLE community meets.

 

  • On 04/09/16 12:22, ken wrote:

    Probably not the correct forum to ask this question but the standard of the

    contributors here suggests I'll get a good response, or at least provoke

    some discussion.

     

    Been around electronics a few years (think 12AX7/ECC83) and I never found a

    satisfactory (to me) answer on what to do with 0 volts, chassis ground and

    protective earth. The equipment I've worked on over the years have used

    different strategies, some tie everything together, others attempt to keep

    things separate, through in two and three pin plugs and it looks a bit of

    an untidy mess.

     

    I'm thinking horsepower BLDC motors, H-bridge drivers, controllers full of

    CMOS and computer power supplies. I've had zaps and seen sparks when

    plugging pieces of gear together which continue to function but I do wonder

    what currents are flowing in a bit of 30AWG ribbon cable, and what's

    happening to noise margins.

     

    Keeping the +V sides of power supplies isolated with optocouplers is

    straightforward but still puzzled about the 0V side. Tie the lot together

    on the board, at the power supply, keep the protective earth separate?

     

    Your thoughts.

     

    This looks like two slightly separate questions, to me.

     

    Firstly, inside a specific box, should the ground rails all be common.

    Usually, the answer is "yes, but...", which is to say you often want to

    "star earth" things.

     

    Secondly, between separate boxes, such as a CD player and amplifier.

    Here, the problem you run into is earth loops. Where both boxes are

    independently grounded as well as connected together, there is a big

    loop of 0V wire, which acts as an inductive pickup. This plays merry

    hell with signals. So the way to avoid this is NOT to connect any of the

    equipment directly to earth, at least not via its mains cable, and have

    at most one ground connection for the circuitry.

     

    This being a potentially troublesome option, especially where the

    equipment is physically quite separate, the alternative is to use solely

    fully floating interfaces. An example of this would be the UTP Ethernet

    standard, which uses balanced lines over twisted pair with isolating

    transformers at each node.

     

    Personally, I tend to keep circuit ground separate from chassis ground

    but couple them with some limiting device (a capacitor or MOV) but

    others will almost certainly have better suggestions.

     

  • Just FYI, as far as I know every PC ever made connects the power supply ground (0V) and the chassis itself to the AC safety ground.

     

    This is not an ideal configuration as described in the first response.