Connect with us

Where to Put Ground on PCB

Discussion in 'PCB Layout, Design and Manufacture' started by Tripp, Jul 3, 2014.

Scroll to continue with content
  1. Tripp

    Tripp

    4
    0
    Jul 3, 2014
    Hi all,

    Big electronics novice here, but im trying to repair an old pcb board that limited current using resistors in parallel and a 6 way 2 pole switch to control the current. The issue is resistors kept frying so I suggested a voltage divider to replace it but I am stuck as to how I put a ground on the pcb board as the only inputs Ive got to work with are two coaxial cables (split to get pos and neg on the board), rather than a nice familiar power supply with +- and ground.

    Can i just connect the ground plane to a large area of copper?

    Regards,

    Tripp
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,651
    2,017
    Nov 17, 2011
    Welcome to our forum.

    There are a few issues with your project:

    1. How do you want to replace the resistors by a voltage divider? Typically a voltage divider is made from high resistance resistors and therefore not suitable for carrying high currents.
    2. Where do you get the ground plane from? I understand that you work on an existing PCB. It will be mechanically difficult to add a ground plane there.
    3. Can you connect ground (plane?) to a large area of copper? We can't know. Where does the area of copper come from? If it already exists, to which potential is it connected?

    A schematic of your proposed circuit and a photo of the location and the copper planes available there can help us understand your project. Also we may ask you more questions before we are able to answer in a satisfactory manner.
     
  3. Tripp

    Tripp

    4
    0
    Jul 3, 2014
    Hey Harald,

    Thank you for the prompt reply.

    I have just this moment found the reason why the resistors were frying. It was being connected after an amplifier instead of before (not by myself I would like to add) so instead of say a few mV it was reciveing about +-45V!! This was something found in a drawer that was used many years ago and my boss wanted to see if it would still work.

    As a result I no longer need to change the pcb board ( not actually a PCB board as i didnt know what to call it, but on one of those copper striped ones with holes in the tracks). However, I would still like to know how or if its possible to have a ground plane if you only have a positive and negative input, see picture, otherwise I dont know how I would have made the voltage divider circuit on that board.

    Last question, the schematic shows that the circuit is just using resistors in parallel to limit current (that I now know is very small) but I couldnt understand what the purpose of Ro was, perhaps some sort of feedback?

    photo 2.JPG IMG_2473.JPG

    Cheers

    Sam
     
  4. BobK

    BobK

    7,673
    1,684
    Jan 5, 2010
    It looks to me like the - is ground, and the resistors are already configured as a voltage divider which is variable depending on which switches are on. What would you do with the "ground" you want to add?

    Bob
     
  5. Harald Kapp

    Harald Kapp Moderator Moderator

    9,651
    2,017
    Nov 17, 2011
    You can't have a ground plane with this kind of PCB (veroboard). You only have the copper stripes to distribute signals and power supply. You probably don't want or need a ground plane. I think what you want is just a ground connection without necessarily a plane attached. A ground is meaningless if you do not have one in your circuit. Ground is only a definition for a reference potential. You can define the - input as ground, if you so like. Or you can define + to be ground. It doesn't really matter which one as long as you refer other voltages in the system (circuit) to this potential. In a circuit that has only one supply voltage it is common (but not necesssarily always so) to define the lowest potential as ground (but I can easily find counter-examples).

    You do not need a defined ground to build a potential divider. You can build a potential divider by connecting two resistors in series between any two voltages. Neither of these voltages needs to be defined as ground.


    It is not eawsy to say what R0 is for. This depends on how this circuit was meant to be used. One possibility is a permanent defined load for the component at the +/- terminals (where the arrows are). But that is just one interpretation.
     
  6. Tripp

    Tripp

    4
    0
    Jul 3, 2014
    Thanks guys that's a great help! I had previously thought I would need a specific ground but you have clearly explained that is not the case. As for R0 , the circuit is used to control the amplitude of an inductor coil and it input is from a computer so its possible that it may be to protect the terminals in some way, but it seemed to me like it would just short itself out, but it works so at the moment Im not questioning its purpose ha!

    Thanks again!
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-