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dual rail supply and ground

Discussion in 'General Electronics Discussion' started by shmeeter, Mar 15, 2013.

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  1. shmeeter

    shmeeter

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    Mar 14, 2013
    Hi all. New to this forum and to hobby electronics in general. I hope this hasn't already been discussed to death. My brief search came up empty, but feel free to point me to another thread if so. :)

    My first project is going to be a dual rail power supply (dual output, either adjustable or with fixed +/-5 and +/-15 volts output - I want to mess with op-amp oscillators and ICs like the 555, logic gates, etc.). Something similar to what you see here: http://www.circuitstoday.com/dual-adjustable-power-supply-using-lm-317-lm337

    My question is about grounding the various parts of the circuit. The common on the components just goes to the center tap on the transformer. Check. Can I really consider this as "ground?" Can I sink DC current into it, for example? And what if I have different loads on the two rails? Like, say I'm using the -15V output for one thing and the +5V output for another, for example, or drawing more current from one rail than the other for some reason. Can I use the common as ground for both loads?

    And do I actually ground the common? Like connect the "ground" in the supply to the common lead from my mains or anything like that?

    Thanks much in advance!
    shmee
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    That circuit essentially creates 2 separate power supplies tied together at the ground connection.

    Whilst the +ve supply cannot sink current, and the negative one cannot source it, they will never be called upon to do so (unless you have an additional supply rail exceeding their voltage).

    Yes you really can consider the center tap to be ground. You can sink current into it from the +ve supply, or source current from it for the negative supply.
     
  3. john monks

    john monks

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    Mar 9, 2012
    In addition to what Steve is saying I think you might be asking about ground loops. If the supplies are used in audio equipment then it is important that all your grounds joint together at one point. What I mean is that R1. R4, C1, C2, C3, C4, and your return lead all join at one common point such as one solder lug. You do not want a return lead to connect directly to the transformer with a long wire between the transformer and R1 or R4 or you will get a buzz (ripple).
     
  4. BobK

    BobK

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    Jan 5, 2010
    Shouldn't the capacitors C1-C8 all be connected to ground where they cross? The schematic does not show them connected.

    Bob
     
  5. john monks

    john monks

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    Mar 9, 2012
    Of course that is what I meant. This is a common problem with schematics. It should show a big dot where the connection is made.
    What I said here is simplistic but it is the easiest way I know to prevent ground loops.
     
    Last edited: Mar 15, 2013
  6. BobK

    BobK

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    Jan 5, 2010
    John,

    I wasn't questioning what you said, I was questioning the correctness of the schematic.

    Bob
     
  7. shmeeter

    shmeeter

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    Mar 14, 2013
    Awesome, this is all great information, thank you for the tips.

    Just to clarify, even though all the components' grounds are physically joined at a common point, there is still of course another wire going from that common point to the center tap of the transformer, right? But no wire from that common point to the common or ground of my mains power, right?

    But I would of course ground the case of the power supply to the ground of my mains, right?

    Thanks again. Sorry for all the questions. I'm sure they seem silly to an expert!
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    yes

    It is not required but you will find many power supplies connect this to the ground of your mains supply. (or, in bench supplies, you may have a separate ground connection that you can link to any of your supply rails -- or none). The ground connection will make a big difference if you want to connect supplies in series or parallel or where you are connecting 2 pieces of equipment where the earths are different.

    If it's metal, yes.

    No, they are not silly.
     
  9. shmeeter

    shmeeter

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    Mar 14, 2013
    Just wanted to say thank you. I really appreciate the advice from all of you. I couldn't find anything anywhere that was specific on these points. Not where I was looking, anyway.

    Cheers,
    Ian
     
  10. Rleo6965

    Rleo6965

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    Jan 22, 2012
    Place bigger wires for +V, -V and GND and as short as possible between IC regulators and Output Jack. Solder R2 and R3 to Output and GND terminal of IC voltage regulators. This is to avoid voltage drop on wires and better voltage regulation.
     
  11. shmeeter

    shmeeter

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    Mar 14, 2013
    Okay. Thanks for the tip.
     
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