Relative GND

Discussion in 'The Projects Forum' started by alecjcook, Jan 2, 2014.

  1. alecjcook

    Thread Starter New Member

    Jan 2, 2014
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    Hello,
    I am trying to create a power bus to feed items in a modular system (I am building a modular analogue synthesizer).
    Before I begin I want my power supply board to provide a stable "+12", "-12" and "GND".

    I have bought a 24V power supply and I have a bunch of Voltage Regulators, one that takes +15v and gives +12v (I can cope with that, easy stuff), and one that takes in -15v and gives -12v...

    Now, I think I have implemented a way of using a voltage divider to work myself some negative voltage (cause all voltage is only relative to its GND right?), but I think I am going to have a problem because I don't think my resulting GNDs are common to each other. Can anyone suggest what I might need to do to achieve this? (please see the diagram)

    Many thanks, Alec
     
    Last edited: Jan 2, 2014
  2. MrChips

    Moderator

    Oct 2, 2009
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    If you are building a system power supply to deliver +12VDC and -12VDC then you will need a dual power supply. A single 24V supply will not cut it.

    Secondly, if you wish precision +12V and -12V to drive opamps you should use a tracking supply, not individual +12V and -12V regulators.
     
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  3. alecjcook

    Thread Starter New Member

    Jan 2, 2014
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    OK, what about if it was +12 and -5?
     
  4. MrChips

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    Assuming both +12V and -5V supplies have to deliver considerable current then you still need two supplies. You can build or buy +12V to -5V DC-to-DC converters but before you go that route consider your amperage requirements.
     
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  5. alecjcook

    Thread Starter New Member

    Jan 2, 2014
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    OK, Thanks :)
    (I thought there might have been a clever trick i was un aware of. I guess not, many thanks.)
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Why? Most opamps I have worked with have Power Supply Rejection Ratio (PSRR) of greater than 85db so why does it matter that the -12V supply is the mirror image of the +12V supply?
     
  7. MrChips

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    You may be correct but I am from an old school where our opamp supplies were always tracking.
     
  8. inwo

    Well-Known Member

    Nov 7, 2013
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    Technically I don't know why you can't do it. ie. use different grounds/commons.

    Just don't think it's a good idea.

    If you use a 12 volt regulator powered from an isolated 24 volt supply. You have 0volts, 12volts, and 24volts available.

    Using +12volts as common. Gives -12, common, and +12.

    May be a hundred reasons this won't work. I'm sure I'll hear of 99.:(
     
  9. panic mode

    Senior Member

    Oct 10, 2011
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    you can use virtual ground if your current draw is insignificant (otherwise add some sort of voltage follower). this is quite common in analog circuits using opamps
     
  10. panic mode

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    suppose you use 7812 to derive 12V from 24V...
    then we choose to call 24V rail "output +12V", the output of 7812 is "output GND" and 0VDC is "output -12V".

    in some cases this may work but there is no guarantee, in fact it is likely to cause problems.

    using multiple power supplies means that each of them can operate independently from others. suppose that load between "GND" and "-12V" is disconnected but we still have load between "+12V" and "GND" (or at least current in this branch is larger).

    that means we have higher load (lower resistance) between 24V rail and output of 7812. but the 7812 is series regulator - it cannot handle current flowing from load into its own output. as a result the "GND" potential rises...

    so you get perhaps +5/-19 instead of +12/-12. worse, the ratio will keep on changing and that is certainly not good.

    using voltage follower, you create circuit that can both sink and source current at terminal declared as virtual ground so this problem does not exist (as long as follower can handle the imbalance current)
     
  11. #12

    Expert

    Nov 30, 2010
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    About 20 years ago, I was building a 6 stage audio processor and I didn't have a -12V chip, so I used a -15V chip. Works just fine and hasn't come back for repairs in 20 years.

    They don't have to track, they just have to be clean, and that is mostly the responsibility of the capacitors.
     
  12. ronv

    AAC Fanatic!

    Nov 12, 2008
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    I think the symmetrical supplies stems from wanting the +/- portion of a sine wave to clip at the same level so you can get maximum output from an amplifier. I've seen virtual grounds at some strange voltages because the op amps would swing to ground but not to Vcc.
     
  13. #12

    Expert

    Nov 30, 2010
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    It's also convenient to use one transformer for both positive and negative voltages because transformers are usually symmetrical.

    (I just happened to be working on a very complicated machine and had plenty of voltages to choose from when I found I hadn't ordered enough regulator chips.)
     
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