Is it possible to float ground and still maintain a low impedance to earth?

Discussion in 'General Electronics Chat' started by coinmaster, Jun 11, 2016.

  1. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    I want to use low voltage capacitors in high voltage circuits to save on cost which would require floating ground to x volts below the supply but I also need this new "ground" to be super low impedance to earth so the circuit can draw its AC current through it and nothing else.

    I can't think of a way to do this, anything I can think of requires a high value resistor and any resistor will screw up the impedance. Zeners won't work.
    Source followers won't work. I thought maybe a push pull current source could work but it refuses to boot up in LTspice so I don't know if that will work or not.

    Can this be done?
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    Conceivably you could capacitively couple your node to ground, but that would depend very much on many different factors that can't even begin to be guessed at without a MUCH better description and schematic of what you are contemplating. And what you are trying to do may well raise serious safety concerns as well.
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Deliberately drawing significant current through an earth connection is a no-no for domestic installations. How much current and why? Earth current plus high voltage don't sound a very safe combination.
     
  4. WBahn

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    Mar 31, 2012
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    Given the kinds of circuits the TS has usually been talking about, I suspect that the "earth" he is referring to isn't actually Earth. But we need a bunch more information (which, as usual, we seem to have to draw out bit by bit).
     
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    If you put more voltage than the rating on a cap it will fail 100%.

    Give us a sketch of just what you are doing and we may have a suggestion.
     
  6. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    I'm referring to filter caps before and the bypass cap after a regulator for an audio amp.

    An audio amp draws AC current from the last cap in the supply assuming it is an AC short to ground so if I was going to use a super capacitor in a 600v supply the grounded half of the capacitor would need to be floated to 595v but still be an effective short to earth ground since the amp itself isn't floating.
     
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    You have an audio amp running off a 600 volt supply???

    If you "float" a cap at 595 volts you leave that 595 volts unfiltered.
     
  8. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    It wouldn't matter, the 595 volts would be "ground" to the cap. The only reason it's at 595 volts is because I would want there to be 5v across the capacitor.
    The problem is the cap won't source AC current into the load if it isn't a short to earth so this 595 volt "ground" would need to be super low impedance to earth.
     
  9. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Yes it would. That 595 volts you are trying to bypass will Still be just as noisy as it ever was.

    So if you are good with less than 1% filtering your scheme may work...
     
  10. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    What did you have in mind?
     
  11. WBahn

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    Mar 31, 2012
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    Could you PLEASE provide a SCHEMATIC for what you are trying to do?!?!
     
  12. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    Sorry I thought I made it obvious. Screenshot_92.png
    Simple regulator example.
    Filter caps and bypass cap.
    Since this is for an audio amp the bypass cap sources all the AC current. None of the AC current comes from anything else, assuming it is an AC short to earth.
    I want to use super capacitors that are rated for 5v which means floating "ground" and also maintaining a short to earth.
     
  13. WBahn

    Moderator

    Mar 31, 2012
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    Why do you want a short to earth (as in, a short to a metal rod stuck several feet into the ground)?

    How are you getting 600 V on that top node? Why wouldn't it be closer to -600 V?

    What is the breakdown voltage of the zener diode?

    If your rail WERE 600 V, how would you ever turn on the PNP transistor?
     
  14. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    Because current loops. If it is not an AC short to earth then the AC current will be drawn from whatever the lowest impedance source to earth is.

    It dosn't matter it's an example schematic. It applies to any power supply with a capacitor and AC current.
    My question is is it possible to float ground to reduce voltage across the capacitors while still maintaining low/no impedance to earth ground.
     
  15. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    I believe we have answered if it is possible to float the ground several times.

    The answer is no, not in any way that is useful.

    You may not agree with that answer and that is your choice.

    I am done here.
     
    Last edited: Jun 13, 2016
  16. BobTPH

    Active Member

    Jun 5, 2013
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    So you want a node that is at 595V and has a low impedance to ground. What impedance, say 1 Ohm? Then you will have 595A flowing continuously from that node to ground. Great idea!

    Or did you mean low AC impedance, with a high DC impedance? That is doable, just use a capacitor (which, of course, defeats your purpose.)

    Bob
     
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