Help with different potentials across a capacitor

Discussion in 'General Electronics Chat' started by Devon, Feb 8, 2014.

  1. Devon

    Thread Starter New Member

    Feb 8, 2014
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    Hi all, I have battery powered 24vdc circuit powering a resistive load (R1). I also have a seperate 12vdc battery powered circuit powering another resistive load (R2). As you can see from the schematic I have taken a conductor from the positive of the 24vdc battery and a ground from the 12vdc battery across a capacitor which is switched into R4.
    My question is, firstly can I use the positive of one circuit and the ground potential from a separate circuit across a capacitor? Secondly if I can, do I need the same voltage potential from both sides?
    What I mean by that is could I step up the 12vdc circuit to 24v with a transformer then bridge rectify it back to dc so I have the same potential across the cap.
    Lastly, if I can do this, could I get rid of C1 altogether and just drive R4 with a positive 24vdc from one circuit and the ground from the other circuit which is now transformed and rectified into 24vdc? Thanks in advance.
     
    Last edited: Feb 8, 2014
  2. R!f@@

    AAC Fanatic!

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    Ur circuit is incomplete. It won't work

    I still don't get what you are trying to do.
     
  3. Devon

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    Feb 8, 2014
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    A few typos, sorry. can u reread please.
     
  4. R!f@@

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    I tried but the more I read the more confusing it is to me.

    Can you explain in step by step of your requirements.
     
  5. Devon

    Thread Starter New Member

    Feb 8, 2014
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    Ok, I have a 24vdc battery supply across a resistive load on one circuit and a separate circuit with the same configuration. All I want to know is: Can I take the positive off one 24vdc circuit and the ground from the other circuit and charge a cap?
     
  6. MrChips

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    No..................
     
  7. Devon

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    Feb 8, 2014
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    Can you explain the science why it cannot be done please Mr Chips if you can? Is it because a cap has to be on the same power source (battery) and you would have to couple the batteries on my schematic into one potential?
     
  8. bertus

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    Hello,

    As drawn, both batteries are floating, there can be no voltage accross the capacitor.
    When the batteries would share a common connection, it would be possible.

    Bertus
     
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  9. MrChips

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    It depends on what you mean by power source.
    A 12V battery is a power source, for example.
    A 12V battery by itself is a "floating" power source. There is a 12V potential across the terminals of the battery. Each terminal by itself is "floating" and could be sitting at 1000V, for example. There is no reference potential.

    If either or both of your two power sources are floating then there is no electrical connection between the two power sources.

    For the capacitor to charge there must be some common connection between the two power sources, i.e a common reference point.
     
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  10. R!f@@

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    In short what are you OP trying to do ?

    It is the same as the cap leaving without any connection.
    Two floating supply has no connection between each other what so ever.
     
    Last edited: Feb 8, 2014
  11. Devon

    Thread Starter New Member

    Feb 8, 2014
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    Thanks for that reply.
    If we look at this from the capacitors point of view what does the capacitor see?
    A cap works by having a positive voltage on one plate and by definition the opposite charge on the other plate separated by the dielectric layer. The potential voltage across the cap is governed by the power in the battery and those dielectric properties i.e its point of dielectric breakdown.
    But my question is, if a cap see's a positive on one of its plates from one side of a floating 12vdc power supply and there is a ground potential on the other plate from another 12vdc floating supply, how does the cap see the difference? Surely the cap cannot see both floating supplies but only see's a positive voltage potential on one plate then it tries to grab the opposite charge on the other plate? If both floating battery supplies are of equal potential how does the cap know which is which? Surely the cap can only see voltage?
     
  12. MrChips

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    Forget about positive and negative voltages.
    Forget about dielectric breakdown.

    If I connect one terminal of a capacitor to the positive terminal of a 12V battery the capacitor does not "see" 12V.

    If I connect one terminal of the capacitor to the positive terminal of a 12V battery
    and connect the other terminal of the capacitor to the negative terminal of the same 12V battery
    the capacitor will experience a 12V potential difference across its terminals.
     
  13. R!f@@

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    And forget about that circuit too.

    For the capacitor to charge it needs to be connected between positive and negative from the same source. Whether the source floats or not does not matter. Electron cannot flow without a complete path
     
  14. Devon

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    Feb 8, 2014
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    That is very true.
    I'm trying to learn how capacitors work and the charge direction of the plates which is confusing me a little.
    Have a look at the schematic below. Figure one (the lower circuit). This is a conventional resister/cap/ battery set up. The charge flow is from negative to positive which I have indicated with a diode. What is confusing me is how voltage builds on the positive plate if the current direction is in the direction of the diode. Does it jump across the dielectric layer? Or does it charge that plate in the opposite direction of the diode marked in blue?
    The upper circuit is my schematic. What i'm confused about is how does C1 'know' it is collecting charge from two seperate floating batteries or the same potential of one battery? How does it see the difference?
     
  15. MrChips

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    We are not making progress.
    If you want to understand how capacitors work, focus on one capacitor and one battery.

    Electrons cannot jump across the dielectric layer.
     
  16. Devon

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    Feb 8, 2014
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    Right, I'll do that. If the current is running from a battery to a cap in a simple circuit and the current runs from the negative of a battery to the positive how does the positive cap plate becomes charged in this schematic?
    If it cannot jump across the dielectric layer and the diode only allows current to flow in one direction how does the positive plate (encircled) become charged?
     
  17. bertus

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  18. inwo

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    I'm trying to think of it as you are.:confused:
    If you are thinking electron flow.

    Then think of the positive plate as being "uncharged" with electrons.
    And the negative plate being charged with electrons.

    No actual flow thru dielectric, but apparent flow as capacitor is charging (storing/filling).

    This can only go on for a short time. Related to the voltage pressure and anything slowing down the charging. Like resistance!
     
  19. MrChips

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    Think of a capacitor somewhat like a mirror (a far fetched analogy). What is on one side of the capacitor is a reflection of the other side, i.e. positive charges are balanced with equal negative charges.

    When you connect a source with a potential difference across the terminals of the capacitor electrons will flow on both arms of the connection until the voltage across the capacitor is equal to that of the source.

    In your example, current will attempt to flow through the back-biased diode. The magnitude of the current is the same on both arms of the supply connections. You cannot suddenly end up with full potential at one side alone.

    Because the diode leakage current is extremely low, the potential difference across the capacitor will be almost zero at the beginning. Over a long period of time, if the leakage current in the capacitor is zero, the potential difference across the capacitor will increase to match the voltage across the battery.
     
  20. Devon

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    Feb 8, 2014
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    I see where you are coming from now. There is a circulating effect that passes through the battery that makes the charge potential on the plates the same as the battery. On this schematic, the conventional battery and capacitor arrangement has this circulating effect from A to B (lower right circuit), on my schematic unless I join both sets of batteries in series then that circulating effect cannot take place between A and B across the cap. I see.
    What I was hoping would happen in my schematic was when I connect S1 the circulating current in circuit 'P' already driving R1 would enter D1 from the negative side of R1 and create a potential on C1. I then thought that circuit 'M' which was already circulating current into R2 would provide a potential for the other cap plate.
    So in essence to sum it up, I thought that I could replace the circulating effect (marked A to B on my schematic) in a capacitor circuit with the two circulating circuits marked P and M. So that the capacitor would be tricked into seeing circuit P and circuit M as the terminals of a battery.
    Guess I was wrong.
     
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