intermediate voltage between capacitor plates

Discussion in 'General Electronics Chat' started by crazyrock, Aug 21, 2009.

  1. crazyrock

    Thread Starter New Member

    Aug 21, 2009
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    ive a setup where i am using a 14 volt DC (thru adapter) between two capacitor plates(they are homebrew, made of alum foil). they are about 10 cm apart. in the middle i placed another "plate". shouldnt i be able to measure a voltage if the voltmeter goes between the middle plate and another terminal? check the figure

    ...this seems too simple tho i cant still figure out why i am reading no voltage at all from the middle plate. i am guessing i need a much bigger middle plate.
     
  2. davebee

    Well-Known Member

    Oct 22, 2008
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    yes, in theory that sounds right. But you'd need to measure the voltage with a voltmeter that doesn't draw any current, because the center plate will be at an extremely high impedance, meaning that practically any current draw will affect its measured voltage.

    If you can set up a FET-input op-amp like an LM356 as a unity-gain buffer, you could try connecting the middle plate directly to the non-inverting pin of the op-amp, with no other conenction to that pin, and measure the op-amp's output. The op-amp might be able to isolate the plate enough to give you a reading.
     
    Last edited: Aug 21, 2009
  3. crazyrock

    Thread Starter New Member

    Aug 21, 2009
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    hmm...wouldnt that mean to measure any voltage source it would have to draw a current. a multimeter can measure a voltage fine from jus one source , but isnt the negative, positive terminals of high impedance in a source as well?
     
  4. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    You CAN measure this with a device called an electrometer.

    When I worked in ionospheric research, we used very sensiitive electrometers to measure electron precipitation.....a "rainfall" of excess electrons that stream down from the sky during an Aurora Borealis. Very cool, actually! :)

    Eric
     
  5. davebee

    Well-Known Member

    Oct 22, 2008
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    A conventional voltmeter measures voltage by applying it to a resistor and measuring the resulting current flow through the resistor, but there are other ways to do it.

    You can connect your point to an isolated plate of an electrostatic oscilloscope and look for the beam's deflection.

    Or you could connect a sensitive current meter to your point, connect the other end of the current meter to a voltage source, then adjust your voltage source until zero current flows. Then the voltage of the point is the same as your voltage source, but you're not drawing any more current from your point.

    In both these, there will be some initial current flow until the circuit adjusts, but after that the current flow will be pretty close to zero.
     
  6. crazyrock

    Thread Starter New Member

    Aug 21, 2009
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    thanks for the suggestion. but i guess there no point of doing it if it cannot drive a dc current.
    i was trying to see if capactive transformers(similar to my setup) would offer any advantage to inductive transfomers. i thought they could tranfer DC
     
  7. crazyrock

    Thread Starter New Member

    Aug 21, 2009
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    i did some thinking and i think i figured out y no current flow. what im doing is effectively choking the metal plate out of electrons(or rather charges).
    another thing to notice is that there is no power transfer between the two circuits in DC.
    i got an idea how about putting a semiconductor p+ region above the top plate. im guessing the P region wouldnt alter the electric fields in a big manner.in this way charges can flow without choking the middle plate out of charges.
    i am not sure how i can test this
     
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