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Capacitor charging

Discussion in 'Electronic Basics' started by Ken, Feb 23, 2006.

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  1. Ken

    Ken Guest

    Hi,

    This is a newbi questions I suppose.
    If I take a voltage source say a battery, I put a capacitor with rating of
    300volts to it, the charge went up to the maximum charge of the battery
    (12.4volts). I thought the charge would accumulate up to 300 volts. Why does
    it go to 300 volt ?

    thank you

    Ken
     
  2. Ken

    Ken Guest

    AA ok good, Thank you
    I guess I need to put a resistor in there to creat current.
    thats answers it.

    ken
     
  3. Ken

    Ken Guest

    yes you are right, Dj said that I could get up to maximum value. I tried
    addidn a resistor in the circuit , but still did not get more then the 12.4
    volts, the battery voltage.
    I guess there is no easy way to charge my capacitor to 300 volts from a 12v
    battery.

    ken
     
  4. DJ Delorie

    DJ Delorie Guest

    No, that rating means if you put more than 300 volts across the
    capacitor, it explodes (or fails in some other way ;).

    If you put a constant CURRENT into the capacitor, it will continue
    accumulating voltage until it reaches its max rating and fails.

    You are not supplying a constant current, you're supplying a constant
    VOLTAGE. The capacitor will charge to that voltage, and then the
    current flow stops and the charge stays at that voltage.
     
  5. The voltage rating on a capacitor tells you the maximum voltage you
    can safely apply to it and still have it function as a capacitor.

    The relation between current and voltage for a capacitor is:

    I=C*(dv/dt), where I is in amperes, C in farads, and dv/dt is the rate
    of change of the voltage across the capacitor in volts per second.

    When you first connect the capacitor across a voltage source, like a
    battery, there is a large pulse of current, limited only by the
    internal series resistance of the source and inside the capacitor.
    But this large current pulse results in a high rate of change of
    voltage and the capacitor quickly charges up till its voltage matches
    that of the source. At that point, you have two equal voltages
    bucking each other, and the current heads toward zero.

    This is something like attaching a small air storage tank to a much
    larger one. There is a brief blast of air through the connecting
    hose, and then, as the small tank pressure approaches that of the
    larger tank, the flow through the hose falls toward zero. You can not
    get the small tank to reach a pressure higher than that inside the
    larger tank that is filling it.
     
  6. DJ Delorie

    DJ Delorie Guest

    A resistor will only *reduce* the current, not *create* it. Thus, the
    capacitor will charge slower, but still stop charging when it's
    voltage reaches that of the power source.

    If, for example, you had a power supply capable of providing 500 V at
    1 mA, you could charge the capacitor - slowly - up to 500 V (or until
    it fails, that is). But a 12 v supply cannot charge it past 12v,
    because at that point the voltage across your resistor is zero, and
    zero volts means zero amps means charging stops.
     
  7. Pooh Bear

    Pooh Bear Guest

    You won't.

    You can't get any more volts that way than there are to begin with without using
    some stup-up circuitry.

    What you're doing is a bit like complaining that diesel fuel poured into a can
    marked gasoline still won't work in a gasoline engine.

    Graham
     
  8. Pooh Bear

    Pooh Bear Guest

    NO.

    A resistor *resists* current.

    You need to read a book about the basics of electricity.

    Graham
     
  9. Guest

    Sure there is. You just need the electronic equivalent of a pump. One
    way works something like an ignition coil does. You apply 12 volts
    across an inductor till the current ramps up to some value, then turn
    that currnet off, very quickly. The inductor produces a large voltage
    in an attempt to keep the current going. You pass that pulse through a
    high voltage rectifier and dump it into your capacitor.

    The flyback transformer in any TV or monitor is such an inductive
    component. Got any junk monitors sitting around?
     
  10. Pooh Bear

    Pooh Bear Guest

    Do you seriously imagine the OP has even half a clue what you mean ?

    Let's start with 'back emf' for example.

    Graham
     
  11. Go for it.
     
  12. Ken

    Ken Guest

    Until now I was looking as voltage multipliers that includes diodes and
    capacitors.
    The idea of back emf is good. A quick search on google led me to some motors
    running on back emf. Is there any truths on John Bedini's motors ?
     
  13. Ken

    Ken Guest

    I guess you tried it and didn't work.
    he is saying I guess, that the back emf gives off voltage (and current) that
    is greater then the one that came in. Seems to defies some laws here...
     
  14. Ken

    Ken Guest

    For what he is saying that his machine is doing. Not for what is really
    going on
     
  15. The motors, themselves tell truth.
    Bedini wouldn't understand that truth if it bit him on the ass.
     
  16. DJ Delorie

    DJ Delorie Guest

    No, same total energy. Recall that inductors store energy as magnetic
    flux. You can create high flux with low voltage and high current,
    then use that flux to create high voltage and low current. The
    wattage is the same, hence conservation of total energy.

    This is very similar to the way a step-up transformer works, except
    you're doing it with only one coil instead of two.
     
  17. Back EMF and any other kind of EMF is just another word for voltage.
    What he doesn't want to understand is the the time-continuous
    integration of the product of volts and amperes. Instantaneous volts
    times instantaneous amperes is instantaneous power. The integration
    of power over time is energy. He measures average or peak volts and
    average or peak amperes, separately, and guesses that energy is being
    created. He doesn't make the measurements necessary, nor does the
    math on those measurements necessary to find out what the answer is.
    If he did, it would ruin his day.
    Ya think!?
     
  18. Guest

    Yes. We must never confuse one of those for the other. ;-)
     
  19. Clinton

    Clinton Guest

    It will if you have a 300 volt battery..
     
  20. Art

    Art Guest

    Must be a "Flux Capacitor" running on banana juice?? Developing voltage from
    quasar sources.
     
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