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Capacitors & Capacitance

Discussion in 'Electronic Basics' started by Richard Harris, Jan 24, 2005.

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  1. Hi,
    Just been reading about capacitors and understand the property of
    capacitance. The problem is I don't see how this property is of use, many
    circuits use capacitors but I don't understand what there role is.

    I know capacitors don't allow current to flow through them and that they can
    be charged and discharged and that the time taken to charge and dischage can
    be calculated. But how are these devices off any real use?

    Thanks for ya time guys.
     
  2. Capacitors are a class of devices that have time dependent response.
    The mathematical description of the relation between the current
    through a capacitor and the voltage across it is
    I=C*(dv/dt)

    With I an amperes, C in farads and dv/dt, the time rate of change of
    voltage, in volts per second.

    Since pure DC has no rate of change, it produces no current through a
    capacitor. but any other voltage waveform from varying DC to
    sinusoidal AC or any other variation in voltage over time causes
    current to pass through capacitors.

    There is also energy stored in any capacitor that does not have zero
    volta across it. The energy is

    E=(1/2)*C*V^2
    With E being the energy in joules or watt seconds, C in farads and V
    in volts.

    The voltage cross a capacitor is also related to the total charge that
    has passed through it since its had zero volts across it.
    Q=C*V
    Where Q is charge in coulombs, C is farads and V is volts across the
    capacitor.

    Capacitors are used for energy storage, filtering (frequency dependent
    response) including resonance with inductors, DC blocking (while
    passing AC riding on the DC bias), and timing circuits the measure the
    time it takes for a specific voltage change caused by a charging
    current. They can also be used to add up the total (integral) of a
    signal over a period of time as a voltage change across the capacitor,
    if the signal can be converted to a proportional current that charges
    the capacitor.
     
  3. current through a capacitor never happens.
    AC will charge and discharge a capacitor, reversr polarity of charge will
    occur based on the frequency of the AC but no current will pass through a
    capacitor.
    Agin if current can't flow through a capacitor how can voltage or a charge?
    Thanks for your time.
     
  4. Tom Biasi

    Tom Biasi Guest

    Hi Richard,
    John has given you a technical and eloquent (as usual) explanation.
    Your response indicated that you may not have comprehended the explanation.
    You say "I know capacitors don't allow current to flow through them ".
    Well if you look at DC you could be considered somewhat correct.
    If you understand the capacitors mechanisms you may see that AC will 'Pass'.
    You also say, "they can be charged and discharged and that the time taken to
    charge and discharge can
    be calculated. ".
    After saying those things you still can not think of any uses for
    capacitors?
    Think some more,
    Regards,
    Tom
     
  5. Gareth

    Gareth Guest

    Well, that is true sort of true. I expect that is why John used the
    quotes. If a capacitor is charging or discharging then it appears that
    current is flowing though it.

    You know that capacitors can be charged and discharged, well the charge,
    Q, stored in a capacitor is:

    Q = C*V [1]

    Where V is voltage and C is capacitance. Now, if you change the voltage
    you must also change the charge on the capacitor. This charge has to
    come from somewhere in the circuit, so charge must be moving. Moving
    charge is current, so we have some current, but only while the voltage
    is changing. If the voltage is constant then no current flows.

    Current is actually the rate of flow of charge. So if we increase the
    charge on a capacitor by dQ (where dQ means a small amount of charge) in
    a time dt (where dt means a short time) then the current which appears
    to flow through the capacitor during the time, dt, is dQ/dt

    so I = dQ/dt [2]

    For example if the charge increased by 1 C (Coulomb) in 1 second, the
    average current flowing though the capacitor in that time would be 1 Amp

    According to [1] a change in charge would also mean a change in voltage,
    dV. Using equations 1 & 2 we can eliminate Q from the equation. This
    gives, as John said:

    I = C*(dV/dt)

    If you use the water flow analogy and think of wires as pipes full of
    water and voltage as pressure, then a capacitor would be a stretchy
    rubber sheet blocking a pipe.

    Now, if you increase the pressure (voltage) the rubber sheet will
    stretch and some water (current) will flow in the pipe. Note that as
    the rubber sheet stretches the water on the other side of the sheet will
    be pushed down the pipe. No water actually crosses the rubber sheet but
    as you increase the pressure water flows into one end of the pipe and
    water flows out the other end. It is different water, but we don't care
    about that, as far as we are concerned we force water in one end, by
    increasing the pressure, and water comes out the other end.

    If the pressure is constant no water will flow. If you reduce the
    pressure the rubber will contract and push water back down the pipe.

    If you constantly increase and decrease the water pressure water will
    flow up and down the pipe. No water will actually cross the rubber
    sheet but water is flowing up and down the pipe.

    A capacitor will do a similar thing.

    Gareth

    --
     
  6. People who run motor windings with capacitors in series would be
    amazed to hear that. While it is true that no particular electron
    that goes in one side ever makes it out the other side, if you push an
    electron in one side, a different one comes out the other side. And
    that is still current.
    Charge flows into one sire and out of the other, even though no charge
    makes it through the insulation between the plates. An electron
    arriving onto the surface of one plate creates an electric field that
    repels an electron on the other side of the insulation to leave the
    other plate and leave the capacitor. Since there are electrons moving
    through both leads, there is effectively current passing through the
    capacitor. All that is required to make this current is to force a
    change in the voltage applied to the capacitor. Once the voltage
    stops changing, the current stops.
     
  7. Andrew Holme

    Andrew Holme Guest

    Current *can* flow through a capacitor - but not forever, in the same
    direction. As it flows, charge is taken from one plate and piled up on the
    other. The greater the current, and the longer it flows, the more charge is
    moved, and the greater the potential difference between the plates. If this
    goes on for too long, the dialetric breaks down.
     
  8. Andrew Holme

    Andrew Holme Guest

    That should be "dielectric" of course.
     
  9. Thanks guys

    My documentation must be inaccurate as it states that AC can not pass.
     
  10. I have two statements I would like to know if they are correct and that my
    interpretation of them is good.

    1.) Capacitance is the property that opposes changes in voltage in a
    circuit.
    There fore a capacitor can be used to steady voltage and keep it constant.

    2.)Inductance is the property that opposes changes in current in a circuit.
    There fore a coil can be used to steady current and keep it constant.

    Thanks
     
  11. If it does then it is.
     
  12. A bit overstated, I think. I would say that a capacitor is a device
    that must pass current in order for the voltage across it to change.
    There are other devices that oppose voltage change by other means than
    capacitance.

    Capacitors are often connected across DC supply rails for the effect
    you describe. If only small and brief currents are involved, then
    small capacitors may do (e.g. .1 uf across the power pins of a logic
    chip). If larger and longer lasting currents are involved, then quite
    large capacitors ar used (following rectifiers in power supplies, for
    instance). In both cases, the voltage is certainly more steady with
    the cap in place than it is without it. This is using caps as
    something like small rechargeable batteries, except that chemical
    batteries can supply current with almost no change in their output
    voltage, while capacitors must always have some voltage change if they
    are going to pass current.
    Again, a bit overstated, but not wrong. Change "the property" to "a
    property".
    There are other things that oppose current change that do not involve
    inductance.
     
  13. bxbxb3

    bxbxb3 Guest

    Hi,
    Its been a bit late, but I hope someone will read this. Could anyone tell
    me, if a capacitor is compared to a rubber sheet connected to a water
    pump, what could be the possible analogy for an inductor. That example was
    pretty good to compare and imagine. Thanks
     
  14. Gareth

    Gareth Guest

    An inductor is a bit like a heavy object on wheels. The current in the
    inductor is analogous to the speed of the object. The force applied to
    the object is analogous to the voltage applied to the inductor.

    If you apply a force to the heavy object on wheels it will slowly
    accelerate. Similarly, if you apply a voltage to an inductor the
    current in the inductor will ramp up smoothly.

    If you stop pushing the heavy object it will slow down due to friction.
    Similarly if you remove the voltage from an inductor current will
    continue to flow through it but the current will decrease due to
    resistance in the circuit (for a perfect frictionless object or a
    perfect inductor with no resistance the motion or current will continue).

    If you try to stop your heavy object quickly when it is moving fast the
    inertia of the object resists this change. Similarly if you try to stop
    the current in an inductor quickly the inductance opposes this change.
    This is why you need to be careful when you switch inductive loads like
    relays and motors.

    The equations for a heavy object are:

    F = ma

    Where F = Force, m = mass, a = acceleration (rate of change of speed)

    E = 1/2*m*(v^2)

    Where E = Kinetic Energy, m = mass, v = velocity

    The equations for an inductor are:

    V = L*dI/dt

    Where V = Voltage, L = inductance, dI/dt = rate of change of current

    E = 1/2*L*(I^2)

    Where E = Energy stored in the inductor, L = Inductance, I = current

    If you think of inductance as mass, voltage as force and current as
    velocity the equations are the same.

    Gareth.

    --
     
  15. If you imagine that voltage is torque and rotational speed is current,
    then inductance is something like the inertia of a flywheel. Apply
    torque and the flywheel steadily increases its rate of rotation (apply
    voltage across an inductance, and current ramps up). It takes a large
    spike of torque the other way to bring the rotation to a halt (it
    takes a large applied reverse voltage to bring an inductive current to
    zero, quickly).

    A nice thing about this analogy is that flywheels turn around an axis
    while current goes around the magnetic field of an inductance.
     
  16. An inductor is like a water pump without a motor, it is driven by the
    water that is pushed through it.

    It needs a pressure and a current to get going, and then it keeps on
    running, pumping water, until the resistance slows down the current.

    This is used in cars for creating a spark, or to create the starting
    spark in a flouroscent tube.

    Voltage is used to get the current running in a coil, and then suddenly
    the connection is cut off, the current still pumps through the coil and
    where is that current going to go? It has nowhere to go so the voltage
    increases until a spark jumps over to ground, and that spark starts the
    car or the fluoroscent tube.

    Inductors have a resistance against quick changes in the current,
    capacitors have a resistance against quick changes in the voltage.

    The resistance changes with frequency, so inductors have little
    resistance at low frequencies and high resistance at high frequencies.

    There is a diagram over these factors which I think is very useful but I
    can only find it in a pdf file from a swedish company.
    http://www.elfa.se/se/fakta.pdf

    This pdf is written in swedish but that doesn't matter because it is
    only the diagram we need. In the index find the word " Induktanser" and
    click on it, then scroll down one page, there is the diagram. Zoom in to
    see the details. It is on page 33 in the pdf file.

    You can see how the horizontal scale is the frequency scale, the vertical
    scale is resistance, diagonally you see inductance and capacitance.

    This diagram tells you what resistance a certain inductance or
    capacitance has at a certain frequency.

    For example, we want to know what inductor is needed for a loudspeaker
    filter, it should have a resistance of 10 Ohms at 200Hz.

    We go into the diagram from the horisontal 10 Ohm line, follow it to the
    (vertical) 200Hz line, there is our working point. From there, follow the
    diagonal line down left towards the border of the diagram and you see the
    value 10uH.

    So, we need a 10uH coil for this purpose.

    I wish there was a better way to find such a diagram, better than to have
    to download a pdf file in swedish and find the diagram. If anybody knows
    about such diagrams in other places on the web, tell us about it.

    These diagrams are useful because you only need your eyes to focus and
    follow lines, there is no need to do calculations or touch anything, I
    have this diagram in front of me all the time at the work bench, and use
    it very often.
     
  17. PeteS

    PeteS Guest

    Although I agree with that desccription, it might not go far enough, in
    a way. (IMO)

    Strictly speaking (and you have to, quite often), capacitance is
    exhibited between any two points where there is a difference in
    potential.
    This leads to the interesting observation that a resistor with current
    flowing through it is, in fact, a parallel RC network (the amount of
    capacitance is roughly proportional to the resistance but
    obviously depends on the physical dimensions of the resistor). This,
    incidentally, is one of the reasons to avoid large resistor values in
    the feedback loop of an opamp (it makes an integrator out of it).

    It also means that there is capacitance between virtually any two
    points on the
    average PCB. That is a point of major importance to practising
    designers. Thousands more examples may be found of where capacitance
    exists (static discharge testing uses, amongst other things, the 'human
    body model').

    When designing filters or PCBs that are low frequency, you usually
    don't have to worry about those effects but as soon as things get above
    a few 10s of MHz or so, (and don't forget transients), they can become
    of overriding importance. Note that in a switching power supply with an
    oscillator of 100kHz (quite low by current standards), board layout
    capacitive effects are of major significance (the transients are in the
    order of 10s of MHz, typically).

    Note my first statement - it is what led to the development of a slick
    little arrangement called 'guarded leakage' used quite a bit in the
    input to sample and hold circuits. You can find that in Horowitz & Hill
    (don't ask for the page, it's been over 20 years since I read it)
    :)

    As with many things electrical and electronic, the model one uses
    depends on the circumstances.

    So the answer to "what is capacitance" is long, and you have to decide
    which parameters are of importance at a given time :)

    PeteS
     
  18. Rodney Kelp

    Rodney Kelp Guest

    It's used in filtering power to smooth it out.
     
  19. Rodney Kelp

    Rodney Kelp Guest

    You should read up on ELI the ICE man.
     
  20. What is ELI and ICE ?
     
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