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Absolute Beginner--Making Sense Of Capacitors

Discussion in 'Electronic Basics' started by Pete Holland Jr., Apr 20, 2006.

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  1. Hey, everybody!

    In the interest of actually getting my understanding of electronics right,
    I'm trying to forget all the notions I had as a kid (not that tough,
    really, there weren't that many). Lamps and resistors, no problem. They
    let current flow through (with varying degrees of success) and give off
    light and/or heat.

    Now, for components that do a little more than just move the train down the
    track. The next thing I'm examining is capacitors. I would like to know
    how close to the truth I am and if there's anything I have wrong.

    1) One of the things I was told was that electricity follows the path of
    least resistance. This puzzled me, since, without micromanaging every
    aspect, devices like computers with multiple circuits and a single power
    source would never work (please note this was the days of SBC's, when a Z80
    CPU was considered hot stuff).

    As I understand it, a capacitor allows current to flow, but how much gets
    through is inversely proportional to the charge it holds--once full, a
    capacitor basically will not allow any more current to flow through it. Is
    this partly how the load is distributed so all components get electricity?

    2) A capacitor acts as a flow control and as a (very short lived) battery
    when hooked up correctly. Are there any other tricks it can do?

    Dobre utka,
    Pete Holland Jr.
     
  2. Bob

    Bob Guest

    Your notions of the characteristics of a capacitor are wrong.

    The only way you can really understand these things is to get a book and/or
    a tutor. You must truly understand:

    electrical energy
    power (easy once you're comfortable with energy)
    voltage
    current


    Then, you can move on to resistors and various networks with them hooked up
    with voltage sources and current sources.

    It's my opinion that you shouldn't even bother trying to understand
    capacitors, inductors, and other devices (e..g., diodes and transistors)
    until you can hook up voltage sources, current sources, and resistors, and
    be able to predict what is happening at every node and every branch of the
    network.

    After that, you'll be ready for your capacitors.

    Bob
     
  3. It is not an all or nothing thing. Current caused by a given voltage
    difference is inverse to the resistance of any path. The lower the
    resistance the higher the current. But higher resistance paths still
    pass some current. This proportionality is captured in Ohm's law. It
    states that the current is proportional to the voltage and inverse to
    the resistance. I=E/R Rearranging this to solve for resistance shows
    that ohms are just another word for volts per ampere. R=E/I
    There is some value to this description, but it is awful approximate.
    A better way to say this is that the current through a capacitor is
    proportional to both the capacitance and the rate of change of voltage
    across it. I=C*(dv/dt) I is in amperes, C in farads, and dv/dt in
    volts per second. Once the voltage across the capacitor matches some
    DC source connected across it, the voltage across the capacitor
    quickly becomes constant (has zero rate of change) so the current
    becomes zero.
    Not really. Resistors pass current continuously in proportion to the
    voltage across them, but capacitors pass current only when the voltage
    across them changes.
    A capacitor has a well defined AC current when AC voltage is applied
    across it, because the AC waveform has a well defined rate of change
    throughout the wave.

    A capacitor behaves a little like a battery, because it supplies
    current when its precharged voltage runs down (that is just another
    example of a rate of change), but the voltage has to be falling for it
    to supply current.

    Batteries produce a roughly constant voltage for a long time, till
    their chemical energy is depleted, and then their voltage decays rapidly.
     
  4. Joel Kolstad

    Joel Kolstad Guest

    Well, the above is a simplification, and given how it's misleading you but
    still keeping in the spirit of simplication, it'd be better to say that
    electricity *prefers* the path of least resistance. But it will flow wherever
    it can -- if you take a 9V battery and connect a 9k and 1k resistor in
    parallel with it, the 9k resistor ends up with (V=IR -> I=V/R) 9V/9k=1mA
    flowing through it while the 1k resistor ends up with 9V/1k=9mA.
    Well... in the ideal capacitor, there's so much thing as "full." What I
    imagine you mean, though, is that if you take something like a 9V battery,
    resistor, and capacitor and wire them all up in series, current will stop
    flowing once the capacitor has reached 9V as well. But this is only because
    the *resistor* has 9V on both sides of it (9V from the battery, 9V from the
    capacitor), so the voltage across the *resistor* is 9V-9V=0V, and hence no
    current flows.
    If you think of the capacitor as being like a big water storage tank,
    somewhere there's a "water supply" (e.g., a river connected to an ocean; this
    corresponds to the "bulk" power supply in a circuit) that's trying to keep the
    water tank at a certain level (e.g., 5V). You can hook up as many devices
    (showers, sinks, etc.) to that water tank and so long as the external supply
    can meet the average water current demand for all the loads, all the loads see
    pretty much exactly the same water pressure (voltage) and work just fine. The
    purpose of the water tank (capacitor) in this case is actually just to smooth
    out what would otherwise be pressure (voltage) fluctuations seen at the
    various loads, since often the river (bulk power supply) is a long distance
    away from the loads and the finite impedance of the river (power supply
    wiring) doesn't allow the ocean itself to quickly "equalize" the pressure
    (voltage).
    Sure... capacitors are frequency-dependent components, so you find them all
    over the place if you're trying to build filters, resonators, oscillators,
    etc. This fact can also be exploited to use them as (typically) integrators,
    to collect current from photdiodes or somesuch, solve differential equations
    in analog computers (granted, not a very common device these days!), etc.
    Additionally, since they store charge (as you allude to by calling them "short
    lived batteries"), you can generate some cool circuits by charging them via
    one set of electrical connections and then discharging them via another; this
    leads to things like switched-capacitor power supplies (including the
    ubiquitous Max232) -- a somewhat "crude" application -- and switched-capacitor
    signal processing -- a more refined application, which many people aren't
    aware of. (For some decades the filters in telephone central offices were of
    the switched capacitor variety -- they could replace physically bulky and not
    particularly ideal inductors... these days those filters are done with DSP
    chips. Here's a very nice implementation of a switched-capacitor low-pass
    filter: http://www.idiompress.com/scaf-1.html)

    There's a ton of web sites that (attempt to :) ) explain electronics. Here's
    one: http://www.geocities.com/talkingelectronics/index.html ... if you find it
    confusing, just seek out another one, since sooner or later you'll find one
    that makes sense to *you*.

    ----Joel Kolstad
     
  5. Joel Kolstad

    Joel Kolstad Guest

    One other thing: When you think about how simply defined an ideal capacitor is
    (I=C*dV/dt -- that's it!) and how real capacitors are actually very good
    approximations of the ideal (at least compared to inductors!), it's truly
    incredible just how many creative ways people have been able to apply them.
     
  6. Guest

    Definitely! And voltage is the tricky one.


    Electricity DOESN'T follow the path of least resistance.

    Instead the rule is the same as for water : For a constant-pressure
    pump, if you make the pipe resistance higher, the water flows slower,
    so the current is smaller. (And if you make the pressure higher, then

    again the water flows faster and you have more current.)

    Ohm's law is pretty simple: the higher the pressure, the faster the
    flow.

    With parallel resistors where the charges split into two paths, if the
    paths have two different resistances ...electricity DOESN'T take the
    path of least resistance. Instead the majority of the flow is in the
    low-
    resistance path, while a proportionally smaller flow is in the high-
    resistance path.

    Capacitors hold zero charge. Capacitors are only ever "charged" with
    energy, while the total charge inside a capacitor never changes.

    Or in other words, whenever you force an electron into one of a
    capacitor's terminals, you're also forcing one electron out of the
    other
    terminal at the same time. Electrons don't build up inside of
    capacitors,
    any more than electrons build up in resistors (or inside wires.)

    Take a look at :

    Capacitor misconceptions
    http://amasci.com/emotor/cap1.html
    http://amasci.com/emotor/stmiscon.html#six

    What actually happens is, as charges flow through a capacitor,
    the capacitor increasingly fights against the charge flow. It does
    this because, as charges flow through it, the voltage ACROSS the
    capacitor terminals rises higher and higher. When the voltage
    across the capacitor gets to the same value as the voltage of the
    power supply which produces the flow, the flow halts. For larger
    capacitors the voltage builds up more slowly. (In engineer-speak
    we'd say that the capacitor voltage is the time integral of charge flow

    per second through the device, divided by capacitance.)


    In that water-capacitor in the link above, the rubber barrier would
    stretch more and more until it managed to slow the current to a
    stop. But if you then increased the power supply's pressure, the
    current would start up again. If instead you decreased the power
    supply's pressure, the rubber would push the water backwards and
    run the pump as if it was a motor. Finally, a stiff rubber barrier
    acts like a capacitor of low value.



    Most of the capacitors you see on an analog circuit board are
    there to pass the AC signals between different circuit sections,
    while at the same time keeping the different DC stuff confined
    to each section. The various setups of DC runs the sections,
    while the signals flow between the sections of circuitry.

    It's harder to design the sections of circuitry so they can connect
    together without needing any capacitors. But it's possible. All
    the sections inside an analog IC must connect without capacitors.


    ((((((((((((((((((((((( ( ( (o) ) ) )))))))))))))))))))))))
    William J. Beaty Research Engineer
    UW Chem Dept, Bagley Hall RM74
    Box 351700, Seattle, WA 98195-1700
    ph425-222-5066 http//staff.washington.edu/wbeaty/
     
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