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Still confused with basics

Discussion in 'Electronic Basics' started by jaydee, Aug 21, 2004.

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  1. jaydee

    jaydee Guest

    Since an electrical outlet might be used for any size appliance it
    must contain a full/max current and then the appliance only uses what
    it needs... right?

    Does the appliance then reduce it? A simple lamp lightbulb uses very
    little but doesn't contain any electronics that would do this. So how
    does it use only a few watts and what happens to the rest?

    Secondly, the electric meter and bill indicates watts used. How does
    this work? This seems to imply that the current is always flowing in
    and out like a two-way road and the meter measures the difference
    (unlike all the water analogies that flow into a dead-end).

    Thanks Jay.
     
  2. John Fields

    John Fields Guest

    ---
    Its resistance keeps it from taking all of it, and what it doesn't use
    is there for anything else that plugs into the outlet.
    ---
    ---
    It measures how much current the appliance uses over time, and it
    knows what the voltage supplied to the load is, so it multiplies the
    voltage, current, and time and comes up with watt-hours, which is what
    you pay for. In the mechanical watt-hour meters, the speed of a
    spinning disk increases as more current is drawn, and that spinning
    causes a series of dials to move which point to ever-increasing
    numbers, so that what happens is that when the readings are taken
    month-to-month, the number from last month is subtracted from this
    month's number and the difference is the energy you used between the
    time the readings were made.
    ---
     
  3. John Larkin

    John Larkin Guest


    To amplify JFs comments...

    Think of the power coming into your house as a rotating shaft.
    (Factories actually used to have one big rotating shaft from a water
    wheel or steam engine, and it ran everything.) If it spins freely with
    no load, no energy is taken and the power bill is zero. Any device can
    be connected to the shaft (they used to use belts or gears) and it can
    use all the horsepower it wants or needs. A mechanical device uses
    torque, and the power it gobbles is torque times speed.

    An electrical system has wires (== shaft), voltage (== rotation of
    shaft), and loads take as much current (==torque) as they need. The
    price you pay is voltage * current * time * some_dollar_factor,
    because that quantity represents how much fuel the utility had to burn
    to make that much energy.

    A low resistance load is like a high-torque load. It may even pull the
    line voltage down a bit, just like a big load might slow down the
    drive shaft some.

    I like that better than the water pressure analogy.

    John
     
  4. Repeat after me...

    VOLTAGE IS THE FORCE THAT CAUSES CURRENT TO FLOW.

    That is, voltage is the cause and current is the effect.

    When you apply voltage to anything, the amount of current that flows depends
    on the resistance of that thing. That's why a big light bulb consumes more
    current than a little one. It has a lower resistance and lets the electrons
    flow through it more easily (thus grabbing more of them per second and
    giving off more heat and light).

    Voltage = the pressure that makes electrons moved.

    Current (amperage) = a measure of the number of electrons actually going by
    per unit of time.
    The same thing that happens to the water that isn't coming out of your
    faucet when the faucet is turned on just a little bit. Those other
    electrons aren't flowing. Pressure is available to make them flow, but
    pressure is just pressure -- not flow -- if you don't let many of them move.
    It indicates watt-hours. That is voltage times current times time. I.e.,
    pressure times number of electrons that were actually moved through your
    house at that pressure.

    It's as if the water company were charging you for the pressure of the water
    as well as for the quantity of it. That would be a reasonable way for them
    to charge you if you were using to turn waterwheels (rather than just
    consuming the water). The power company is charging you for the energy of
    the moving electrons, rather than for the electrons themselves, because you
    give them all back, exactly as if you were using water to turn waterwheels
    and then giving it back. They'd charge you for pressure and for the volume
    that flows by, because the product of these is the amount of energy you've
    received. (Not the amount of water.)
     
  5. John Fields

    John Fields Guest

    ---
    Yeah, me too.

    The only thing that isn't taken care of is the oscillatory nature of
    the AC, which would correspond to an oscillating shaft coming into
    your house. For heaters and lamps it wouldn't matter, you just grab
    the shaft and friction does the trick, no matter which way it's going,
    but to get the unidirectional rotation you need to run compressors and
    simulate motors you'd need some kind of a mechanical converter. Maybe
    like those little manually propelled railroad cars with the push-pull
    bars used?
     
  6. John Larkin

    John Larkin Guest


    Right, it's not a great mathematical analogy to AC. And a shaft has
    "inductance" (torsional flex) and "capacitance" (mass) but no
    equivalent of resistance, unless you make it out of taffy.

    If you apply a sudden twist to a long shaft, it propagates just like a
    step on a transmission line.

    I did design a mechanical full-wave rectifier once, so a ship shaft
    rotation indicator would count revs in both directions. It used a
    couple of shafts and some belts, and some really cool one-way
    mechanical couplings that look just like bearings and act like diodes.
    The MEs were impressed.

    John
     
  7. John Fields

    John Fields Guest

    ---
    Don't you have it backwards?

    That is, since mass fights anything trying to make it, or stop it,
    from moving, isn't that like inductance, which fights tooth and nail
    to keep any change of current through it from happening?

    And isn't torsional flex like capacitance, which will absorb as much
    energy as you can push into it until it starts to fill up and fight
    back?

    Also, doesn't it have resistance? No matter what you make it out of
    it'll still get hot if you flex it, I think. Just like a cap has ESR
    and a choke has winding resistance to contend with, a shaft has grain
    boundaries which rub against each other and and spoil what would
    otherwise be a perfect Q.
    ---
    ---
    Yes, nice.

    And the shaft will have a characteristic impedance, just like a
    transmission line, but I think those guys call it its "moment of
    inertia"
    ---
     
  8. jaydee

    jaydee Guest

    Thanks guys, I'm seeing the light (a little).

    So an appliance built with low resistance takes more current and an
    appliance like a light bulb contains some material causing high
    resistance.

    Regarding the meter... it's not measuring the difference between what
    came in and what's going out, but simply the flow into the house
    caused by some applicance usage.

    If yes, then why do we need the neutral wire? Whats the extra wire
    leaving the house for? I thought I understood the loop concept.

    Thanks again,
    Jim
     
  9. John Larkin

    John Larkin Guest

    Right. A light bulb with a fat filament will conduct a lot of amps
    (let a lot of electrons rush through) so use a lot of power and get
    very bright. A skinny filament throttles down the current to a
    trickle, so uses less power. If you weld ten skinny filaments together
    side-by-side, you make a fat filament that conducts 10 times as well.
    That's "putting resistors in parallel." Weld them end-to-end, and the
    current really trickles down, resistors in series.
    Right. Actually it integrates the product of voltage * current. The
    voltage isn't supposed to vary much from 120, but the meter accounts
    for it anyhow, just to be accurate.
    Current needs a loop, just as a shaft needs a foundation connecting
    the motor and the load to return the torque. Without a shared
    mechanical anchor for both ends, the motor or the load housing would
    just spin and no power would be delivered. We use the neutral wire as
    the reference or foundation to conduct the return current. We could
    (electrically and mechanically!) use the earth (ground) as the return
    path, but it's safer to use a neutral wire, and use the third (ground)
    wire as a safety shield.

    John
     
  10. John Miller

    John Miller Guest

    It's because your home service has *two* 110-volt circuits, which can be
    used individually for 110V, or combined for 220V. The neutral is the
    return for either/both of the 110V legs, and is not necessary for 220V.

    --
    John Miller
    Email address: domain, n4vu.com; username, jsm

    Life is like an onion: you peel it off one layer at a time, and sometimes
    you weep.
    -Carl Sandburg
     
  11. John Fields

    John Fields Guest

    ---
    It's like this:


    HOT--> +------------------+----------------+------>TO MORE HOUSES
    | | |
    | | |
    +------|-------+ +-----|-----+ +-----|-----+
    | | | | | | | | |
    | [GENERATOR] | | [TOASTER | | [ANY LOAD]|
    | | | | | | | | |
    +------|-------+ +-----|-----+ +-----|-----+
    | UTILITY | YOUR | ANY
    | COMPANY | HOUSE | HOUSE
    NEUTRAL--> +------------------+----------------+------>TO MORE HOUSES
     
  12. Rich Grise

    Rich Grise Guest

    The electricity doesn't pile up at your house like water, or go
    into the light bulb and dissipate as light or anything like that.
    (not the electricity itself - its _energy_ comes out as light.)
    For the water model, it still comes in one pipe and goes out the
    other.

    The energy gets transferred by the movement of the water. How
    hard does the power station have to push it (the pressure, or
    voltage) times how much it has to push (the flow == current).
    That force times motion is the work done, or energy transferred,
    and the power is the rate at which that energy is getting
    transferred. The water, or electric charge, is just the medium
    for moving that energy from one point to another.

    Hope This Helps!
    Rich
     
  13. jaydee

    jaydee Guest

    The light is getting brighter!

    I just thought of another analogy... If you had a well connected to
    your water heater... you would be paying to heat the water, but not
    for the water itself.

    My real objective was to understand enough to hook up a generator to
    the breaker box and reduce my bill. Transfer switches do the opposite
    of what I need... they work when the utility power is off and
    disconnect when the power comes back on.

    I've also read someplace that home generated power isn't pure (?)
    enough to blend with the power company's. If so my only alternatives
    would be to use extension cords or a sub-panel for the few circuts I
    could feed (yes/no).

    Thanks for the education,
    Jay
     
  14. John Fields

    John Fields Guest

    ---
    Actually, it's backwards from that in that when the utility power is
    ON, that power actuates the switch and causes it to connect power to
    the loads in your house. When the utility power goes off, the
    transfer switch reverts to its de-energized position and a set of
    normally closed contacts connect the alternate power source to the
    loads in your house.
    ---
    ---
    I don't understand what you mean. Are you talking about selling
    energy back to the power company or running some things on your house
    on your own power at the same time you're running some others on power
    company electricity, or connecting your power source in parallel with
    the power company's so that you can run your house on both at the
    same time?
    ---
     
  15. What that probably meant is in connection with those systems that actually send
    power back into their system from your remote location and that in these cases
    the power company needs your generation facility to be "compatible" with theirs.
    Some power companies will make the meaning of "compatible" to be a nearly
    impossible goal to reach, just to be a pain. Some will encourage you and help
    you understand the details. But you can't just hook a generator up and start
    feeding power back into the lines without synchronizing up and without the right
    pieces in place. If all you are doing is to use your locally generated power to
    defray your own use and are NOT trying to push power back into the system of
    your provider, these issues don't arise.

    At least, that's my guess about it.

    Jon
     
  16. My real objective was to understand enough to hook up a generator to
    Can you generate power more cheaply than the power company?

    When dealing with AC, you have to synchronize your oscillations with
    theirs...
     
  17. You MUST NOT connect your local generator in parallel with the power
    company!!!

    You will need a manual transfer switch to transfer those circuits you
    want to power from your generator between the generator and power
    company.
    "purity" isn't the problem. AC power is continually varying in
    voltage, so, if you want to connect two AC sources in parallel, you
    must ensure that both sources are "in phase" (varying exactly in step
    with each other) and of the same voltage.

    --
    Peter Bennett, VE7CEI
    peterbb4 (at) interchange.ubc.ca
    new newsgroup users info : http://vancouver-webpages.com/nnq
    GPS and NMEA info: http://vancouver-webpages.com/peter
    Vancouver Power Squadron: http://vancouver.powersquadron.ca
     
  18. Rich Grise

    Rich Grise Guest

    I was working at a temp job as a document coder for big litigations.
    One of the litigations was about some company who was going to make
    a big profit that year that the government mandated that the power
    companies have to buy power from the customers. They had all kinds
    of schemes for excuses to generate power, like gas turbine generators
    with the exhaust used to heat a greenhouse, and crap like that.

    They went broke.

    Good Luck!
    Rich
     
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