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AC sine wave: What does increasing the frequency do?

Discussion in 'Electronic Basics' started by Commander Dave, Nov 26, 2004.

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  1. The Phantom

    The Phantom Guest

    Of course; I didn't pick up on the zc part.
     
  2. John Fields

    John Fields Guest

     
  3. I love this newsgroup because a simple, one sentence question can
    generate days of technical exchange that teaches quite a few of us
    things we knew but didn't realize. :)
     
  4. The Phantom

    The Phantom Guest

    Yes. And all without any name-calling and foul language. Much
    more pleasant.

    John Larkin brings up some more interesting points that deserve a
    new thread. I'm going to have to think about them and perhaps in a
    day or so, start a thread. He said:

    "a. For a sinusoidal source, a time-varying resistive load can have a
    load current with a non-zero fundamental phase shift, hence a reactive
    load component. This load component can be expressed as an equivalent
    inductance or capacitance.

    b. For a sinusoidal source, a time-varying reactive load can have a
    load current with a non-quadrature phase shift, hence a real load
    component. This real component can be expressed as a positive or
    negative equivalent resistance. This is why a varicap can be used as a
    parametric amplifier.


    In case a, it takes no power to vary the resistance (as say moving a
    pot wiper or switching resistors in or out) because the synthesized
    reactance doesn't dissipate power. In case b, power must be involved
    in varying the reactance (spinning the shaft of a variable cap, or
    pumping a varactor) because we're synthesizing a real resistance."
     
  5. It amazes me that my simple question on power spawned this much
    discussion! I followed a little of it, but for the most part it made me
    realize how much I have to learn... :)

    Cheers!
    -Commander Dave
     
  6. The wheels are turning.
     
  7. You too?
     
  8. john jardine

    john jardine Guest

    message >
    ......
    The classes of gyrators here, has made for an interesting and
    *understandable* thread.
    Sometime this kind of stuff will be written up in depth, (maybe already
    has!). I'll bet that during the process 'understanding' will be #1 item to
    fall by the wayside :)
    regards
    john
     
  9. John Larkin

    John Larkin Guest

    Tons of theoretical work has been done on time-varying capacitances,
    mostly in the 60's and such when two-terminal devices (varactors,
    tunnel diodes, step-recovery diodes) were the rage.


    I like to try to avoid equations until I can really feel what's going
    on. Being able to do the math doesn't mean you understand it, just
    that you can push some symbols around. This is risky of course,
    because instincts are often wrong about stuff like this. But the guys
    who just do the math can make ghastly blunders, too, and they
    sometimes don't have the instincts to recognize an absurd result when
    they see it.

    Things like Fourier transforms can be visualized and sort of done by
    inspection, but not many EE courses try to track that along with the
    math.

    To a creature that was sufficiently intelligent, everything would be
    intuitively obvious.

    John
     
  10. peterken

    peterken Guest

    'scuse my ignorance, but doesn't an incandescent lamp behave more like say a
    triac or a set of zeners after the startup cycle?
    this means, the gas ignites at a specific voltage (think it was about 80V
    somewhere) thus lowering the lamps impedance from that point on.
    that's also the reason of the coil in the circuit, the impedance reduces
    lamps current, it allows the lamps voltage to drop to the burning point.
    the gas stays ignited until again a specific voltage where it stops
    conducting, and re-ignites again at a specific voltage the other half of the
    cycle.
    only other thing in the circuit is a coil, so i don't see the "capacitor"
    anywhere
    (view drawing in notepad using fixed font)

    |mains
    | . .
    | . .
    |. .
    .-----------.------------.------
    | . .
    | . .
    | . .
    |
    | ..
    | . .
    | . .
    | . .
    |....----- .....-------.......--
    | . .
    |lamp . .
    |current . .
    | ..
    |
     
  11. What you are talking about is not an incandescent lamp, it is a
    fluoroscent lamp.

    An incandescent lamp has a thin wire inside which glows, it is the most
    common lamp in the world. But fluoroscents with their higher efficiency
    are taking increasing parts of the market.

    It is difficult to keep all these technical terms apart in english if you
    are not born in an english-speaking country.
     
  12. peterken

    peterken Guest

    oops, sorry, my mistake :-(
    indeed I'm dutch, thereof the mistake
     
  13. Khwaj

    Khwaj Guest

    This is a mechanical problem. At the most basic level the generators are
    unable to maintain their rpm while heavily loaded.
     
  14. John Larkin

    John Larkin Guest

    So, could a motor-driven variable capacitor, paralleled with an
    inductor, become an oscillator? Seems like it.

    John
     
  15. Don Kelly

    Don Kelly Guest

     
  16. Bill Bowden

    Bill Bowden Guest

    Wasn't it Tesla that proposed the 60 Hz. standard?
    And probably because there are 60 minutes in an hour,
    and 60 seconds in a minute, and therefore 60 cycles
    in a second, and 60 was high enough to avoid flicker,
    and maybe some other reasons.

    What was the reasoning for 50 Hz, other than
    slightly better transmission efficiency?

    -Bill
     
  17. Don Kelly

    Don Kelly Guest

    ---------
    The difference between 50 and 60 Hz will make some gain in a reduction of
    iron in machines at 60 Hz and, on the other side, some increase in
    transmission capability (not necessarily efficiency) at 50 Hz. However, it
    appears that the areas which originally had longer transmission distances
    went to 60 Hz. so where's the logic.
    Probably a choice of "the Brits chose 50Hz so we will choose 60Hz" (or the
    opposite with Yanks substituted for Brits ).

    As for flicker at 25 Hz ( Not 24) with incandescents. It exists and can be
    noticed. I have seen it. Subtle but there- somewhat similar to a computer
    monitor refresh rate of 55 to 60Hz. It can be annoying if you are not used
    to it..
     
  18. Rich Grise

    Rich Grise Guest

    About time somebody noticed! ;-)

    Actually, I've been following the thread, and about the only contribution
    I can make is that I was visualizing the water-hose model of current,
    where the lightbulb goes "OOF!!" when it gets hot, stopping up the
    current, but when it cools, it goes, "Aaaah!", and lets the current flow
    through again.

    So you see why I didn't try to contribute to the actual science of the
    thing! ;-)

    Thanks!
    Rich
     
  19. Rich Grise

    Rich Grise Guest

    Oh, this is nothing compared to the thread I instigated a couple of years
    ago about running a 120V bulb off 240V mains by just putting a diode in
    series!

    Cheers!
    Rich
     
  20. Rich Grise

    Rich Grise Guest

    I don't even know what a Fourier analysis _is_ other than a way of
    translating a data set from the time domain to the frequency domain
    (transforming?), but my "gut-feeling" is that the zero-crossings would be
    identical _of the actual source waves_ - the fundamental would be out of
    phase, in the plot of the Fourier-transformed resolved fundamental, but
    the zero-crossing would be brought back into sync by the harmonics after
    you added them back together.

    It's like a sine wave with the half-cycles tilted to one side.

    But wouldn't that act more inductive?

    Thanks,
    Rich
     
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