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What is the importance of phase shift???

Discussion in 'Electronic Basics' started by sf1, Oct 11, 2005.

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  1. sf1

    sf1 Guest

    i study RLC circuits in school but i just dont understand why i stud
    it for..
    voltage leads current and current leads voltage 90 degrees, what doe
    that mean and why is it important to study it
  2. Lord Garth

    Lord Garth Guest

    Good old ELI the ICE man! It's just there to weed out those that aren't
    about electronics.

  3. Right now, you are just learning what the components do (how the
    voltage across them relates to the current through them. Once you get
    into amplifiers with feedback networks, one important effect of such
    phase shifts is that it can turn negative feedback into positive
    feedback and turn an amplifier into an oscillator. Very useful when
    you need it, very bad when you don't want it. You will find that any
    component with a sense of time (resistors don't remember anything
    except a severe overload) involves phase shift between its voltage and
    current. This is part of its memory of what has been going on,
    recently. The present value of the relationship is a combination of
    the present signal and the memory of earlier signals.
  4. Kitchen Man

    Kitchen Man Guest

    On Mon, 10 Oct 2005 19:35:54 -0500,
    You study it in order to get a diploma that pays well for a lifetime.
  5. Bob

    Bob Guest

    First, try to really understand what a resistor does. It's pretty easy,
    because the relationship between the voltage across a resistor and the
    current through that resistor is always fixed -- at all points in time, and
    for all types of applied signals. The current through a resistor is ALWAYS
    in phase with the voltage across the resistor regardless of the applied
    waveshape of the voltage (or current).

    Also, understand that resistors do NOT store electrical energy, they only
    radiate the energy as heat. Disconnect them from a circuit and you will not
    be able to recover any of the energy that has been radiated by them.

    Inductors and capacitors ARE electrical energy storage devices. If an
    un-energized L or C is somehow enticed to store some electrical energy then
    it takes time to change that stored energy from one level to another
    (regardless of whether you're increasing or decreasing its energy level).

    Here's an easy way to understand a capacitor:

    If you start with an un-energized C (i.e., its initial voltage = 0V), and
    then connect that C to a constant current source, then the voltage across
    the capacitor will increase linearly with time. The voltage is "lagging" the
    current because it takes time for the voltage (and energy) to build up.

    If you start with an un-energized C, and then connect that C to a current
    source that fluctuates with a sinusoidal characteristic, then the voltage
    will also have a sinusoidal characteristic, but it will have a phase shift
    of 90degrees with respect to the current through that C. This 90degree
    relationship is ONLY true for a sinusoidal stimulus. The voltage is said to
    "lag" the current by 90degrees (for a sin stimulus).

    A similar thing happens with an inductor. If you connect that L to a
    constant voltage source then the current will increase linearly with time.
    If you apply a sinusoidal voltage then the current will have a phase shift
    of 90degrees with respect to the voltage across that L. This 90degree
    relationship (yada, yada, yada...). The current is said to "lag" the voltage
    by 90degrees (for a sin stimulus).

    Think and experiment.

  6. PeteS

    PeteS Guest

    Further to John's post

    RC networks have certain characteristics (phase shift amongst them).
    These are found everywhere (even when you don't expect it) and you need
    to understand what is happening - to understand that, you need to
    understand the basics, of which RC networks is merely a part.

    Apart from that, we deliberately exploit the effects of RC networks, so
    a thorough understanding of what is *going* to happen is required.

    As some food for thought (where you can have a capacitor without
    knowing it) - capacitance exists between any two points of differing
    potential (voltage).


  7. redbelly

    redbelly Guest


    I do hope the OP appreciates the time you have spent on a thoughtful
    post. My own feelings on the matter are either
    1. He's a troll
    2. He is severely lacking the math prequisites (i.e., familiarity with
    the sine and cosine functions) needed to understand AC circuits.

    If he were a home hobbyist trying to teach himself electronics, it
    might be different. But he is taking a class in school, surrounded by
    people whom he could ask.


  8. Bob

    Bob Guest


    You may be right. There's always that chance that he's a troll, but I didn't
    sense that.

    These concepts are difficult, at first. There may be nobody around that can
    explain them in such a way that they "click". When I was in school, I had
    the luxury of working with some experienced engineers. They tought me most
    of the fundamentals. The books and teachers just confirmed what they were
    telling me.

  9. Bill Bowden

    Bill Bowden Guest

    i study RLC circuits in school but i just dont understand why i study
    Well, think about current charging a capacitor. If you decrease the
    charge current, the capacitor still charges and the voltage continues
    to rise because current is still flowing into the capacitor. The only
    way to stop the voltage from rising is to reduce the current to zero,
    or go negative.

    That means the maximum voltage will be reached when the current is
    zero, and visa versa. Thus, you get a 90 degree phase difference
    between the capacitor voltage and current.

  10. redbelly

    redbelly Guest

    Now I believe you're right, a troll would have been more active with
    follow up responses in the thread. In that case, I wish him luck in
    his studies.
    Getting things to click is the key. If somebody asked me in person
    about phases, I could sketch a graph of two sine waves with a phase
    offset, and say "Look ... this wave leads the other wave".
    Good one! :)
  11. Rich Grise

    Rich Grise Guest

    The "click" came for me when the teacher said, "A capacitor opposes
    a change in voltage, and an inductor opposes a change in current."

  12. ehsjr

    ehsjr Guest

    Studying electronics without studying RLC circuits would be like
    studying to be an MD without studying the skeleton. To be effective
    at designing or working on electronic circuits, you need to
    understand what each (resistance, inductance, capacitance) does
    in the circuit.

  13. BobG

    BobG Guest

    A big click of the lightbulb up above the head came when someone said
    the cap in an RC lo pass filter was just a 'frequency dependent
    resistor', and the filter had an R in series and this magic resistor to
    ground, just like a voltage divider or a pot. At hi frequencies, the
    bottom R is real small, and the voltage divider is turned all the way
    down. AT DC, the bottom R is real big, and the voltage divider is all
    the way up. Great intuitive seat of the pants explanation that I never
    heard in 5 years of engineering school and maxwells equations and a
    bunch of other stuff. To the guy wondering whats important: I get A LOT
    of mileage out of ohm's law.... like use it every day. Learn it.
    Memorize it.
  14. YD

    YD Guest

    It didn't "click" for me until I realized you could force the
    _current_ and measure the voltage. Until then I had only thought in
    terms of applying a voltage and measuring the current.

    - YD.
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