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Impedance Low/High

Discussion in 'Electronic Basics' started by Uriah, Jun 3, 2006.

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  1. Uriah

    Uriah Guest

    I seem to be real slow in grasping these simple concepts. Another
    question on Impedance.
    What is low impedance? and what does it mean? Can you have low
    impedance with high voltage or does it only apply to current? Is it
    just a relative thing. When dealing with digital circuits does
    low/high impedance imply certain things? What things?
    thanks again
  2. Impedance, simplified, is just a characterisation number of a component
    that determines what current is caused by an applied voltage.

    It is resistance (impedance) R = V/I, V is the applied voltage, I is
    the current.

    There is a little complication for AC circuits, which is where
    resistance -> impedance, but this does not change the basic concept of

    Impedance can be any value from 0 - infinity. So, yes you can have low
    impedance and high voltage. This will produce a high current.

    Impedance can effect the speed and frequency response of a system. The
    basic figure of merit is T=RC, with F~1/T. C is the capacitance. Lower
    impedances imply faster speed, and more current.

    There is a general principle here. That is, for a given accuracy, if you
    increases speed, you need to increase the current. This is expresed by
    F(error, speed, power) = 0

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.

    "There are none more ignorant and useless,than they that seek answers
    on their knees, with their eyes closed"
  3. Ralph Mowery

    Ralph Mowery Guest

    It is a relative thing depending on who is doing the talking and what the
    subject is. For audio circuits (microphones) low impedance may imply a mic
    that has a dynamic element and an impedance of under 50,000 ohms, more like
    under 1000 ohms. A high impedance may imply a crystal element and an
    impedance of over 1 meg ohms. Lots of times you can drive a high impedance
    load with a low impedance source and it will work. If you try to drive a
    low impedance load with a high impedance source, the source will be
    overloaded and not do very well. You may think of it as a battery. If you
    put several AA batteries in series to get 12 volts and compare it to a car
    battery at 12 volts , the car battery will have a lower impedance than the
    AA batteries as it can deliver more current due to the internal impedance of
    the batteries.

    You can have high voltage and high current at the same time, but that takes
    lots of power. Usually low impedance indicates low voltage and high current
    and high impedance indicates the opposite.

    With digital circuits it does come into play. You may see it as fan in and
    fan out. A low impedance output can drive a small amount of low impedance
    loads, but a greater number of high impedance loads. As a general rule the
    low impedance circuits are less subseptical to random noise than the high
    impedance circuits.
  4. John Fields

    John Fields Guest

    Impedance is nothing more than resistance to the flow of charge.

    Likening it to the flow of water in hoses with equal lengths but
    different diameters connected to sources of water with the same
    heads, the smaller diameter hose will allow less water to flow,
    therefore it presents a higher impedance to the source than the
    larger diameter hose.

    The name says a lot. Impedance is the tendency of a device or circuit to
    IMPEDE or resist current flow. It is not simply resistance but the "sum" of
    REACTANCE and RESISTANCE. I put the word "sum" in quotes because impedance
    is a vector sum of reactance and resistance not a simple arithmetic sum.


    While resistors resist current by dissipating power in the form of heat
    (like the brakes in your car), reactive components such as inductors and
    capacitors resist current by storing energy. In physics you should have
    learned about two forms of energy: POTENTIAL ENERGY and KINETIC ENERGY.
    Potential energy is energy possessed by things not due to motion such as the
    energy contained in compressed springs, rocks on a cliff or even fire
    crackers before exploding. Kinetic energy however is energy possessed by
    things only by virtue of their motion such as moving automobiles, falling
    rocks or exploding firecrackers. Reactive devices store energy in two ways:

    1. Inductors store energy in the form of charge (usually electrons) motion
    or current.

    -Inductors store kinetic energy.

    2. Capacitors store energy in the form of electrostatic force or voltage.

    -Capacitors store potential energy.

    To understand how a reactive device such as an inductor can impede current
    think about a flywheel. A flywheel performs the same function in the
    mechanical realm that inductors perform in the electronic realm. If you
    attempt to spin a flywheel it resists being turned because you must first
    overcome its inertia. Once you get a flywheel spinning however it contains
    stored energy and you must first dissipate this stored energy before the
    flywheel will stop. In fact you might get hurt trying to stop a flywheel all
    at once by simply grabbing it. The same thing happens in inductors.
    Inductors resist current flow because they posses a certain amount of
    electrical "inertia" in the form of inductance. Because inductors store
    kinetic energy you cannot stop the current through an inductor instantly.
    Many electronic professionals and enthusiasts are aware of the sparks that
    can fly at the switch when devices such as a motors, coils or electromagnets
    are disconnected from their power source suddenly.

    To understand how capacitors can impede current think about an air
    compressor tank. An air compressor tank resists being pressurized because it
    is storing energy. This energy can be retrieved later by releasing air from
    the tank. You cannot change the pressure in the tank, to some arbitrary
    pressure, instantly because you must push air into the tank at some finite
    rate to change its pressure and the tank is constantly resisting attempts to
    push more air into it. Anybody who's ever watched a pressure gage on an air
    compressor, waiting for it to reach a certain pressure, knows you must wait.

    All of the hot air above was to introduce the concept of reactance to those
    who have problems with the concept. Remember there are three main types of
    opposition to current flow:

    1. Resistance - electrical energy converted to heat

    2. Reactance - electrical energy stored in electrostatic form (capacitor) or
    magnetic form (Inductor)

    3. Impedance - combination of Resistance and Reactance (vector sum).

    Dorian McIntire EE, PE

  6. Impedance is a word that means the frequency dependent ratio of the
    voltage applied across two nodes divided by the current that voltage
    forces through those two nodes. If frequency doesn't make any
    difference, the impedance is a resistance.

    Low(er) impedance implies a relatively lower volts per ampere, while
    high(er) impedance implies a relatively higher volts per ampere, at
    some frequency.

    For example, an 8 ohm impedance speaker takes 8 volts across it to
    force an ampere through it at some specified audio frequency, perhaps
    1 kHz.
  7. Rich Grise

    Rich Grise Guest

    Dewd! I like this a _lot_! :) :) :)

    The way I heard it once, "An inductor opposes a change in current, a
    capacitor opposes a change in voltage".

  8. I wanted to add to my last post that impedance and reactance, as a general
    rule, are only observed in AC and not DC circuits. Resistance alone is only
    observed in steady state (nothing changing) DC circuits. Reactance and
    impedance effects can only be observed in DC circuits during transients such
    as when power is first applied or the voltage is changed suddenly.

    The key thing to remember is that changes have to occur in a circuit for
    reactance or impedance effects to be observed.

    Dorian McIntire
  9. John Larkin

    John Larkin Guest

    If there's a voltage source of X volts, and you hook a load to it, if
    the voltage droops just a little, it's a low impedance source. If it
    loses a lot of voltage, it's a high impedance source.

    If you connect your car battery to the starter motor, it might drop
    from 12 to 8 volts with the 100 amp starter load; that's a source
    impedance of 4/100 = 0.04 ohms (it lost 4 volts when it was delivering
    100 amps.) If you connected a string of eight AAA batteries (also 12
    volts at no load) to the starter motor, you'd see near zero volts,
    maybe an amp or so of current, and the car *would not* start.

    Imagine a tiny motor spinning 1000 RPM, and a huge motor also spinning
    1000 RPM. Now imagine grabbing the shaft of the little one, then
    grabbing the shaft of the big one. You could feel the lower mechanical
    impedance of the big motor; it might rip your hand off, but it
    wouldn't slow down much; it just doesn't care. It's the difference
    between being hit by a pillow going 30 MPH, or a cement truck going 30
    MPH. Same thing.

  10. Pooh Bear

    Pooh Bear Guest

    You wanted to *add* some more ? Goodness.

    My definition of impedance for newbies is 'like resistance but takes into
    account the effect on ac signals of inductance and capacitance too'. If the guy
    wants a lecture he can always ask.

  11. John Larkin

    John Larkin Guest

    DC resistance is impedance.

  12. Ralph Mowery

    Ralph Mowery Guest

    DC resistance is resistance, not impedance. AC has impedance. If you have
    a coil, it will have a DC resistance and an AC impedance which will
    normally be much differant.
    The same with a capacitor, it will have a DC resistance (normally very high
    for low values) and an AC impedance.
  13. John Larkin

    John Larkin Guest

    Gosh, that's not what they taught me in engineering school. They
    claimed that impedance is the vector sum of resistance and reactance.

    Thanks for straightening me out. I'll ask for my money back.

  14. I think you have misunderstood impedance. Impedance is a two
    dimensional value and frequency dependent. Resistance is a special
    case of impedance, where the second dimension (the imaginary part that
    is frequency dependent) is zero. Pure inductance or pure capacitance
    have an impedance that has no real part (that dimension has a zero
    value), but only an imaginary part (that is frequency dependent).

    Impedance is a general concept that includes all combinations of
    resistance and inductive or capacitive reactance. Impedance carries
    an implied if not stated frequency dependence, except for the case of
    pure resistance, which is frequency independent.
  15. Yes, learning is like that. You must use at least a few words to explain
    things and I choose very few words for this particular topic. You haven't
    experienced a lecture if you think my post was a lecture. If people want to
    learn they have to watch, listen and realize there are no short cuts.

    If the original poster or any others for that matter only want quick,
    useless, opinions about electronics they just need to let me know and I'll
    stop posting.

    Saying something takes into account the "effect" of something doesn't quite
    explain what the "effect" is. Does it?

    Since you mentioned it the effects are as follows:

    The impedance for a perfect capacitor (no leakage) approaches infinity (open
    circuit) as the frequency goes to zero (DC) and its impedance approaches
    zero (short circuit) as the frequency approaches infinity.

    What the above really means is that a capacitor is an open circuit for DC
    and just the opposite for extremely high frequencies.

    The impedance for a perfect inductor (zero resistance) approaches zero
    (short circuit) as the frequency approaches zero (DC) and its impedance
    approaches infinity (open circuit) as the frequency approaches infinity.

    What the above really means is that an inductor is a short circuit for DC
    and just the opposite for extremely high frequencies

    In practice both capacitors and inductors have resistances and other
    properties that make them less than perfect.

  16. John Fields

    John Fields Guest

    Wrong. Impedance is defined as:

    Z = sqrt (R² + (Xl - Xc)²),

    and, since capacitive reactance is defined as:

    Xc = -------

    and inductive reactance is defined as:

    Xl = 2pifL,

    at DC, then, the "f" term will be zero, so Xc will be infinite and
    Xl will be zero. Plugging that into:

    Z² = R² + (Xl - Xc)²,

    gives us:

    Z² = R² + (0 - infinity)²,

    = R² + infinity

    = infinity,

    So Z = sqrt (infinity)

    = infinity.

    Looking at it another way, with DC in the circuit the capacitor will
    not pass any current other than the initial transient and whatever
    leakage current manages to get through it, in series with the
    resistance of the resistor and the inductor, so the
    resistance/impedance of the circuit at DC is essentially infinite

    In a parallel circuit, DC will flow through the inductor and will be
    limited only by the resistance of the inductor in parallel with the
    resistance of the resistor and the ESR of the capacitor, so the
    impedance/resistance of that circuit at DC will be essentially zero
  17. John,

    Looks like your second post and my third post almost crossed in the system
    and say essentially the same thing expressed two different ways.

  18. Indeed you should:)

    Kevin Aylward B.Sc.
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.

    "There are none more ignorant and useless,than they that seek answers
    on their knees, with their eyes closed"
  19. Er... this is not accurate. Impedance is indeed observed in DC circuits.
    Impedance is a *generlised* term that covers both DC and AC at *any*
    frequency. The DC condition is when f=0.

    To wit:

    Z = R + jXz

    Now set jXz to zero.

    Resistance is an impedance, but all impedances are not resistances.
    Er... no...see above.
    The key thing is remembering the actual definition of impedance:)

    Kevin Aylward B.Sc.
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.

    "There are none more ignorant and useless,than they that seek answers
    on their knees, with their eyes closed"
  20. Setting the imaginary term to zero leaves only resistance which is what I
    stated can only be observed in DC circuits. This is merely a semantic
    argument so I'll leave you with the Wikipedia definition for impedance (not
    my first choice but readily available):

    "Electrical impedance, (sometimes Electric impedance) or simply impedance,
    is a measure of opposition to a sinusoidal electric current. The concept of
    electrical impedance generalizes Ohm's law to AC circuit analysis. Unlike
    electrical resistance, the impedance of an electric circuit can be a complex
    number. Oliver Heaviside coined the term impedance in July of 1886"

    Dorian McIntire EE, PE
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