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DC Wave Questions

Discussion in 'Electronic Repair' started by [email protected], Jun 10, 2005.

  1. Guest

    2 questions about a fully DC Sine Wave....let's suppose you have a DC
    Sine wave which varies from +5V to +15V peak-to-peak going into a load
    with R, L, and C components.....

    Question #1:
    Is the load's impedance a function of R, L, and C (and wave frequency)
    or is it simply just R (i.e. Z=R)? In other words does non-resistive
    impedance (L + C) really only matter with an AC signal OR anytime
    voltage varies periodically (even if it is all DC)?

    Question #2:
    Would a "regular" negative peak detector ciruit, like shown here: Circuits/NEGATIVE PEAK DETECTOR.htmgative

    work for the DC Wave described? Will it output +5V or do negative peak
    detectors only work for AC signals?

    Thank you.
  2. NSM

    NSM Guest

    One answer. Sine waves aren't DC.

  3. Guest

  4. Bob Eldred

    Bob Eldred Guest

    Impedance varies with frequency if there are reactive components, L's and
    C's. Since you haven't told us whether this is a series or parallel circuit
    of L's, R's and C's, We don't know what the impedance is at DC, zero
    frequency or any other frequency for that matter. If it's a parallel circuit
    the DC impedance is zero unless there is resistance in series with the L as
    is the usual case. In that case, the impedance is R at DC. If it is a series
    circuit, the DC impedance is infinite. SO, you have three choices, Zero
    ohms, Infinite ohms or R ohms depending on the connection.

    A peak detector will have to work on the range of voltages expected on it's
    input. I can't get to the URL, sorry.
  5. Dave

    Dave Guest

    That is not always true.

    1) A resistor of resistance R in series with a capacitor of capacitance C.

    2) Another identical resistor of resistance R, but in series with an
    inductor L.

    Make R=sqrt(L/C)

    and put 1 and 2 in parallel and measure the impedance across that
    combination. The impedance is always R, and is independent of frequency.

    A useless fact I would admit!!
  6. Terry

    Terry Guest

    Varying DC? i.e. DC varying in amplitude a manner similar to an AC sine
    If it goes into plus and minus regions I guess we are getting pretty close
    to an AC waveform?
  7. Andrew Holme

    Andrew Holme Guest

    There is no such thing as a "DC sine wave." I suspect you mean what would
    more correctly be described as a 10 volt peak-to-peak sine wave with a +10
    volt DC offset.
    The principle of superposition applies: the currents and voltages in the
    circuit will be the sum of those that would result if the DC voltage and the
    AC sine wave were applied to it seperately.
    That circuit (I've fixed the link) exploits the fact that the LM139
    comparator has an open-collector output. It runs off a negative rail, and
    cannot produce a positive output voltage.
  8. Guest

    Since you haven't told us whether this is a series or parallel circuit
    O.K. here's the combinatrics:

    Combo 1:
    DC Sine Wave + (R+L in series with C parallel)

    Combo 2:
    DC Sine Wave + (R+C in series with L parallel)

    Combo 3:
    DC Sine Wave + (L+C in series with R parallel)

    Combo 4:
    DC Sine Wave + (R, L, and C all in parallel with each other)

    Combo 5:
    DC Sine Wave + (R, L and C all in series)

    O.K., so can I correctly infer from your response that a negative peak
    detector will yield a value of +5V for a sine wave which varies from
    +5V to +15V?
  9. Guest

    There is no such thing as a "DC sine wave." I suspect you mean what would more
    Not that it's that important, but I don't see why a "DC sine wave" is
    an impossible concept, considering the definition of DC as a current
    which flows in one direction:

    A "DC Sine wave" doesn't say that current reverses direction, only that
    the current flow wanes and a river is still a river even
    though its flow varies with rainfall...
    O.K. - now we're getting're saying the current and
    voltage (and the implied impedance Z = V/I) of the "DC sine wave" is
    the sum of the respective current and voltage of a +10V DC signal and a
    -5V/+5V AC signal going into the same load.

    DC +10V into load produces 1 Amp, therefore implied resistance = 10
    AC -5V/+5V (and given frequency) into load produces 0.5 amps, therefore
    implied impedance = 20 ohms,

    then what would the superposition prinicple predict as the resulting
    combined current and impednace?
  10. Guest

    read the original post - talking about a sine wave bouncing between +5V
    and +15V - no where near negative
  11. NSM

    NSM Guest

    That's an AC wave with a DC offset.

  12. Andrew Holme

    Andrew Holme Guest

    The current is simply the sum of the AC and DC components
    e.g. 0.5 amps peak-to-peak AC with a 1 amp DC offset
    Max. instantaneous current = 1.25 amps
    Min. instantaneous current = 0.75 amps

    Impedance can be represented as a complex number:
    real part = reisitance = R = 10 ohms
    imaginary part = reactance = X

    Total impedance Z = R + jX

    To work out the imaginary part, you have to do a vector addition because
    current and voltage in a reactance are 90 degrees out of phase:

    Ipk = Vpk / sqrt(X*X + R*R)

    0.25 = 5 / sqrt(X*X + 10*10)

    X = sqrt(300) = 17.3

    i.e. Z = 10 + j*17.3
  13. Don Bowey

    Don Bowey Guest

    Your posts have all the characteristics that indicate you are a troll. If
    you aren't I suggest you quit being combative and learn from what the
    posters are saying.

    And re the link; that refers to an inverter that uses a DC input and outputs
    a sinewave. You must be troll.
  14. Don Bowey

    Don Bowey Guest

    So the 10V p-p sinewave is riding on 10VDC. There is no requirement that a
    sinewave must have an absolute negative component.
  15. ehsjr

    ehsjr Guest

    Question 1: A capacitor "capacitates" whether it sees
    AC or DC. An inductor "inducts" whether it sees AC or
    DC. A resistor resists whether it sees AC or DC. You
    might find it beneficial to think of what happens to
    each component on a component level rather than thinking
    of total impedance. Understand what each component
    does, and circuit impedance will make more sense.

    Question 2: 404 file not found error
    That said, you can peak detect on a varying DC
    sine. As someone else said, its AC with a DC offset.

  16. Bob Monsen

    Bob Monsen Guest

    The impedance of a set of passive devices is independent of the voltage
    across them. It only depends on R, L, C, and f. The fact that there is a
    DC component makes no difference.

    An inductor will pass DC current as if it were a wire. Only differences
    in current cause a voltage across it. A capacitor will not pass DC, so
    the DC does not matter. Obviously, a resistor is a resistor, and cares
    nothing for ac vs dc.

    This is only true for ideal components. In the real world, inductors,
    caps and resistors have voltage limitatations. They are usually well
    beyond 15V, though.
    Your link has crap on the end. Here it is without the crap: Circuits/NEGATIVE PEAK DETECTOR.htm

    With this circuit, the input at V+ will always be outside the power
    rails. Thus, it will not work.

    NOTE: I changed the followup-to field to sci.electronics.basics, because
    that is where this thread belongs. I hope you don't mind.
  17. Rich Grise

    Rich Grise Guest

    If you think that the term "fully DC Sine Wave" even means anything,
    then you have not understood the coursework. Either your teacher is
    incompetent, or you have been spending too much time partying and not
    enough time studying.

    Good Luck!
  18. Rich Grise

    Rich Grise Guest

    Bullshit. This kid is not a troll, by any means. He's just a student
    desperate to weasel answers to his final without having to learn the
    material he was supposed to have learned while partying and chasing tail.

    A troll is a much more serious matter. This is just a child who needs
    to fail the course, have Mom and Dad scold him, and next semester,
    pay attention in class.

  19. According to Fourier analysis, any repeating waveform can be
    decomposed into harmonically related and appropriately phase shifted
    sine waves and also a DC component. If all the components involved
    are linear, then they react to each of these components,
    independently, and the result is the linear sum of all those
    reactions. So the capacitors react to the DC component as open
    circuits, and the inductors as short circuits. At all frequencies,
    the resistances follow ohms law, and at each AC harmonic, the
    inductances and capacitances react in their normal frequency dependent

    Throw in one nonlinear component, like a diode, and you have to do a
    completely different kind of analysis.
  20. Fred Bloggs

    Fred Bloggs Guest

    That is not a "regular" peak detector, it is a comparator used as an
    overcompensated opamp follower and exploits the open collector output
    characteristic of fast discharge and slow ( 10 second) charge of the
    capacitor. In concept it will work for a varying "DC sine wave" by
    replacing "-Vcc" with "GND" and all "GND"'s with "+15V" in that circuit
    diagram only. Then Vout= "Vpk,neg" =+5V.
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