# 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:

http://www.elektroda.net/cir/index/Detector 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. ### NSMGuest

One answer. Sine waves aren't DC.

N

4. ### Bob EldredGuest

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.
Bob

5. ### DaveGuest

That is not always true.

Take
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. ### TerryGuest

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

7. ### Andrew HolmeGuest

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:

http://www.answers.com/topic/direct-current

A "DC Sine wave" doesn't say that current reverses direction, only that
the current flow wanes and waxes.....like a river is still a river even
though its flow varies with rainfall...
O.K. - now we're getting somewhere......you'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.

Example:
DC +10V into load produces 1 Amp, therefore implied resistance = 10
ohm.
and
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. ### NSMGuest

That's an AC wave with a DC offset.

N

12. ### Andrew HolmeGuest

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 BoweyGuest

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 BoweyGuest

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

15. ### ehsjrGuest

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.

Ed

16. ### Bob MonsenGuest

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:

http://www.elektroda.net/cir/index/Detector 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 GriseGuest

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!
Rich

18. ### Rich GriseGuest

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.

Cheers!
Rich

19. ### John PopelishGuest

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
ways.

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

20. ### Fred BloggsGuest

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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