AC or DC?

Discussion in 'Homework Help' started by solaris9123, Dec 31, 2014.

  1. solaris9123

    Thread Starter New Member

    Dec 23, 2014
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    can you explain, and provide visual aides for why rectified, filtered AC is different from DC?

    * use an oscillator to invert phase of direct current, you have pulsating DC, which to me is the same thing as half-wave AC

    * use a single diode to make a half-wave rectifier
    * use a diode bride and capacitor to make a full wave rectifier and filter
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi 9123,
    Is this a Homework assignment.?
    Hint: would you say the full wave rectified image you have posted is an Alternating Voltage.?

    E
     
  3. solaris9123

    Thread Starter New Member

    Dec 23, 2014
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    yes, I would say it is alternating
     
  4. solaris9123

    Thread Starter New Member

    Dec 23, 2014
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    isn't it a half wave though?
    if a voltage is fully rectified, shouldn't there be no waveform at all?
     
  5. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    Alternating current means the Current is changing direction in the circuit loop.

    Is the rectified Current changing direction.????
     
  6. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Alternating current vs direct current......
     
  7. MrChips

    Moderator

    Oct 2, 2009
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    There are strict (rigid) definitions of AC and DC then there is common usage.

    In the strict sense DC refers current that flows in one direction with no reversal.
    AC refers to current that reverses direction.

    Now let us see how the terms AC and DC are commonly used in circuit design and analysis, and in practice in general.

    We can create a DC that is periodic, positive flow of 1A for one second, zero for another second. By the strict definition this is still DC.

    What is the average current? It is 0.5A.

    Mathematically, we can reconstruct this signal as a constant 0.5A DC with a ±0.5A AC.
    Hence we have substituted a DC + AC signal for the original DC signal.

    Thus we can conclude that all signals, DC or AC can be reconstructed with infinite AC signals ranging from 0Hz to ∞Hz. The DC component is simply the component that happens to be at 0Hz.
     
  8. profbuxton

    Member

    Feb 21, 2014
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    For goodness sakes , MrChips, I'm sure there is no need for such a complicated answer!
    Solars9123, your voltage waveform does not go below the zero volts axis, even though it is "pulsing" at the 2x mains frequency. It is a DC voltage!! Even if it is unfiltered!
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    Oh yeah? Then how do you explain to someone how you would use the AC/DC selector option on the input channel of an oscilloscope?

    My 24VDC power supply has a 100mV ripple. How do I observe this on the oscilloscope?
     
    Last edited: Dec 31, 2014
  10. LDC3

    Active Member

    Apr 27, 2013
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    The answer you are obviously looking for is to AC couple the input, but you could also add -24V to the input and magnify the result to see the ripple.
     
  11. solaris9123

    Thread Starter New Member

    Dec 23, 2014
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    so I get that If the current/ voltage is moving in a positive direction and switching, it is somewhat oscillating, or changing direction because the current will flow from positive to ground? there is a passive, negative charge that can be seen on a scope as the trough? of a wave
     
  12. LDC3

    Active Member

    Apr 27, 2013
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    For the rectified and filtered voltage, there is a ripple to the output which can be seen. The current never changes direction (since the voltage is always positive), but it does fluctuate with the changing voltage. For some ICs, this ripple can cause unpredictable output from the IC. Voltage regulators can reduce the ripple significantly.
     
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