Why is this wrong on rectifier

Discussion in 'Homework Help' started by bwd111, Nov 4, 2013.

  1. bwd111

    Thread Starter Member

    Jul 24, 2013
    Voltage output of rectifier would be known as pulsating direct current. And was told I was wrong! Why
  2. MaxHeadRoom


    Jul 18, 2013
    I thought we covered this here?

    You were probably told this by someone who believes it is AC, whereas by my definition, AC by is where the current actually reverses or alternates direction or polarity, not just pulsates.
  3. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
    Well the DC is not solid because it keeps pulsating on and off producing the DC voltage from the Alternating AC voltage...
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
    Look at the attached simulation.
    First: A Time-varying voltage is not the same as a "alternating" voltage.
    Second: look at V(f). Do you ever see it go below zero Volts?
    Third: The average value of V(f) is 5.1006V DC. The average value of an AC wave is zero.
  5. wayneh


    Sep 9, 2010
    Because whoever said "wrong", is wrong. Your answer was correct. If only two choices were offered, AC or DC, then it's DC.

    Few here would answer "AC" without qualification words such as "an AC component on a DC carrier" or such. Likewise calling it "DC", while closer, benefits from a qualification too, such as your inclusion of "pulsating". This differentiates it from, say, smooth DC from a battery.

    Perhaps to some the definition of "DC" includes the concept of noise, including ripples and pulsation, since ANY voltage can be found to contain noise.
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    Being pedantic, perhaps one should say "Voltage output of rectifier would be known as pulsating direct voltage"
  7. WBahn


    Mar 31, 2012
    Most of the black-and-white definitions given do not allow for the fact that many waveforms exist that do not fit either. Yet it is often implied that signals have to be one or the other.

    For instance, one claim is that an AC waveform must involve current that changes direction otherwise it is DC, while another claim is that AC waveforms have zero average value. So what about a waveform that involves current that changes direction but that also has a non-zero average value? Is it AC or DC?

    If a waveform that changes amplitude but never changes sign is DC and not AC, then when we apply it to a "AC-coupled" circuit that "blocks DC but lets AC pass", then no signal should make through -- after all, we have a circuit that blocks DC and we have a signal that we have declared as being DC.

    And then there's the whole can of worms surrounding whether the 'C' and AC and DC can only refer to "current" or does it, more generally, apply to voltage as well, keeping in mind that the two are not always related by a constant. It is not hard at all to have a current that changes direction while having a voltage that does not -- heck, all you have to do is change the reference node by which your voltage is measured!

    This is another case of where there are a few common definitions that are not perfectly in agreement. The actual meaning depends, in large part, on the context of both the specific topic being discussed and the audience discussing it.

    Perhaps the most general description would be to use four categories of waveform:

    DC : A fixed, constant value
    AC : A periodic signal with zero average value
    Pulsating DC : A signal that changes value but never sign.
    Time-varying : A signal that, at least can, change both value and sign.

    But even here things can get murky. In many contexts, the mention of "AC" implies sinusoidal waveforms. Not triangular, not squarewave, but sinusoidal.

    When dealing with signals (not power, but signals), it is quite common to consider ALL signals as time-varying (even if they just don't happen to actually ever change) and to us DC and AC not to describe the overall signal, but rather to describe the components of the signal as a DC component and an AC component. In some contexts, the DC component refers ONLY to the average value of the waveform and the AC component refers to everything else. In other contexts, the DC component consists of all of the frequency components below a certain frequency and AC refers to all of the frequency components above a certain frequency.

    Since the point is effective communication in the real world, it is pointless to try to insist on rigid definitions of terms such as these since the real world is simply going to ignore you and continue to use the "soft" definitions common to the context of the discussion at hand. You are far better served by being flexible and able to adapt to using the soft definitions as actually used -- you might describe it as being able to move between dialects of a root language in which the various dielects mean somewhat (or even significantly) different things by the same word that they all of "in common".

    As for the OP's initial question. I would have no problem with using the term "pulsating DC" to describe the waveform. Not knowing the detail context of the material in which the question is asked, I don't know whether it is a "right" or "wrong" answer within that context.
  8. wayneh


    Sep 9, 2010
    So true. I've noted here before that, in my opinion, the problem is that our classification of a source signal as AC or DC often depends more heavily on the load - the application - than on any characterization of the source properties. We call a rectified wall wart charging a battery, "DC". The same signal fed into an audio amp would surely sound like "AC". Both are right.