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Does an AC current have to cross zero?

  1. Oct 22, 2015 #1
    So my question is simple.
    If the current doesn't change sign, is it considered AC or DC?

    I seem to recall we called it DC when I briefly saw it.
    However after a quick google search I'm confused. Some said that for example an AC signal offset with a DC signal to show this behaviour should be called AC because there is an AC component.
    But then there was the odd example of a capacitor discharging. I absolutely do not agree with that being an AC signal.

    In fact I'm reluctant to accept the first one too. Say we have 2 DC sources which we can add together.
    If we let one of them be turned on and off periodically (simple example would be an on/off switch).
    How can you say it's got an AC component there?

    Is there any consensus on this problem of nomenclature?

    Best regards,

  2. jcsd
  3. Oct 22, 2015 #2
    That's more a matter of definitions, which can vary.

    Most DC sources that originate from a wall outlet (through various electronics) will have a small AC ripple. No one calls these AC.

    AC sources can also have an offset so that their peak to peak is not centered relative to zero voltage or zero current.

    AC and DC are more conventions than rigorous universal definitions that can be used to categorize less standard sources and signals.
  4. Oct 22, 2015 #3


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    Staff: Mentor

    As Dr, C says, there is probably not a universal agreement on the answer to your question. I'm in the camp that considers a varying current or voltage to be AC, with a potential DC offset.

    One thing that helps to justify this is the way that most signal generators are set up. As you can see on the HP 33120 front panel below, you have separate settings for the (AC) Amplitude and DC Offset. :smile:

    http://www.us-instrument.com/objects/catalog/product/image/img1807.jpg [Broken]
    http://www.us-instrument.com/objects/catalog/product/image/img1807.jpg [Broken]
    Last edited by a moderator: May 7, 2017
  5. Oct 22, 2015 #4
    Indeed that might be more natural given how equipment handles these thing.

    What about the second example where a capacitor discharges (once).
    Would you say it's AC or DC?
    I figure this would be DC since it's not really sharing the wavelike nature (in my eyes that is).

    I'm mainly asking this because for a pedagogics class we have to "design a lesson on how AC-generators work".
    To anticipate I was thinking how to actually differentiate between AC and DC. The latter they know.
    AC has been introduced in a way that many won't remember (as an aside inside the textbook).

    Better safe than spread misinformation.
  6. Oct 22, 2015 #5


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    Staff: Mentor

    That's a "transient" waveform. :smile:
  7. Oct 22, 2015 #6
    I shall tread lightly with respect to this.
    Luckily they don't see capacitors, I believe.

    Thanks a lot both of you!
  8. Oct 27, 2015 #7
    For most practical purposes, DC current with high ripple is still considered DC unless it goes negative. This is important with capacitors. Capacitors can naturally handle AC current but not AC voltage (for types like electrolytic, tantalum, etc). So a DC voltage with some AC ripple is fine as long as the voltage doesn't reverse. The current will naturally be AC (charge up and discharge) but the voltage must never reverse.
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