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Circuit to change dc to ac?

  1. Jan 24, 2015 #1
    I have only just found out about inductors, and am trying to figure out how a circuit with an inductor and a capacitor could change dc current to ac current.

    With the set up of the circuit, would it suffice just to have the dc supply with a 'gate' that stops/allows through the dc current with a frequency of 50 Hz, and then an inductor connected to this is series and a capacitor conected to the inductor in series? Would the output be ac? I don't quite understand how to complete the circuit either...

    If the above is a good way to go, is there a way to show mathematically that sinusoidal current is the output? I am still trying to get my head around this, but from what I understand the capacitor will initially be charging with a large current and the current will be decreasing. It would be decreasing exponentially, but the inductor resists the change in current so that it ends up decreasing sinusoidally, and then when the dc current is off the capacitor discharges with an initial high current which decreases exponentially, but the inductor decreases this change so that it is sinusoidal?

    I have spent many hours trying to find a good YouTube video or link, but with no luck! I think it has something to do with RLC as this seemed to crop up a lot, although I don't really understand these circuits...
    Thank you in advance for any help! :)
  2. jcsd
  3. Jan 24, 2015 #2


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    a little more involved that that

    google DC to AC inverter circuits

  4. Jan 24, 2015 #3

    jim hardy

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    see if this helps


    it's just harmonic motion, like a spring-mass system

    establish an initial condition of current through inductor or voltage across capacitor..

    That's just a textbook solution though, to make useful AC you'll need more parts, as dave said...
  5. Jan 25, 2015 #4


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    If you 50-50 gate DC into a load with a switch you get a squarewave that goes from 0 to V. It has an average value of V/2. You can think of it as a +V/2 to -V/2 squarewave sitting on top of a V/2 DC level. Put a capacitor in series with that and you block the DC, leaving the AC squarewave. Now, in reality it's a bit trickier than that since the capacitor size and the load interact with a time constant Tau=RC, which changes the waveform. But, the result is, indeed, AC.

    If you put an inductor in series with the capacitor it gets Much more complicated since the energy storage of the capacitor and the inductor play against each other. They can behave in ways that are over-damped, under-damped, and critically damped, and can result in natural frequencies (ringing) that are at a different frequency than your switching frequency. That gets into the RLC circuit responses.

    Jim pointed you to a good article that describes stand-alone LC circuits that resonate at a single frequency, which is indeed a sinewave.

    If you think about this circuit (from http://www.facstaff.bucknell.edu/mastascu/eLessonsHTML/Circuit/Circuit4A.htm) being driven by a sweeping AC source:

    The capacitor blocks low frequencies, and shifts phase in one direction, and the inductor blocks high frequencies and shifts phase in the other. In the middle range there is a frequency when the L and C have the same impedance and opposite phase shifts. As a result, their impedances "cancel" at that frequency, called the resonant frequency. That is the basic concept used by simple radio tuners. It can be called a bandpass filter.

    Trying to imagine what happens when the source is 50 HZ switched DC. It can get messy. But, only AC can make it through the capacitor. That much you can count on. You don't need an inductor to remove the DC from a signal. Just a series capacitor.
  6. Jan 25, 2015 #5
    Thank you so much to everyone that has replied! I really understand the role of the capacitor and inductor now, however I am confused about the rest of the circuit. meBigGuy, could you explain what the earthing does and how it works in this circuit? There is also the circuit on this link
    http://www.physics.ox.ac.uk/olympia...2_Paper_2011_ Solutions_selectedl_answers.pdf
    and the circuit is under question 2c)

    If there is earthing, would the current not all just flow to the earth when the ac has a positive voltage, and flow from the earth when the ac has a negative voltage to completely cancel out any current? If you have any links for explanations of the role of earthing in circuits, I would also be grateful.
  7. Jan 25, 2015 #6
    I was wondering if earthing has any analogy to the use of a reservoir in thermodynamics so that a system remains isothermic? And also why it seems like earthing always occurs through a resistor?
  8. Jan 25, 2015 #7
    Earthing in electrical schemes serves to denote the zero reference voltage point (= 0 V). Every electrical circuit can work with no earthing: all it suffices is circuit to be closed. Earthing in a real world has a multipurpose function, and doesn't always occur through a resistor.
  9. Jan 25, 2015 #8
    Thank you for your reply! But I am still a bit unsure- I thought it might have something to do with completing the circuit because in the link I gave above, http://www.physics.ox.ac.uk/olympia...2_Paper_2011_ Solutions_selectedl_answers.pdf, just having an inductor and capacitor would just give single wire and not a loop. In these circuits, like the one provided by meBigGuy, and in the link I gave, there is always earthing. It seems like it is playing a pretty important role in these circuits...
  10. Jan 25, 2015 #9

    jim hardy

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    hmmm sometimes that's an okay thought, as in lightning protection

    but one must be very careful to not fall into the beginner's trap of thinking earth is some infinite sink with a magical affinity for current.
    When studying circuits, earth is "just another wire that goes nearly everywhere".
    And like any other wire you can have a potential difference across a length of it.
    And Kirchoff dictates any current you pull out of earth you must return to earth, also just as with any other wire..

    Now - A circuit that's earthed will of course be at potential of earth in that vicinity... So your analogy is i think okay.
  11. Jan 25, 2015 #10

    jim hardy

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    Just be aware that in Thermo you have an absolute zero for temperature
    here on earth all potentials are relative to something on earth ,, there's no absolute zero potential that i know of.
    i think that's why students get confused about "earth", thinking of it as an infinite sink.
  12. Jan 25, 2015 #11
    Thank you for your replies! I see now why the analogy isn't the best... In the circuits above, there would be current flow to/from Earth, right? I am just wondering why that is significant, or necessary, in these dc to ac circuits...
  13. Jan 25, 2015 #12
    Forget about Earth. As said, all it suffices is the circuit to be closed. You can use a perfect wire to form a closed loop (closed circuit) instead of earth.
  14. Jan 25, 2015 #13
    But then if you have a closed loop with this periodic dc current, an inductor and a capacitor, where in the circuit will the current/voltage be ac and where will it be dc? The supply remains dc, so you cannot have an ac current entering the power source and a dc current leaving it?

    And also, if the purpose of this conversion is to produce an ac supply that can go to homes, them how can you create a complete circuit? You wouldn't have a wire going from this gated dc supply, through an inductor, through a capacitor, to a house, and back to the dc supple again... For the doc supply in this case, you should only have one wire coming out of it, and so the circuit cannot be complete! I think that is where earthing comes in, and I don't want to just ignore the earthing because I know that it is an important concept that I should try to understand...

    So far, my impression is that if you have two wires coming in to this gated dc supply, and you Earth the wire that is the 'input' and then the output wire goes through an inductor, capacitor and then to a house circuit and then it is earther from the house circuit, then this would form a complete circuit? Although this doesn't match the diagram given by meBigGuy- in that diagram the earther wire is after the inductor and capacitor, and also I still do not understand where in the circuit the current stops being dc and becomes ac?

    EDIT: I realise that because current cannot be stored at a point in a wire, it must be ac all the way through. There cannot be a transition from gated dc to ac across some component of the circuit...
  15. Jan 25, 2015 #14
    In AC circuit parts you have AC current, and in DC parts of the circuit you have DC current. Current which leaves the power source must return to it and for this circuit must have the return path. IOW, the circuit must be closed in some way or you will have accumulation of charge and voltage increase in the circuit. Will you use earth groundings for return path , to connect pole of the source and pole of the load, or wire is irrelevant from theoretical point of considerations (but important in practical realisations). How is AC converted to DC and vice versa in circuits is the matter of the commutation theory in electrical circuits.

    What conversion? Dominant electrical system in most of the countries is still AC all the way, from the power plant to the residental home networks.
  16. Jan 25, 2015 #15
    If you were to generate dc current in a solar panel and want to convert it to 240V, 50Hz ac to use in homes? Or if you simply had a dc power supply and wanted to convert it to ac for some reason...
  17. Jan 25, 2015 #16
    Then, you use DC-AC converter (inverter) as the interconnection, function of which is governed by commutation laws.
  18. Jan 25, 2015 #17


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    The AC input to a house consists of two (or maybe three) AC lines. Earth (ground) may or may not be supplied from the power station. If it is not supplied, it is used as a protection for human beings. Otherwise, a short circuit to the metal shell of a stove or a washing machine could become very unpleasant when you touch it.

    Otherwise, in an electronic circuit you have power supply lines. Tradition has it that the supply line connected to the negative pole is called "ground" or "earth" even if it is not connected to "Protective ground". Handling "earth" in an electronic product is an art in itself, and gets harder all the time (FCC requirements to radiation play a large part here).
  19. Jan 25, 2015 #18
    Thank you for your reply! So it seems like earthing is never really necessary, it is just a safety measure?

    What about in this circuit though:
    It seems to me like the earthing to get 0V is necessary in this circuit, even though I don't know why...
  20. Jan 25, 2015 #19

    jim hardy

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    seems to me you've already fallen into the "earth as absolute zero" mental trap.

    Would your fig 1.3 gizmo work in an airplane, or on that asteroid that's due to fly by here in a couple days? Sure it would.
    It needs no connection to earth.
    Definition of voltage is potential difference
    ie between two points of your choosing.
    Voltmeters have two probes for that reason.

    It was a revelation for me when i learned definition of potential,
    the work required to bring a unit charge from some point to wherever you are.
    That "some point" is your point of reference.
    The closest we EE's can come to "absolute potential" would be the work required to bring that charge from infinity to where we are.
    Since we have no way to do that we do not know earth's "absolute potential".*****
    so we always use terms potential , and voltage, to mean referred to someplace nearby
    There's no reservoir of charge that's at absolute zero voltage.

    Practice this for a few days
    "whenever I say "voltage" i will append the phrase "from (measured pointA) to (reference pointB)"
    pointA being the place where i have connected my meter's red wire
    pointB being the place where i have connected my meter's black wire

    in saying it right you will come to think of it right.

    ***** even though some guy got a patent on an apparatus to measure it, http://www.google.com/patents/US4839581
  21. Jan 25, 2015 #20

    jim hardy

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    Here's the simplest way to get AC out of DC
    you keep two and only edit: two switches closed at any time,
    S1 & S4 for a while
    S2 & S3 for the next while

    motor M sees its applied voltage reverse every time you change the switches.

    Now - what will your voltmeter report during each of the above "while's" if you connect its + lead to M's left side and - lead to M's right side?

    While S1 & S4 closed : ?
    While S2 & S3 closed : ?

    Next - what will your voltmeter report during each of the above "while's" if you connect its + lead to M's right side and - lead to that bottom wire joining S2 & S4 & voltage source?

    While S1 & S4 closed : ?
    While S2 & S3 closed : ?

    and with meter + connected to other side of motor, - to same bottom wire ?
    While S1 & S4 closed : ?
    While S2 & S3 closed : ?

    Keep at it until your mind thinks like a voltmeter.
    Then realize you've made a giant step for physics students, you no longer need that mental crutch of earth as an electrical anchor.

    If a circuit is not earthed, its voltage to earth is indeterminate, that is it's unknown. Think on that and see what questions it brings up.
    Good luck in your studies. Your questions are interesting; someone like you who probes the very basics can come to a most solid understanding of them.

    old jim
    Last edited: Jan 25, 2015
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