Center-tapped fullwave rectification without diodes?

Discussion in 'General Electronics Chat' started by Vorador, Oct 20, 2012.

  1. Vorador

    Thread Starter Member

    Oct 5, 2012
    87
    1
    Hello everyone!

    This is a question that has been bothering me for some weeks now. I am having trouble understanding why we need diodes at all for a center-tapped fullwave rectificer circuit. My flawed understanding about electricity is telling me that rectification should be achieved even if I replace the diodes with resistors (forgive me for unashamedly making such a preposterous statement).

    I have attached two diagrams - one for the positive cycle of the AC input and the other for the negative one - of what I think should happen in a center-tapped full-wave rectifier circuit consisting of three resistors, an AC source and a transformer.

    In the diagram, the resistor in the middle is my output. I have drawn current directions in the diagrams according to my understanding. As you can see, in both the diagrams i.e during one full cycle of the input AC cycle, the current through the middle resistor is invariably unidirectional. But that's obviously not what would actually happen in a real circuit. Resistors can't rectify, but I am speaking on the basis of the circuit geometry alone, which suggests (to me) that the current through that middle resistor must be unidirectional!

    I can't see how it is possible for current to reverse its direction through the load in this particular circuit? That's what I need help in understanding.

    Please help me realize the silly mistake I am making. It really upsets me when I can't understand some theoretical principle even after giving it much time and thought.

    I'd be extremely grateful for your time, patience and response.

    Thank you! :)
     
    • ag.JPG
      ag.JPG
      File size:
      8 KB
      Views:
      98
    • ag1.JPG
      ag1.JPG
      File size:
      8 KB
      Views:
      69
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The resistor connected to the center tap has no current through it and no voltage across it because when the voltage on one outer wire goes positive then the voltage on the other wire goes negative with the same voltage then the voltages cancel.
    Also when the voltage on the first outer wire goes negative then the voltage on the other outer wire goes positive with the same voltage then the voltages cancel.
     
    Vorador likes this.
  3. Papabravo

    Expert

    Feb 24, 2006
    10,142
    1,790
    Resistors conduct current in both directions. Diodes conduct current in one direction only. Why on Earth would you expect them to be the same? Especially for an AC circuit. Probably the same reason people are so easy separate from their money in a financial scam like the Pigeon Drop.
     
  4. Vorador

    Thread Starter Member

    Oct 5, 2012
    87
    1
    I don't expect them to be the same. It's just that analyzing that PARTICULAR circuit made me conclude that they weren't really necessary here.

    Thanks Audioguru, it makes more sense now! :)
     
  5. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    Vorador likes this.
  6. Vorador

    Thread Starter Member

    Oct 5, 2012
    87
    1
    Thanks BillB3857!

    So I take it, in the circuit in my first post, the load resistor would get no current at all through it, right?
     
  7. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    If the two resistors are the same value, then the load resistor gets no current. If they are not the same value, then the load resistor gets some portion of the AC current that runs through the two "diode" resistors.
     
    Vorador likes this.
  8. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    Everything has a tolerance limit. In addition to what kubeek just posted, there is a tolerance in the voltage from center tap to each end. Any difference there will also produce SOME current through the load resistor.
     
    Vorador likes this.
  9. Vorador

    Thread Starter Member

    Oct 5, 2012
    87
    1
    Hmmm. I understand that the two "diode" resistor won't be getting a DC current, but still don't quite grasp how the load resistor is getting AC current.

    I think it's because I see the center-tap as ground which only serves as a destination for the current and not as a terminal capable of "forcing" the current away from it at any point during the entire cycle (if you get what I mean).
    Surely, this is where I'm wrong?

    Thank you so much everyone!
     
  10. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    The center tap is nothing but a connection point in the circuit. It has no special properties compared to the other points. The ground symbol in the circuit is there just so the simulator can do it´s job, you can place it wherever you like and nothing really changes.

    Now imagine the CT transformer as two batteries in series, in one half wave with positive polarite and in the other with negative polarity. Also I assume that either the voltage on the transformer is unbalanced or the two "diode" resistors, otherwise there would be no current through the load. See what happens to the voltage across the load and the direction of current through it.
     
    • ct1.png
      ct1.png
      File size:
      3.5 KB
      Views:
      49
    • ct2.png
      ct2.png
      File size:
      3.6 KB
      Views:
      52
    Vorador likes this.
  11. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Also, The circuit makes a resistive divider. When the current through the two "diode" resistors is AC, the current through the load has to be AC as well.
     
  12. Vorador

    Thread Starter Member

    Oct 5, 2012
    87
    1
    Oh wow! I never thought it like that. I always thought of the transformer as being a single polarity-reversing battery with one terminal connected to the top wire and the other to the bottom.

    Thank you so very much, kubeek! I appreciate it tremendously! Thanks a heaps!
     
  13. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Well yes, but with a center tapped transformer you got two of those connected in series and both change polarity at the same time.
     
    Vorador likes this.
Loading...