I'm confused about current direction of transformer

Discussion in 'General Electronics Chat' started by Dong-gyu Jang, Jan 15, 2016.

  1. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015
    115
    4
    Hello.

    Current direction of transformer.jpg

    For typical battery (nothing but capacitor), current direction is clear as electrons can't not flow across the two plates in battery so they can only choose wire as a path from cathode to anode.

    But in secondary side in transformer, basically electron also can go through the "connected" coil so I guess electron can choose this path. Maybe it is more favorable as it is short path from cathode to anode.

    The Ohm's law on resistor forces me to think that current path should be the same to case of the battery but It doesn't physically convince me.

    How we can determine current direction and why?
     
    Last edited: Jan 15, 2016
  2. crutschow

    Expert

    Mar 14, 2008
    16,186
    4,323
    The transformer plus and minus signs do not indicate absolute polarity since the signal is AC, they indicate the polarity (phasing) with respect to the other windings (in your diagram you need a + and - sign on the primary otherwise the secondary polarity signs have no meaning).
    This phasing is more commonly indicated by a dot at one end of each winding. The phasing dot indicates that the phase of the signal at those two points are the same with respect to the other side of the windings (i.e if one dot has a positive going signal applied then the other dot will also have a positive signal).

    As far as what direction the electrons flow, that's determined by the instantaneous polarity of the AC signal out of the transformer.
    The electrons will always flow out of the terminal that's negative at any instant in time.
    This reverses at the AC frequency of the signal.
     
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  3. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015
    115
    4
    Thanks to give me practical guide of transformer notation.

    I'm still not convinced why electrons always flow "out" of the terminal, rather than "in" (across the coil here). For battery, electrons can not flow vacuum or insulator between two plates so direction "in" (across the plates here) is prohibited. Easy to understand. But for coil, what does force electron not go "in"? Coil is nothing but conductor which is electron flow path.
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,993
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    The standard on flow of electrons is confused. Folk regularly squabble about it on this site. Understand there are two standards, and with some exceptions that deal with physics it doesn't usually matter.

    Phase on transformers matter when you are trying for feedback (as with oscillators) and signal canceling (like on telephones).
     
  5. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015
    115
    4
    Am...so for transformer, current direction is just following the convention of what most people just accept? Does it mean we really don't know true direction of current? It is so wired for me..
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    Only AC passes through a transformer. The AC signal can be inverted or not, depending on how you attach the wires of the secondary. So the current goes both ways.

    If you want a power supply then you use diodes to convert the AC to pulsating DC. So you know which way the current is going on an instantaneous level, but the correct answer is it goes both ways.

    When phase matters I put a phasing dot on my schematics. Most cases it does not matter.

    AC = Alternating Current.

    https://en.wikipedia.org/wiki/Alternating_current
     
  7. profbuxton

    Member

    Feb 21, 2014
    311
    129
    I always use the convention that current flows from positive to negative. Very very rarely to I have to concern myself about electron flow since I don't do quantum mechanics or design IC etc.
    For 99.9% of electrical/electronic work postive to negative works just fine.
     
  8. WBahn

    Moderator

    Mar 31, 2012
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    5,650
    A transformer only works in an AC circuit (though that does not mean that it has to be sinusoidal, but merely that it won't work with a DC input). For the sake of simplicity, let's stipulate that we are talking about sinusoidal AC. As such, the electrons move first in one direction and then in the other, changing direction twice per cycle in response to the reversing magnetic field in the secondary due to the reversing current in the primary.
     
  9. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    4,165
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    Getting the electrons to 'move' requires work. It takes an expenditure of energy to make electric current move. Something has to charge a capacitor. The work is expended on driving the generator which makes electrons enter a capacitor. That energy is now stored in the capacitor in the form of an electric field. With transformers and AC. The work is stored in the magnetic field.
    Electricity is a car without a motor. To make it move you have to get behind it and push. :)
     
  10. withoutego

    New Member

    Dec 22, 2015
    23
    4
    I have the model in my head of current being two flows. One is positive to negative
    and the other is negative to positive. One is electrons the other is holes. For every
    electron that moves left to right a hole must move right to left.

    or, to be practical, current moves against the arrow of a diode...neg to pos, most
    cases this is the "pull-up" from a negative ground to the positive rail. Forward biased
    diodes pointing toward the neg ground.

    Electricity is not dependent on our models. True understanding may be beyond our
    little minds. The model you choose just needs to match reality.
     
  11. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015
    115
    4
    Hey guys.

    Thanks to give me feedback. I was thinking this issue seriously and got clear answer. The attached picture made it briefly clear.

    Transformer3d_col3.svg.png


    In this case, I was confused by + and - notation in secondary wining. I instantaneously thought electric field direction was + through coil to - like capacitor. It was wrong. The induced field direction is from - through coil to +!

    If voltage is developed from space charge separation like capacitor, my original imagination is right. However here, + and - notation really doesn't relate to true field direction! They're just indicating current direction in conventional way.

    I think this post is clearly finished now:)
     
    Last edited: Jan 15, 2016
  12. j4e8a16n

    Member

    May 1, 2013
    34
    0
    AC current has a direction. This is why nowadays there is a large pin and a small pin in wall inlets.
    No?
     
  13. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    What direction would that be?
    [​IMG]
     
  14. MaxHeadRoom

    Expert

    Jul 18, 2013
    12,887
    3,521
    No, that is to ensure that the Neutral and power pin maintain polarization.
    As to the direction they are exactly the same, AC, just that one side is referenced to earth ground.
    Max.
     
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  15. BobaMosfet

    Senior Member

    Jul 1, 2009
    137
    19
    No-- as a law of physics, electrons always flow from the greater negatively charge pile to the lesser negatively charged pile of electrons (relatively speaking). No exceptions. This is contrary to the 'conventional' thinking that current is + to - (which is wrong, and known to be wrong).

    Beyond that, what no one is explaining to you is this:

    The direction that current flows in the secondary side of the transformer is dependent solely on the direction it is traveling in the primary side.
     
  16. crutschow

    Expert

    Mar 14, 2008
    16,186
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    Not wrong.
    The convention is that conventional electric current flows from positive to negative.
    Electrons flow from negative to positive.

    upload_2017-5-7_19-2-50.png
     
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  17. Tonyr1084

    Well-Known Member

    Sep 24, 2015
    1,264
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    Consider just a battery for a moment. It has a positive end and a negative end. What's in the battery? An electrolyte that consists of atoms. These atoms consist of Protons, Neutrons and Electrons. The neutron is the largest part of the atom, followed closely in size by the proton. The electron is very small and can be made to move easily. The positive end of the battery has a surplus of protons and an absence of electrons. It's natural that the electrons want to migrate back to the protons to satisfy the need of the atom to be in balance. Different chemical solutions that make up the electrolyte may have different properties, but for ease of understanding, the electrons (negative end of the battery) will bump into each other and knock the next electron (through a wire) ever closer to the proton (positive end of the battery). This is called "Electron Flow", and on a physics level, exactly is what happens.

    So why do we use a conventional current flow over the physical flow? Because when electricity was being discovered and beginning to be understood, someone made a decision that current flowed from positive to negative. Which was an error. However, so much of electronics has been built using conventional current that it remains a standard part of our understanding, and we consider that current flows from positive to negative.

    In a transformer, at any given instant, whatever the output is - electrons flow from negative to positive. But in keeping with conventional consideration it's considered to flow from positive to negative.

    Some advice: Don't try to design or build using electron flow. Use conventional flow because you will be able to understand it far better than trying to understand the physics of it.
     
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