Transformer Phasing: The Dot Notation and Dot Convention

 Transformer Phasing: The Dot Notation and Dot Convention

The Dot Notation
Generally, when we study about Transformers, we assume that the primary and secondary voltage and currents are in phase. But, such is not always the case. In Transformer, The phase relation between primary and secondary currents and voltages depends on how each winding is wrapped around the core.
The Dot Notation and Dot Convention

Refer to fig (1) and (2), you may see that the primary sides of both transformers are identical i.e. primary windings of both transformers wrapped in the same direction around the core.

But in fig (2) you may notice that the secondary winding is wound around the core in the opposite direction from the secondary winding in fig (1).
Consequently, the voltage induced in the Secondary winding in fig (2) is 180° out of phase as compared with the induced voltage in secondary in fig (1) and the direction of secondary current (IS) is opposite from the primary current (IP)
Transformer Phasing: he Dot Notation and Dot Convention
So we see that
  1. The primary and secondary voltage and current are in phase in fig (1)
  2. The primary and secondary voltage and current are 180° out of phase in fig (2)
Dot Convention
To eliminate any confusion in the phase relation between primary and secondary voltage and current, a dot convention has been adopted for transformer schematic diagrams. Dots are placed on the top of primary and secondary terminals as shown in fig (3) and (4)
In fig (3), we see that dots are placed at the top in both primary and secondary terminals. It shows that the primary and secondary current and voltages are in phase. Moreover, the primary and secondary voltages (VP and VS) have similar sine wave, also the primary and secondary (IP and IS) currents are same in direction.
The story is opposite in fig (4). We can see that one dot is positioned at the top in primary terminal and the other one (dot) is placed at bottom of secondary terminal. It shows that the primary and secondary current and voltages are 180° out of phase. In addition, the primary and secondary voltages (VP and VS) sine waves are opposite to each other. Also the primary and secondary currents (IPand IS) are opposite in direction.
The Dot Notation and Dot Convention

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  1. kashif hussain says:

    why the transformer output is 180 degree out of phase in theoretical phenomena

    1. shambel Abera says:

      this description of electrical system is very interesting page

  2. Jack Wilborn says:

    How is the ‘dot’ indication used when you have a schematic like the Armstrong oscillator at this link:


    Since they are both at the top of the inductor coil, does that mean they are in phase? The explanation of the oscillator there leaves something to be desired…


    Jack Wilborn

    I have built a similar device as is described by this link:


    Makes it much easier to see the results. I use a atmega328p (@ 16Mhz) to drive the switch. I only use one pulse and trap it on my DSO. I bring this up, as I have a few baluns that I will try and catch the output of different windings.. Never thought of that before… Thanks for the information…

  3. Jack Wilborn says:

    Funny, I looked at the entry I just posted and it has the times tamp as

    10/20/2016 at 10:32 pm (copied and pasted)

    It was actually at 10:32 am on 10/20/2016, unless this site is on the other side of the world..



  4. Now it all make sense, It was all about winding and in what direction, was never mentioned in any of my books. Now I understand the dot convention

  5. Eesh… I don’t know anything about transformers. I have some learnin’ to do.

  6. Abdur Rab says:

    Good Day!

    Many Thanks for such a helpful information regarding Dot notation of a Transformer in schematics.

    I did never come across this for the last many many years.

    Thanks Again.

    All the Best.

    Abdur Rab

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