I have two questions about dot convention for the ideal transformer. I have understood how to use them in writing the voltage drops on the coils, but I have some doubts:

1) Which is the physical cause of their necessity? Intuitively, I think it is due to the fact that the direction of the magnetic field generated by a coil connected to a voltage source depends not only on how it is connected to it, but also on its winding sense.

2) If we write the ideal transformer equation: V2/V1 = I1/I2, do we apply the dot convention as we do for evaluating the voltage drops on the coils?

If it is true, we would write for instance:

V2/V1 = - I1 / I2

for the following scheme, since I1 flows in the dot and then in the inductor, and I2 the opposite.

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  • \$\begingroup\$ What did your google search reveal? \$\endgroup\$
    – winny
    Commented Jun 24, 2019 at 15:26
  • 1
    \$\begingroup\$ When the voltage at the primary dot is on it's positive half-cycle, so is the voltage at the secondary dot. Alternatively, when primary current is flowing in towards the dot (ie a sink), secondary current is flowing out from the dot (ie a source). \$\endgroup\$
    – Chu
    Commented Jun 24, 2019 at 16:32

1 Answer 1


Reality is this - if you apply a voltage source to the primary of a transformer then the phase difference between primary voltage (dot end) and secondary voltage (dot end) is zero. In other words the dots tell you about the phase relationship between primary wires and secondary wires.

Because of this, if you have a load resistor connected to the secondary, load current flowing into the dotted primary wire is matched by a secondary current flowing out of its dotted secondary wire. This of course is a 180 degrees shift.

The above analysis ignores magnetization current.

  • \$\begingroup\$ Thus a positive-going sin on the primary "+" will cause a positive-going sin on the secondary "+"?? \$\endgroup\$ Commented Jun 24, 2019 at 15:55
  • \$\begingroup\$ Correct @analogsystemsrf \$\endgroup\$
    – Andy aka
    Commented Jun 24, 2019 at 15:59
  • \$\begingroup\$ But which is the physical cause of this different behaviours? When I put two coils close together, how do I decide if the two coils are in phase or not? \$\endgroup\$
    – Kinka-Byo
    Commented Jun 24, 2019 at 16:15
  • \$\begingroup\$ The direction of the turns. If you wrap the primary clockwise, and the secondary counter clockwise, they will be out of phase. If the turns are going in the same direction they will be in phase. \$\endgroup\$
    – Stiddily
    Commented Jun 24, 2019 at 16:47
  • \$\begingroup\$ Perfect! Thank you very much \$\endgroup\$
    – Kinka-Byo
    Commented Jun 24, 2019 at 16:59

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