# What physical construction features in a transformer control the dot polarity?

As we know, the dot convention is used on transformer symbols to denote whether the transformer "inverts" the signal passing through it or not. However, I am not clear on what in the physical construction of a transformer determines the dot polarity -- is there something physically different between two otherwise identical transformers that have opposite dot polarity conventions? (I.e. one has the dots on the same side and the other has the dots on the opposite side) Or are these two transformers physically identical and interchangeable?

• Draw an arrow each on two pieces of paper. Line up the pieces of paper so that the arrows are parallel. Are the arrows pointing in the same direction, or the opposite? Commented Jun 17, 2016 at 11:51
• The dots are relative and give the relative phase (so directions of the windings) of the transformer. Commented Jun 17, 2016 at 11:52
• The winding direction (clockwise or anticlockwise) of the coils. And if a winding appears to have the wrong direction, start at the other end...
– user16324
Commented Jun 17, 2016 at 12:16

The primary of a transformer and the transformer core are a electromagnet. If you put current thru the primary one way, the core is magnetized one way. If you reverse the current, the magnetization direction is reversed. Likewise, the secondary produces a voltage based on how this magnetism changes.

Between how the primary and secondary are connected to their circuits, there are two opportunities to flip overall polarity. You can flip the primary connections, or the secondary connections. Each causes a flip, so flipping both gets you back to where you were.

The winding direction of a coil determines the magnetism direction for a fixed current direction. Just like flipping the two leads of a winding, you can get the same effect by unwrapping the winding, then wrapping it back with the opposite twist.

To actually answer your question, the dots indicate relative polarity between the primary and secondary. This is fixed for a particular transformer by the direction each coil is wound, and by how the leads are labeled.

Winding direction determines the output polarity.

simulate this circuit – Schematic created using CircuitLab

Figure 1. Double-primaries and double secondaries on transformer.

The dots tell you that the primaries and secondaries are wound the same direction starting from that end of the coil. When connected in series their voltages will add.

• This is not an answer to the question. The OP understands what dot notation means; he's asking about how the physical construction determines the placement of the dots. Your first sentence is too vague to be useful. Commented Jun 17, 2016 at 12:06
• @DaveTweed - The question asks what physically determines the dot polarity. But the only thing that determines said polarity, is the winding direction. Do correct me if I'm wrong.
– Bort
Commented Jun 17, 2016 at 12:14
• But what does "winding direction" actually mean? Direction along the axis of the core, or direction around the core? See Olin's answer for a clearer description of the distinction. Commented Jun 17, 2016 at 12:22
• it's direction around the core. for example the dots could be placed on placed on all the ends that start clockwise around the core. Commented Jun 19, 2016 at 4:24