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I've already seen this and this. But I can't decide how to correctly wind a SMPS transformer. This is the transformer:

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I mantain the same turns direction (CCW if you look at transformer from the top).

I start winding the first half of primary. I consider this the dotted end.

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Now I wind secondary in the same direction. Which is the dotted end? On the dotted end I must connect the anode of the fast rectifier diode and I don't know which one is it.

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Now I wind the auxiliary. Considering the dot at start is correct?

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And finally the second half of secondary. But now I'm winding from the top of the transformer towards its bottom. Is this correct?

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The drawings do not reflect the actual number of turns.

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2 Answers 2

up vote 4 down vote accepted

Winding from top to bottom or bottom to top does not change the "dot" (which represents the direction of the magnetic north/south pole for a given current direction).

You can use the "right hand rule" to keep this straight.

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Place your right hand around the coil with your fingers in the direction of current flow. Your thumb points to the North magnetic pole. The ends with the "dot" will be the ones that create the same direction of magnetic field when current flows from that end to the other.

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When you say "winding from top to bottom", I say this does not change the convention if you wind from "bottom to top" instead.

The dot convention ensures this: -

If an AC voltage is applied to a coil, the other coil(s) will be in phase with that AC voltage when (for instance) you use an O-scope with the probe at the dot ends.

BTW, it looks like you are winding the transformer correctly although the pictures and labels took a little deciphering. Your last diagram fooled me but it's clear you are continuing to wind the 2nd half of the primary in CCW to match the 1st half.

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