Dot rule for magnetically coupled circuits ( baluns and transformers)

In a magnetically coupled circuit like a balun or a transformer the phasing of currents and voltages is commonly indicated by the dot rule. Place a dot at either end of the primary. Drive this end positive ( for example). Measure the voltage at the secondary ends. The end of the secondary which is also positive ( or generally in phase ) with the primary end is also indicated by a dot. The other end of the secondary will be 180 degrees out of phase. Current flowing into the dotted terminal at the primary end will cause the current to flow out of the dotted end of the secondary and vice versa. Visit our website for more information and articles of interest at

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