Transformer Cheat Sheet

·        A transformer is used to transform from one voltage to another. 

·        The voltage ratio is proportional to the turns ration.  Current is inversely proportional to the turns ratio.

·        The power ratio of an ideal transformer is 1:1.  In other words, if the voltage goes up, the current must go down.

·        Transformers only work with AC, Alternating Current.



Dots and phase

·       The dots can be thought of as voltage polarity.

·       When current flows into the dot on the primary side, an induced current flows out of the dot on the secondary.

·       The voltage is in phase at the dots, but the current is flowing in the opposite direction.


Impedance Matching


Common Mode Choke

When configured as a common mode choke, a transformer is seen as a low impedance to differential current and high impedance to common mode current. 


·        When common mode current flows through the windings, the flux adds, creating more inductance.

·        When differential current flows through the windings, the flux cancels, reducing the inductance.