The number of turns of the two sets of transformers is N1 and N2, N1 is the primary and N2 is the secondary. An alternating voltage is applied to the primary coil, and an induced electromotive force is generated at both ends of the secondary coil. When N2>N1, the induced electromotive force is higher than the primary applied voltage. This transformer is called a step-up transformer: When N2 U1/U2=N1/N2

In the formula, n is called voltage ratio (turn ratio). When n>1, then N1>N2, U1>U2, and the transformer is a step-down transformer. The opposite is the step-up transformer.

Another current ratio I1/I2=N2/N1

Electric power P1=P2

Note: The above formula is only valid when the ideal transformer has only one secondary coil. When there are two secondary coils, P1=P2+P3, U1/N1=U2/N2=U3/N3, the current must be determined by the relationship of electric power, when there are multiple, and so on.