Calculating Secondary Current

            Ideally specking, transformers are 100 % efficient. It means that the ideal transformer may transfer 100 % of its input power to the secondary. So by formula we might say

                                                             P₂ = P₁

            As power is equivalent to the product of voltage & current

                                                            V₂I₂ =V₁I₁

                                                                      &

                                                            I₁/I₂ =V₂/V₁

            The current ratio is inverses to the voltage ratio. It means that

                                    (i) For step down, I₂>I₁

                                    (ii) And For step up, I₂<I₁

In other terms, current varies opposite to the variation in voltage. If voltage enhances, current decreases and vice versa.

As the Voltage ratio of a transformer is equivalent to the turn ratio so, the earlier equation may be written like

                                                 I₁/I₂ =N₂/N₁

                                                 I₂ = (N₁/N₂)I₁