Currents

For an ideal transformer (no energy losses), by conservation of energy the electrical energy supplied to the primary equals that delivered to the load in the secondary, so input power = output power:

V_p I_p = V_s I_s, so I_s / I_p = V_p / V_s = N_p / N_s

• The current is inversely proportional to the number of turns: the coil with more turns carries the smaller current (made of thinner wire); the coil with few turns carries the larger current (thick wire).
• A step-up transformer steps current down.
• If the secondary has twice the turns, its current is half that of the primary.