Principle and structure

• A transformer is a device based on mutual induction that steps an alternating voltage up or down.
• It has a primary coil (N_p turns) and a secondary coil (N_s turns), both wound on a closed core of laminated soft iron (thin sheets insulated from each other, often silicon iron).
• The core makes the flux lines of the primary pass through the secondary.
• When AC is applied to the primary, the changing flux in the core links both coils and induces an alternating emf of the same frequency in the secondary.

For a negligible loss of flux (no stray lines), the same flux change links both coils: V_s = N_s ΔΦ/Δt and V_p = N_p ΔΦ/Δt, so

V_s / V_p = N_s / N_p

• N_s > N_p: step-up transformer (V_s > V_p). For example if N_s = 2N_p then V_s = 2V_p.
• N_s < N_p: step-down transformer (V_s < V_p).
• A transformer does not work with DC (the flux does not change, so no emf is induced in the secondary).
• Step-up transformers are used at power stations; step-down transformers are used in the distribution zones.