In a step-down transformer, which winding has more turns?

• A. Secondary winding
• B. Primary winding
• C. Core
• D. Source winding

Answer: (B)

The key idea is that a transformer's voltage is set by the turns ratio between its windings. For an ideal transformer, Vp/Vs = Np/Ns. To step the voltage down, the winding connected to the higher voltage source (the primary) must have more turns than the winding delivering the lower voltage (the secondary). Therefore the primary winding has more turns. The core is not a winding, and the term “source winding” isn’t used for a separate winding. In a step-down, Ns is fewer than Np, which produces a smaller output voltage while drawing more current from the source to keep power roughly constant.