*Question G5C05:
Why is the primary winding wire of a voltage step-up transformer usually a larger size than that of the secondary winding?*  

*Correct Answer is  B: To accommodate the higher current of the primary*  

### AI Response: (may contain errors)  

In a voltage step-up transformer, the primary winding is designed to handle a higher current than the secondary winding. This is because the power input to the transformer must equal the power output (minus any losses), and power is the product of voltage and current.

1. **Correct Answer (B): To accommodate the higher current of the primary**

   - In a step-up transformer, the primary winding operates at a lower voltage and higher current compared to the secondary winding, which operates at a higher voltage and lower current. The wire size in the primary must be larger to handle this higher current without overheating or excessive resistive losses.

2. **Why the other choices are incorrect:**

   - **Choice A: To improve the coupling between the primary and secondary**
     - Coupling between the windings is primarily determined by the core material and the physical arrangement of the windings, not the wire size.

   - **Choice C: To prevent parasitic oscillations due to resistive losses in the primary**
     - Parasitic oscillations are more related to the design of the circuit and the components used, rather than the wire size. While resistive losses are a concern, they are not the primary reason for using larger wire.

   - **Choice D: To ensure that the volume of the primary winding is equal to the volume of the secondary winding**
     - The volume of the windings is not a critical factor in transformer design. The focus is on electrical characteristics like current capacity and insulation.

3. **Useful Formula:**

   - Power in the primary ($P_{\text{primary}}$) is equal to power in the secondary ($P_{\text{secondary}}$), assuming no losses:
     $$ P_{\text{primary}} = V_{\text{primary}} \times I_{\text{primary}} = V_{\text{secondary}} \times I_{\text{secondary}} $$

   - In a step-up transformer, $V_{\text{secondary}} > V_{\text{primary}}$, which implies $I_{\text{primary}} > I_{\text{secondary}}$. Therefore, the primary winding must handle more current, necessitating a larger wire size.