Two separate coils are available to build a step-up transformer. Coil 1 has 100 more turns than Coil 2. To achieve a step-up action (higher secondary voltage), which coil should serve as the primary?

Introduction / Context:
Transformer step-up or step-down behavior depends directly on the turns ratio between secondary and primary windings. Choosing which physical coil to use for primary or secondary determines whether the output voltage increases or decreases compared to the input.

Given Data / Assumptions:

• Coil 1 has more turns than Coil 2 (by 100 turns).
• We desire a step-up transformer: V_secondary > V_primary.
• Ideal transformer assumptions: negligible losses and leakage.

Concept / Approach:
The ideal transformer ratio is V_secondary / V_primary = N_secondary / N_primary. To step up voltage, N_secondary must exceed N_primary. Since Coil 1 has more turns, it should be used as the secondary, leaving the fewer-turn coil (Coil 2) as the primary.

Step-by-Step Solution:
Let N1 = turns of Coil 1, N2 = turns of Coil 2, with N1 = N2 + 100.For step-up, choose N_secondary > N_primary.Assign N_secondary = N1 and N_primary = N2 to satisfy N_secondary > N_primary.Therefore, Coil 2 should be the primary.

Verification / Alternative check:
Example: if N1 = 600 and N2 = 500, then V_secondary / V_primary = 600/500 = 1.2, a 20% step-up. Reversing roles would give a step-down.

Why Other Options Are Wrong:
Claiming identical windings are required is incorrect; any ratio other than 1:1 can step voltages.

Using Coil 1 as primary produces step-down behavior, not step-up.

“Too small to matter” is false; even modest differences change output voltage.

Turns count directly affects voltage ratio; it cannot be ignored.

Common Pitfalls:
Confusing which side should have more turns for step-up. Always place the higher-turns winding on the secondary side to raise voltage.

Final Answer:
Coil 2 should be the primary.