What is the output voltage if an input signal is applied to the secondary winding of a 4:1 voltage step-down transformer instead of the primary winding?
The correct answer is A: The input voltage is multiplied by 4. If an input signal is applied to the secondary winding of a 4:1 voltage step-down transformer instead of the primary winding, the input voltage is multiplied by 4. A 4:1 step-down transformer (primary:secondary = 4:1) becomes a 1:4 step-up transformer when reversed (secondary:primary = 1:4, which is 4:1 step-up). For amateur radio operators, this explains transformer reversibility. Understanding this helps when using transformers.
Exam Tip
4:1 step-down reversed = input voltage multiplied by 4. Think '4':1 step-down reversed = '4'× step-up. A 4:1 step-down (primary:secondary = 4:1) becomes 4:1 step-up when reversed (secondary:primary = 1:4 = 4:1 step-up). Not divided, not resistance needed - just multiplied.
Memory Aid
"4:1 step-down reversed = input voltage multiplied by 4. Think '4':1 step-down reversed = '4'× step-up. Transformer ratio reverses when input/output are swapped. Transformers work in both directions."
Real-World Application
A 4:1 step-down transformer (primary 4 turns, secondary 1 turn). If you apply input to the secondary (1 turn) and take output from the primary (4 turns), it becomes a 4:1 step-up transformer. 120V input becomes 480V output (multiplied by 4). Transformers work in both directions.
Key Concepts
Why Other Options Are Wrong
Option B: Incorrect. Input voltage isn't divided by 4 - when reversed, a 4:1 step-down becomes a 4:1 step-up, so voltage is multiplied, not divided.
Option C: Incorrect. Additional resistance in series with primary isn't needed - the transformer works in reverse without additional resistance. Resistance isn't the issue.
Option D: Incorrect. Additional resistance in parallel with secondary isn't needed - the transformer works in reverse without additional resistance. Resistance isn't the issue.
题目解析
The correct answer is A: The input voltage is multiplied by 4. If an input signal is applied to the secondary winding of a 4:1 voltage step-down transformer instead of the primary winding, the input voltage is multiplied by 4. A 4:1 step-down transformer (primary:secondary = 4:1) becomes a 1:4 step-up transformer when reversed (secondary:primary = 1:4, which is 4:1 step-up). For amateur radio operators, this explains transformer reversibility. Understanding this helps when using transformers.
考试技巧
4:1 step-down reversed = input voltage multiplied by 4. Think '4':1 step-down reversed = '4'× step-up. A 4:1 step-down (primary:secondary = 4:1) becomes 4:1 step-up when reversed (secondary:primary = 1:4 = 4:1 step-up). Not divided, not resistance needed - just multiplied.
记忆口诀
4:1 step-down reversed = input voltage multiplied by 4. Think '4':1 step-down reversed = '4'× step-up. Transformer ratio reverses when input/output are swapped. Transformers work in both directions.
实际应用示例
A 4:1 step-down transformer (primary 4 turns, secondary 1 turn). If you apply input to the secondary (1 turn) and take output from the primary (4 turns), it becomes a 4:1 step-up transformer. 120V input becomes 480V output (multiplied by 4). Transformers work in both directions.
错误选项分析
Option B: Incorrect. Input voltage isn't divided by 4 - when reversed, a 4:1 step-down becomes a 4:1 step-up, so voltage is multiplied, not divided. Option C: Incorrect. Additional resistance in series with primary isn't needed - the transformer works in reverse without additional resistance. Resistance isn't the issue. Option D: Incorrect. Additional resistance in parallel with secondary isn't needed - the transformer works in reverse without additional resistance. Resistance isn't the issue.
知识点
Transformer reversal, Step-down transformer, Voltage multiplication, Transformer ratio
Verified Content
Question from official FCC General Class question pool. Explanation reviewed by licensed amateur radio operators.