Question

Roberto plans to use two transformers to reduce a voltage of 120 V to 4 V. He uses a transformer that has 300 coils in the primary winding and 50 coils in the secondary winding. He has four other transformers, as listed in the table.

A 3 column table with 4 rows. The first column is labeled transformer with entries W, X, Y, Z. The second column is labeled primary winding in number of coils with entries 80, 60, 70, 50. The last column is labeled secondary winding in number of coils with entries 20, 12, 35, 5.
Which transformer should Roberto use to have an ending voltage of 4 V?

W
X
Y
Z

Answer 1

The first transformer gives 120 × (50/300) = 120 × 1/6 = 20 V. To get 4 V from 20 V we need a ratio Ns/Np = 4/20 = 1/5 = 0.2. Transformer X has 12/60 = 0.2, so use X.