Taizhou medical equipment transformer factory

A. only when S 1 changes from 2 to 1, n 1 becomes smaller. According to the transformation ratio formula U 1u2 = N 1n2, the output voltage increases, so the output current increases and the output power increases. The input power is equal to the output power, so the input power becomes larger and the input current becomes larger, so A is wrong;

B, n2 becomes smaller only when S2 changes from 4 to 3. According to the transformation ratio formula U 1u2 = N 1n2, the output voltage becomes smaller, so the output current becomes smaller and the output power becomes smaller. The input power is equal to the output power, so the input power becomes smaller and the input current becomes smaller, so B is correct;

C, only S3 changes from closed to open, and a branch is missing, but the voltage of the branch connected in series with resistors R2 and R3 remains unchanged, so the current and voltage flowing through resistor R2 remain unchanged, so C is wrong;

D, only the sliding contact of rheostat R3 moves up, and the total load resistance becomes larger, so the output current becomes smaller and the output power becomes smaller; The input power is equal to the output power, so the input power decreases, so choose D.

So choose BD.