To supply a 120 VA load connected to the secondary of a 250 VA transformer, how much power must be supplied on the primary?

To determine the power that must be supplied on the primary side of a transformer for a given load on the secondary side, it is important to consider the transformer's efficiency and the relationship between the input and output power.

In ideal circumstances, assuming the transformer operates under ideal conditions (with no losses), the power on the primary side should equal the power on the secondary side. This principle stems from the law of conservation of energy, where the power in equals the power out in a transformer under no-load conditions, meaning that the losses (like heat) are neglected.

When you have a load of 120 VA connected to the secondary of a transformer rated at 250 VA, the output power is directly utilized by that load. Therefore, the ideal amount of power that needs to be supplied by the primary side to support this load would still be 120 VA.

In practical applications, actual transformers may experience variances due to efficiency ratings, but in this exercise context, and for determining the minimum power required, the answer remains 120 VA on the primary side to match the secondary load effectively.