Physics for IGCSE & O level - Magnets And Currents (Section 5 - No. 47)
What happens when the secondary circuit of a transformer is open?
Primary current increases to a very high value.
Primary current decreases to almost zero.
Output voltage increases significantly.
Transformer stops working.
Explanation
An open secondary means no current can flow, so the primary current, ideally, reduces to the level needed to magnetize the core.