What Happens When a Transformer is Connected to the DC?

When a DC voltage is applied to the primary winding of a transformer, due to low resistance, the winding acts as a short circuit across the terminals of the DC source that leads to the flow of heavy current through the winding resulting in overheating of the winding.

Eventually, the source or the winding will be damaged¹.

The effect of current on winding depends on the applied DC voltage, rating of the transformer, winding resistance, and how powerful the DC source is¹.

Therefore, it is not recommended to connect a transformer to a DC supply.