As mentioned before, transformers do not allow DC input to flow through. This is known as DC isolation. This is because a change in current cannot be generated by DC; meaning that there is no changing magnetic field to induce a voltage across the secondary component.

Can transformer work on DC supply?

A transformer is a device which step-up or step-down the level of AC current or voltage without changing the primary (i.e. input source) frequency. Transformer only works on AC and can't be operated on DC i.e. it has been designed to be operated only and only on alternating current and voltage.

What happens if we give DC to transformer?

When we apply DC voltage to the transformer, the reactance of its winding will be zero since reactance is frequency-dependent and the frequency of a DC supply is zero. Flux produced by the flow of direct current will be constant.