Question

Why cant dc voltage be stepped down?

Answer 1

The priciple of transformer is mutual induction.that is when the current passing through the primary changes(AC changes its direction alternatively) it induces a emf in the secondary.as DC is a steady flow.no change in current. Hence no induced emf.
If the primary of a transformer is connected to the DC supply, the flux produced in the primary will be of constant magnitude because of DC Supply. Hence, no EMF will be induced in the primary, because emf is induced only when if either the flux is changing or the conductor is moving i.e. the emf will be induced in the winding only when if there is any relative motion between flux and conductor. Hence no emf will be induced either in the primary or in secondary.hence can't step down DC using transformers.