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The transformer which increases a low voltage AC emf to a high voltage AC e.m.f. is called a step - up transformer. In step up transformer the number of turns in the secondary coil is greater than that of primary coils. Thick wire is used in the primary coil and thin wire is used in the secondary coil. These two coils AC are wound on an insulated soft iron core

The transformer used to change a high voltage AC to a low voltage AC is called a step-down transformer. In step down transformer the number of turns in the secondary coil is less than that of primary coil and thin wire is used in the primary and thick wire is used in the secondary. Step -down transformer is used for the distribution of electric power for house -hold uses.