Transformers are related to the properties of inductors. If an inductor with an AC voltage across it is placed in parallel to second inductor, the electromagnetic field of the first inductor will induce an AC voltage into the second inductor. When the two coils referred to as windings are placed on core, they become a transformer. The input voltage is to the primary winding and the induced voltage is take off the secondary winding. If the number of turns on the secondary is greater than the number of turns on the primary, the secondary voltage will be greater than voltage on the primary, referred to as step-up transformer. If the number of turns on the secondary is less, the voltage is less, referred to as a step-down transformer.
There are many configurations on a transformer. There are air core(Fig 01), iron core(Fig 02)(increase the mutual inductance between windings) and phased windings(Fig 03), since the secondary voltage is 180。Out of phase with the primary voltage. An autotransformer has only three leads(Fig 04), and one is c (common) connection. Some transformers have center-tapped secondary(Fig 05) and multiple secondaries( Fig 06) to develop various voltages. They are also available in various voltage ratings, sizes and shapes.
Transformers are used to step-up or step-down voltage for power supplies, for electrical isolation, for impedance matching, and to couple signals between circuits.
Normally, DC is not useful on a transformer unless it is varying and it can saturate the windings. Therefore, a transformer is usually considered an AC device.
