Transformers can provide desired voltages to a circuit by modifying the number of turns in the secondary windings.

If the ratio of the number of turns in the secondary winding to that of the primary winding is greater than one, then the transformer is said to be a step-up transformer. In a step-up transformer, the voltage at the secondary winding is greater than the voltage applied at the primary winding.

However, if this ratio is less than one, the transformer is said to be a step-down transformer. In a step-down transformer, the voltage obtained at the secondary winding is lower than that of the voltage applied at the primary winding. However, the current in these transformers follows a reverse trend. The current transformation equations show that if the voltage is stepped up, the current is stepped down, and vice-versa. By replacing the secondary current in the current transformation equation with a ratio of secondary voltage to load resistance, the equation for the transformation of resistance is obtained. This equation shows that the input voltage "sees" not a load resistance but an equivalent resistance, which equals the product of load resistance with the square of the turn ratio in secondary to primary windings. Therefore, a transformer transforms voltage, current, and resistance in an electrical circuit using electromagnetic induction.

Transformers have many applications. Step-up transformers are used at a power generating station where the primary windings are connected to the power source, and the secondary windings are connected to the transmission lines, giving the desired high voltage for transmission. At the consumer end, step-down transformers lower the voltage to a desired value for home appliances or industrial applications. An AC adapter used to charge laptops or power desktop computers has a small step-down transformer that converts line voltage to a lower value, typically 3 to 12 volts.