A popular electrical component used on many control circuits is the step-down transformer. It is the power source for a separate lower-voltage circuit which typically controls the operation of a system. This lesser voltage-control circuit allows the system to use smaller capacity controls and wire sizes to operate the system. This allows the system to be built using less expensive controls. It also makes it much easier to install as contractors or manufacturers do not need to run heavy-gauge wiring for the control circuit.
A step-down transformer reduces the voltage by means of electromagnetic induction. It has no moving parts; it consists of two windings — a primary winding and a secondary winding — enclosed in one body. The two windings are electrically separated and will be of different sizes. When an ac voltage is applied to the primary winding it will induce a voltage in the secondary winding. In a step-down transformer, the primary winding will have more turns of wire compared to the secondary winding. It is this relationship that determines the change in voltage. For example, if the ratio is five-to-one, the primary winding has five more turns of wire than the secondary winding. If 120 V were applied to the primary winding, 24 V will be induced onto the secondary winding.