A transformer is a component that transfers current from one circuit to another in order to increase or decrease electrical voltage. The transfer occurs through the process of induction where electromagnetic fields induce current in a conductor.
A transformer can be broken down into three main parts: a primary winding, secondary winding, and a core. When current is applied to the primary winding the result is the production of a magnetic field. The magnetic field causes current to be produced in the secondary coil. Turn ratios are the ratio of turns in each winding. The turn ratio determines the properties of the transformer. If the primary winding has more turns than the secondary winding then the voltage is decreased. On the other hand if the primary winding has fewer turns than the secondary winding then the voltage will be increased. Transformers that increase voltage are called "step up" transformers while ones that decrease voltage are called "step down" transformers. The turn ratio can be used to calculate how much the voltage will be altered. To do this divide the number of turns on the primary winding by the number of turns on the secondary winding and multiply this by the input voltage. This will give you the output voltage of the transformer.