A transformer is essentially a pair of coils wrapped around iron cores, which are called primary windings and secondary windings for input and output, respectively. When current passes through the primary coil, it creates a magnetic field which then acts as an inductor to create voltage in the second coil. Transformers can be used to increase voltage, and thereby reduce current, for long distance transmission, or they can decrease voltage and increase current. The ratio of input windings to output windings will determine the output of the transformer.
Determine the number of windings of the input and output sides of the transformer. If it is a step down transformer it has fewer secondary windings than primary windings. A step up transformer, conversely, has more primary windings than secondary windings.
Determine the source voltage. In the United States, the voltage coming from a typical home power outlet is 110 volts. If you are unsure of the input voltage, you can measure it by touching the positive terminal of a volt meter to the positive wire leading into the transformer and connecting the ground terminal to the transformer's ground.
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Solve the equation Vs/Vp = Ns/Np where Vs is the secondary voltage, Vp is the primary voltage, Ns is the number of secondary windings and Np is the number of primary windings. Divide the number of secondary windings by the number of primary windings, and multiply the source voltage by this ratio. This will give you the output voltage. For example, a voltage source that sends 240 volts through a transformer with 500 primary windings and 100 secondary windings would have an output voltage of 240 * (100/500) = 48 volts.