Transformers are a pair of coils or solenoids that are usually wrapped around an iron core. Step-up transformers are created specifically to “step up” or increase voltages. This is useful in numerous applications. They operate on the principle of electromagnetic induction, where a changing magnetic field creates a voltage.
An AC voltage at the first coil creates a magnetic field inside of it, causing a current in the second coil. The induced current causes the second coil to have a changing magnetic field that in turn generates a voltage.
The voltage on the primary coil is called the input, and the one on the secondary is called the output. The primary has fewer coils than the secondary, and so an input voltage induces a larger output voltage. The number of coils is also called turns.
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The ratio of the number of input to output coils controls the output voltage size. For example, a 1:3 transformer means that 5 volts at the input will induce 15 volts at the output, since 5 x 3 = 15.
The ratio of input to output coils also controls the output current. In the above example, a 1:3 transformer means that 1 amp at the input will induce 0.33 amps at the output, because the current will be reduced by 1/3.
Power companies use them to help transmit electricity over long distances. They are also useful in devices such as air bags and power supplies.