Transformers are often used in electrical circuits to change the voltage of electricity flowing through a circuit. The coils of wire on each side of the transformer have different numbers of turns, causing the voltage out to be different from the voltage in.
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Voltage Transformers in Electric Circuits

Transformers (sometimes called "voltage transformers") are devices used in electrical circuits to change the voltage of electricity flowing in the circuit. Transformers can be used either to increase the voltage (called "stepping up") or decrease the voltage ("step down").

A transformer is just a piece of iron with a pair of wires coiled around it - one with many more turns in the coil than the other. The coils of wire are not physically connected. The iron core is immersed in an insulating oil bath which does not conduct electricity well.

The basic physical process underlying the operation of transformers is electromagnetic induction. If a conductor, such as a copper wire, is sitting in a magnetic field that is changing, a current will flow in the conductor. This current will not be steady but will also be changing. Alternatively, if a changing current is present, it will produce a changing magnetic field. The typical current flowing in our homes and in power distribution networks changes direction 60 times every second. Around every wire, through which this current flows, a magnetic field is produced.

A transformer works only with AC (alternating current) circuits. The changing AC current enters the primary coil of wire in the transformer. A magnetic field is produced that is concentrated in the iron core of the transformer. A secondary coil of wires (also conductors) is wrapped around the iron core, not physically touching the first set of wires. The changing magnetic field produced by the first coil is experienced by the second coil and current begins to flow in these wires as well. The second coil has many more turns of wire and offers a higher resistance to the current flow than the first coil. The greater resistance means that a larger voltage drop (than is present across the first coil of wire) is produced from one end of the coil to the other. Therefore a low voltage enters the transformer and a high voltage exits, or vice versa.

Transformers are passive devices - they don't add power. Power is equal to the product of the voltage and the current. If the voltage increases the current drops. A high voltage and low current exits the transformer carrying almost the same amount of power along the transmission lines that the initial low voltage and high current did. Most transformers operate at high efficiency, under normal conditions, transmitting about 99% of the power that enters them. (About 1% of the power is lost in heating the transformer.)

Last modified February 25, 2009 by Randy Russell.

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