Transformer : electrical device consisting of one coil of wire placed in close proximity to one or more other coils, used to couple two or more alternating-current (AC) circuits together by employing the induction between the coils (see  Electricity). The coil connected to the power source is called the primary coil, and the other coils are known as secondaries. A transformer in which the secondary voltage is higher than the primary is called a step-up transformer; if the secondary voltage is less than the primary, the device is known as a step-down transformer. The product of current times voltage is constant in each set of coils, so that in a step-up transformer, the voltage increase in the secondary is accompanied by a corresponding decrease in the current.


II. Power Transformers


Large devices are used in electric power systems, and small units in electronic devices (see  Electronics). Industrial and residential power tranformers that operate at the line frequency (60 Hz in the U.S.), may be single phase or three-phase, and are designed to handle high voltages and currents. Efficient power transmission requires a step-up transformer at the power-generating station to raise voltages, with a corresponding decrease in current. Line power losses are proportional to the square of the current times the resistance of the power line, so that very high voltages and low currents are used for long-distance transmission lines to reduce losses. At the receiving end, step-down transformers reduce the voltage, and increase the current, to the residential or industrial voltage levels, usually 115 to 600 V.

Power transformers must be efficient and should dissipate as little power as possible in the form of heat during the transformation process. Efficiencies are normally above 99 percent and are obtained by using special steel alloys to couple the induced magnetic fields between the primary and secondary windings. The dissipation of even 0.5 percent of the power transmitted in a large transformer generates large amounts of heat, which requires special cooling provisions. Typical power transformers are installed in sealed containers that have oil or another substance circulating through the coils to transfer the heat to external radiatorlike surfaces, where it can be discharged to the surrounding atmosphere.


III. Electronics


In electronic equipment, transformers with capacities in the order of 1 kw are largely used ahead of a rectifier, which in turn supplies direct current (DC) to the equipment . Such electronic power transformers are usually made of stacks of steel alloy sheets, called laminations, on which copper wire coils are wound. Transformers in the 1- to 100-W power level are used principally as step-down transformers to couple electronic circuits to loudspeakers in radios, television sets, and high-fidelity equipment . Known as audio transformers, these devices use only a small fraction of their power rating to deliver program material in the audible ranges, with minimum distortion. The transformers are judged on their ability to reproduce sound-wave frequencies (from 20 Hz to 25 kHz) with minimal distortion over the full sound power level .

At power levels of 1 milliwatt or less, transformers are primarily used to couple ultrahigh-frequency (UHF), very-high frequency (VHF), radio-frequency (RF), and intermediate-frequency (IF) signals, and to increase their voltage. These high-frequency transformers usually operate in a tuned or resonant circuit , in which tuning is used to remove unwanted electrical noise at frequencies outside the desired transmission range.