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A transformer is an electrical device that transfers energy from one electrical circuit to another by means of magnetic coupling. It typically transforms between high and low voltages and accordingly between low and high currents. It works only with current that changes, and most often it is used with alternating current. An ideal transformer has perfect magnetic coupling, so that all the power fed into the device is available at its output, but in practice there are losses.

A transformer usually has two induction coils or windings. As the changing current flows through the powered or primary winding, it produces a changing magnetic field that grows through the unpowered or secondary windings. This changing magnetic field induces a current in the secondaries. (The changing magnetic field also causes mechanical forces that make parts of the transformer vibrate, which is why transformers hum. The hum consists of a fundamental note at twice the mains frequency, plus harmonics. The energy converted to motion is one form of core loss[?].)

The winding with fewer turns of wire has higher current, at a lower voltage. The winding with more turns of wire has less current, at a higher voltage. The ratio of voltages is proportional to the ratio of the numbers of turns of wire. For example, a 5-to-1 step-down transformer might take 1200-volt 60 Hz alternating current, and change it to the 240-volt 60 Hz alternating current distributed to houses. It might have 500 turns of wire on the primary, and 100 turns on the secondary. Many electronic appliances have several secondaries to produce different voltages needed in the appliance.

The high-current low-voltage windings have fewer turns of thicker wire. The thicker wire helps carry more current. The high-voltage, low-current windings have more turns of thinner wire. The thinner wire carries less current, but at a higher voltage. Transformer designers will optimize the wire sizes so that each winding will have the lowest resistance while keeping the winding size as small as possible, in an effort to reduce copper losses[?].

Some transformers have equal numbers of windings on both coils. These "isolation" transformers are used to prevent direct current flow between electric circuits, while transferring power.

An autotransformer[?] has only a single winding, which is tapped at some point along the winding. AC or pulsed DC power is applied across a portion of the winding, and a higher (or lower) voltage is produced across another portion of the same winding. Autotransformers are commonly used as spark coils in automotive engines, and as high-voltage flyback transformers in television sets and computer monitors.

A Tesla coil is a high-voltage air-core resonant transformer generating very high voltages at high frequency.

The transformer was invented by three Hungarian engineers: Ottó Bláthy, Miksa Déri and Károly Zipernowsky in 1885.

Uses of transformers

  • If electrical power needs to be transmitted over long distances, the loss is much lower if high voltage is used. But high voltage is dangerous in the home, so transformers are employed to step the voltage up at the power station and back down at the consumer's premises.

  • Some transformers are designed so that one winding turns or slides, while the other remains stationary. These can pass power or radio signals from a stationary mounting to a turning mechanism, such as a machine tool head or radar antenna.

  • Some moving transformers are precisely constructed in order to measure distances. Most often, they have several primaries, and electronic circuits measure the shape of the wave in the different secondaries.


  • Transformers often have silicon steel cores to channel the magnetic field. This keeps the field more concentrated around the wires, so that the transformer is more efficient. The core also keeps the field from being wasted in nearby pieces of metal.

  • Laminated cores are made of many stamped pieces of thin steel. This prevents eddy currents from forming in the cores and wasting power by heating the core. Other types of core are made of nonconductive magnetic materials, such as a ceramic material called ferrite.

  • High-frequency transformers in low-power circuits where moderate losses are acceptable may have air cores. These save weight and cost.

  • Power transformers are usually more than 98% efficient which makes them the most efficient devices man can make. The higher-voltage transformers are bathed in nonconductive oil that is stable at high temperatures. This used to be polychlorinated biphenyl, the famous toxic waste, "PCB". Nowadays, nontoxic very stable fluorinated hydrocarbons are preferred. The oil cools the transformer, and helps prevent short circuits. It has to be stable at high temperatures so that a small short or arc will not cause a breakdown or fire.

  • Follow this link for instructions on how to make a very simple transformer suitable for demonstration of the principles in a school classroom setting. Making a transformer

See also: Milan Vidmar, Mihajlo Pupin.

Unrelated link: Transformers (television series), an animated television series

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