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Engineering >> Transformer >> Transformers
This article is about transformers as used in electrical and electronics applications. For other meanings, see Transformers

Three-phase pole-mounted step-down transformer.

A transformer is an electrical device that transfers energy from one circuit to another by magnetic coupling with no moving parts. A transformer comprises two or more coupled windings, or a single tapped winding and, in most cases, a magnetic core to concentrate magnetic flux. An alternating current in one winding creates a time-varying magnetic flux in the core, which induces a voltage in the other windings. Transformers are used to convert between high and low voltages, to change impedance, and to provide electrical isolation between circuits.





The transformer is one of the simplest of electrical devices. Its basic principles have not changed over the last one hundred years, yet transformer designs and materials continue to be improved. Transformers are essential for high voltage power transmission, which makes long distance transmission economically practical. This advantage was the principal factor in the selection of alternating current power transmission in the "War of Currents" in the late 1880s.
Audio frequency transformers, (at the time called repeating coils), were used by the earliest experimenters in the development of the telephone. While some electronics applications of the transformer have been made obsolete by new technologies, transformers are still found in many electronic devices.

Transformers come in a range of sizes from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge gigawatt units used to interconnect large portions of national power grids. All operate with the same basic principles and with many similarities in their parts.

Single phase pole-mounted step-down transformer

Transformers alone cannot do the following:

  • Convert DC to AC or vice versa
  • Change the voltage or current of DC
  • Change the AC supply frequency.






Those credited with the invention of the transformer include:

  • Michael Faraday, who invented an 'induction ring' on August 29, 1831. This was the first transformer, although Faraday used it only to demonstrate the principle of electromagnetic induction and did not foresee the use to which it would eventually be put.
  • Lucien Gaulard and John Dixon Gibbs, who first exhibited a device called a 'secondary generator' in London in 1881 and then sold the idea to American company Westinghouse. This may have been the first practical power transformer, but was not the first transformer of any kind. They also exhibited the invention in Turin in 1884, where it was adopted for an electric lighting system. Their early devices used an open iron core, which was later abandoned in favour of a more efficient circular core with a closed magnetic path.

  • William Stanley, an engineer for Westinghouse, who built the first practical device in 1885 after George Westinghouse bought Gaulard and Gibbs' patents. The core was made from interlocking E-shaped iron plates. This design was first used commercially in 1886.
  • Hungarian engineers Károly Zipernowsky, Ottó Bláthy and Miksa Déri at the Ganz company in Budapest in 1885, who created the efficient "ZBD" model based on the design by Gaulard and Gibbs.
  • Nikola Tesla in 1891 invented the Tesla coil, which is a high-voltage, air-core, dual-tuned resonant transformer for generating very high voltages at high frequency.

Many others have patents on transformers

Transformer designs


An autotransformer has only a single winding, which is tapped at some point along the winding. AC or pulsed voltage is applied across a portion of the winding, and a higher (or lower) voltage is produced across another portion of the same winding. While theoretically separate parts of the winding can be used for input and output, in practice the higher voltage will be connected to the ends of the winding, and the lower voltage from one end to a tap. For example, a transformer with a tap at the center of the winding can be used with 230 volts across the entire winding, and 115 volts between one end and the tap. It can be connected to a 230 volt supply to drive 115 volt equipment, or reversed to drive 230 volt equipment from 115 volts. As the same winding is used for input and output, the flux in the core is partially cancelled, and a smaller core can be used. For voltage ratios not exceeding about 3:1, an autotransformer is cheaper, lighter, smaller and more efficient than a true (two-winding) transformer of the same rating.

In practice, transformer losses mean that autotransformers are not perfectly reversible; one designed for stepping down a voltage will deliver slightly less voltage than required if used to step up. The difference is usually slight enough to allow reversal where the actual voltage level is not critical.

By exposing part of the winding coils and making the secondary connection through a sliding brush, an autotransformer with a near-continuously variable turns ratio can be obtained, allowing for very small increments of voltage.

Polyphase transformers

For three-phase power, three separate single-phase transformers can be used, or all three phases can be connected to a single polyphase transformer. The three primary windings are connected together and the three secondary windings are connected together. The most common connections are Y-?, ?-Y, ?-? and Y-Y. A vector group indicates the configuration of the windings and the phase angle difference between them. If a winding is connected to earth (grounded), the earth connection point is usually the center point of a Y winding. There are many possible configurations that may involve more or fewer than six windings and various tap connections.

The Y-? transform is a mathematical technique to simplify analysis of an electrical network.

Resonant transformers

A resonant transformer is one that operates at the resonant frequency of one or more of its coils and, usually, an external capacitor. The resonant coil, usually the secondary, acts as an inductor, and is connected in series with a capacitor. If the primary coil is driven by a periodic source of alternating current, such as a square or Sawtooth wave , each pulse of current helps to build up an oscillation in the secondary coil. Due to resonance, a very high voltage can develop across the secondary, until it is limited by some process such as electrical breakdown. These devices are therefore used to generate high alternating voltages. The current available from this type of coil can be much larger than that from electrostatic machines such as the Van de Graaff generator and Wimshurst machine. They also run at a higher operating temperature than standard units.


  • Tesla coil
  • Oudin coil (or Oudin resonator; named after its inventor Paul Oudin)
  • D'Arsonval apparatus
  • Ignition coil or induction coil used in the ignition system of a petrol engine
  • Flyback transformer of a CRT television set or video monitor.
  • Electrical breakdown and insulation testing of high voltage equipment and cables

Other applications of resonant transformers are as coupling between stages of a superheterodyne receiver, where the selectivity of the receiver is provided by the tuned transformers of the intermediate-frequency amplifiers.

A voltage regulating transformer uses a resonant winding and allows part of the core to go into saturation on each cycle of the alternating current. This effect stabilizes the output of the regulating transformer, which can be used for equipment that is sensitive to variations of the supply voltage. Saturating transformers provide a simple rugged method to stabilize an ac power supply. However, due to the hysteresis losses accompanying this type of operation, efficiency is low.

Instrument transformers

Current transformers

Current transformers used in metering equipment for three-phase 400 ampere electricity supply

A current transformer is a type of "instrument transformer" that is designed to provide a current in its secondary which is accurately proportional to the current flowing in its primary.

Current transformers are commonly used in metering and protective relaying to facilitate the measurement of large currents and isolation of high voltage systems which would be difficult to measure more directly.

Current transformers are often constructed by passing a single primary turn (either an insulated cable or an uninsulated conductor (copper or aluminum are typical in electric utility applications) through a well-insulated toroidal core wrapped with many turns of wire. Current transformers (CTs) are used extensively in the electrical power industry for monitoring of the power grid. The CT is described by its current ratio from primary to secondary. Common secondaries are 1 or 5 amperes. The secondary winding can be single ratio or multi ratio, with five taps being common for multi ratio CTs. Typically, the secondary connection points are labeled as X1, X2 and so on. The multi ratio CTs are typically used for current matching in current differential protective relaying applications. Often, multiple CTs will be installed as a "stack" for various uses (for example, protection devices and revenue metering may use separate CTs). For a three-stacked CT application, the secondary winding connection points are typically labeled Xn, Yn, Zn.

Specially constructed "wideband current transformers" are also used (usually with an oscilloscope) to measure waveforms of high frequency or pulsed currents. One type of specially constructed wideband transformer provides a voltage output that is proportional to the measured current. Another type (called a Rogowski coil) requires an external integrator in order to provide a voltage output that is proportional to the measured current.

Care must be taken that the secondary of a current transformer is not disconnected from its load while current is flowing in the primary, as this will produce a dangerously high voltage across the open secondary.

Voltage transformers

Voltage transformers (also called potential transformers) are another type of instrument transformer, used for metering and protection in high-voltage circuits. They are designed to present negligible load to the voltage being measured and to have a precise voltage ratio to accurately step down high voltages so that metering and protective relay equipment can be operated at a lower potential. Typically the secondary of a voltage transformer is rated for 69 or 120 Volts at rated primary voltage, to match the input ratings of protection relays.

The transformer winding high-voltage connection points are typically labelled as H1, H2 (sometimes H0 if it is internally grounded) and X1, X2, and sometimes an X3 tap may be present. Sometimes a second isolated winding (Y1, Y2, Y3) may also be available on the same voltage transformer. The high side (primary) may be connected phase to ground or phase to phase. The low side (secondary) is usually phase to ground.

The terminal identifications (H1, X1, Y1, etc.) are often referred to as polarity. This applies to current transformers as well. At any instant terminals with the same suffix numeral have the same polarity and phase. Correct identification of terminals and wiring is important for proper operation of metering and protection relays.

Pulse transformers

A pulse transformer is a transformer that is optimised for transmitting rectangular electrical pulses (that is, pulses with fast rise and fall times and a constant amplitude). Small versions called signal types are used in digital logic and telecommunications circuits, often for matching logic drivers to transmission lines. Medium-sized power versions are used in power-control circuits such as camera flash controllers. Larger power versions are used in the electrical power distribution industry to interface low-voltage control circuitry to the high-voltage gates of power semiconductors such as TRIACs, IGBTs, thyristors and MOSFETs. Special high voltage pulse transformers are also used to generate high power pulses for radar, particle accelerators, or other pulsed power applications.

To minimise distortion of the pulse shape, a pulse transformer needs to have low values of leakage inductance and distributed capacitance, and a high open-circuit inductance. In power-type pulse transformers, a low coupling capacitance (between the primary and secondary) is important to protect the circuitry on the primary side from high-powered transients created by the load. For the same reason, high insulation resistance and high breakdown voltage are required. A good transient response is necessary to maintain the rectangular pulse shape at the secondary, because a pulse with slow edges would create switching losses in the power semiconductors.

The product of the peak pulse voltage and the duration of the pulse (or more accurately, the voltage-time integral) is often used to characterise pulse transformers. Generally speaking, the larger this product, the larger and more expensive the transformer.

RF transformers (transmission line transformers)

Coils with tickler.

For radio frequency use, transformers are sometimes made from configurations of transmission line, sometimes bifilar or coaxial cable, wound around ferrite or other types of core. This style of transformer gives an extremely wide bandwidth but only a limited number of ratios (such as 1:9, 1:4 or 1:2) can be achieved with this technique.

The core material increases the inductance dramatically, thereby raising its Q factor. The cores of such transformers help improve performance at the lower frequency end of the band. Older style RF transformers sometimes used a third coil (called a tickler winding) to inject feedback into an earlier (detector) stage in antique regenerative radio receivers.


Baluns are transformers designed specifially to connect between balanced and unbalanced circuits. These are sometimes made from configurations of transmission line and sometimes bifilar or coaxial cable and are similar to transmission line transformers in constructuion and operation. This style of transformer is frequently used as an impedance matching balun to convert from 300 ohm balanced to 75 ohm unbalanced in FM receivers.

Audio transformers

Transformers in a tube amplifier. Output transformers are on the left. The power supply toroidal transformer is on right.

Transformers are used in valve (vacuum tube) audio circuits to match the high impedance of the valve to the lower impedance of the load. This is no longer necessary with transistor circuits, which can always be made with a lower output impedance than that of the load, and so these circuits use impedance bridging instead. [1]

Audio transformers are usually the factor which limit sound quality; electronic circuits with wide frequency response and low distortion are relatively simple to design.

Transformers are also used in DI boxes to convert impedance from high-impedance instruments (for example, bass guitars) to enable them to be connected to a microphone input on the mixing console.

Output transformers

A particularly critical component is the output transformer of an audio power amplifier. Valve circuits for quality reproduction have long been produced with no other (inter-stage) audio transformers, but an output transformer is needed to couple the relatively high impedance (up to a few hundred ohms depending upon configutation) of the output valve(s) to the low impedance of a loudspeaker. (The valves can deliver a low current at a high voltage; the speakers require high current at low voltage.)

For good low-frequency response a relatively large iron core is required; high power handling increases the required core size. Low distortion requires iron of adequate properties; special cores with oriented magnetic domains are used for best results. Good high-frequency response requires carefully designed and implemented windings without excessive leakage inductance or stray capacitance. All this makes for an expensive component.

Output transformerless audio power valve amplifiers are possible (e.g., a design by Julius Futterman), but were rarely used due to other disadvantages.

Early transistor audio power amplifiers often had output transformers, but they were eliminated as designers discovered how to design amplifiers without them.

Speaker transformers

In the same way that transformers are used to create high voltage power transmission circuits that minimize transmission losses, speaker transformers allow many individual loudspeakers to be powered from a single audio circuit operated at higher-than normal speaker voltages. This application is common in public address (e.g., Tannoy) applications. Such circuits are commonly referred to as constant voltage or 70 volt speaker circuits although the audio waveform is obviously a constantly changing voltage.

At the audio amplifier, a large audio transformer may be used to step-up the low impedance, low-voltage output of the amplifier to the designed line voltage of the speaker circuit. (For high-powered amps, the amplifier transformer may not be needed.) Then, a smaller transformer at each speaker returns the voltage and impedance to ordinary speaker levels. The speaker transformers commonly have multiple primary taps, allowing the volume at each speaker to be adjusted in a number of discrete steps.

Use of a constant-voltage speaker circuit means that there is no need to worry about the impedance presented to the amplifier output (which would clearly be too low if all of the speakers were arranged in parallel and would be too complex a design problem if the speakers were arranged in series-parallel). The use of higher transmission voltage and impedance means that power lost in the connecting wire is minimized, even with the use of small-gauge conductors (and leads to the term constant voltage as the line voltage doesn't change much as additional speakers are added to the system). Also, the ability to adjust, locally, the volume of each speaker (without the complexity and power loss of an L pad) is a useful feature.

Uses of transformers

  • For supplying power from an alternating current power grid to equipment which uses a different voltage. May be followed by a rectification circuit, if direct rather than alternating power is needed.
    • Adaptation of electrical equipment to supply voltages for which it was not made. For example, to use U.S. equipment, designed for 117 V AC, in European countries with 230 V AC. A transformer or autotransformer may be used, or electronic voltage changers which do not use transformers.
    • Use inside solid-state equipment which requires low voltages to reduce the main electricity voltage to the required value.
    • Use as an external adapter to power low-voltage solid-state equipment from higher-voltage main electricity.
  • Electric power transmission over long distances.
  • High-voltage direct-current HVDC power transmission systems
  • Large, specially constructed power transformers are used for electric arc furnaces used in steelmaking.
  • Rotating transformers are designed so that one winding turns while the other remains stationary. A common use was the video head system as used in VHS and Beta video tape players. These can pass power or radio signals from a stationary mounting to a rotating mechanism, or radar antenna.
  • Sliding transformers can pass power or signals from a stationary mounting to a moving part such as a machine tool head. See linear variable differential transformer.
  • Some rotary transformers are used to couple signals between two parts which rotate in relation to each other.
  • Other rotary transformers are precisely constructed in order to measure distances or angles. Usually they have a single primary and two or more secondaries, and electronic circuits measure the different amplitudes of the currents in the secondaries. See synchro and resolver.
  • Small transformers are often used internally to isolate and link different parts of radio receivers and audio amplifiers, converting high current low voltage circuits to low current high voltage, or vice versa.They are usually tuneable, and labelled filter/bandpass, though technically being transformers/operating on the same electromagnetic principes. See electronics and impedance matching. See also isolation transformer and repeating coil.
  • Transformers may be used as external accessories for impedance matching; for example to match a microphone to an amplifier. These were frequently required with valve equipment, but solid-state electronics are more capable of matching a wide range of impedances without the need for a transformer.
  • Balanced-to-unbalanced conversion. A special type of transformer called a balun is used in radio and audio circuits to convert between balanced circuits and unbalanced transmission lines such as antenna downleads. A balanced line is one in which the two conductors (signal and return) have the same impedance to ground: twisted pair and "balanced twin" are examples. Unbalanced lines include coaxial cables and strip-line traces on printed circuit boards. A similar use is for connecting the "single ended" input stages of an amplifier to the high-powered "push-pull" output stage.
  • Safety. Transformers are used in home electronics, such as computers, to decouple them from the power grid that they are connected to. This is called Galvanic isolation.

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