In electronics, a center tap is a connection made to a point half way along a winding of a transformer

Transformer

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

 or inductor

Inductor

An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...

, or along the element of a resistor

Resistor

A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...

 or a potentiometer

Potentiometer

A potentiometer , informally, a pot, is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider. If only two terminals are used , it acts as a variable resistor or rheostat. Potentiometers are commonly used to control electrical devices such as volume controls on...

.
Taps are sometimes used on inductors for the coupling of signals, and may not necessarily be at the half-way point, but rather, closer to one end. A common application of this is in the Hartley oscillator

Hartley oscillator

The Hartley oscillator is an electronic oscillator circuit that uses an inductor and a capacitor in parallel to determine the frequency. Invented in 1915 by American engineer Ralph Hartley, the distinguishing feature of the Hartley circuit is that the feedback needed for oscillation is taken from...

. Inductors with taps also permit the transformation of the amplitude of alternating current

Alternating current

In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

 (AC) voltage

Voltage

Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

s for the purpose of power conversion, in which case, they are referred to as autotransformer

Autotransformer

An autotransformer is an electrical transformer with only one winding. The auto prefix refers to the single coil acting on itself rather than any automatic mechanism. In an autotransformer portions of the same winding act as both the primary and secondary. The winding has at least three taps where...

s, since there is only one winding. An example of an autotransformer is an automobile

Automobile

An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...

 ignition coil

Ignition coil

An ignition coil is an induction coil in an automobile's ignition system which transforms the battery's 12 volts to the thousands of volts needed to create an electric spark in the spark plugs to ignite the fuel...

. Potentiometer tapping provides one or more connections along the device's element, along with the usual connections at each of the two ends of the element, and the slider connection. Potentiometer taps allow for circuit functions that would otherwise not be available with the usual construction of just the two end connections and one slider connection.

Volts center tapped

Volts center tapped (VCT) describes the voltage output of a center tapped transformer. For example: A 24 VCT transformer will measure 24 VAC across the outer two taps (winding as a whole), and 12VAC from each outer tap to the center-tap (half winding). These two 12 VAC supplies are 180 degrees out of phase with each other, thus making it easy to derive positive and negative 12 volt DC power supplies from them.

Common applications of center-tapped transformers

• In a rectifier
  Rectifier
  A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
  
  , a center-tapped transformer and two diode
  Diode
  In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
  
  s can form a full-wave rectifier that allows both half-cycles of the AC waveform to contribute to the direct current, making it smoother than a half-wave rectifier. This form of circuit saves on rectifier diodes compared to a diode bridge
  Diode bridge
  A diode bridge is an arrangement of four diodes in a bridge circuit configuration that provides the same polarity of output for either polarity of input. When used in its most common application, for conversion of an alternating current input into direct current a output, it is known as a...
  
  , but has poorer utilization of the transformer windings. Center-tapped two-diode rectifiers were a common feature of power supplies in vacuum tube
  Vacuum tube
  In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
  
   equipment. Modern semiconductor diodes are low-cost and compact so usually a 4-diode bridge is used (up to a few hundred watts total output) which produces the same quality of DC as the center-tapped configuration with a more compact and cheaper power transformer. Center-tapped configurations may still be used in high-current applications, such as large automotive battery
  Battery (electricity)
  An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
  
   chargers, where the extra transformer cost is offset by less costly rectifiers. Center-tapped transformers are also used for dual-voltage power supplies. When a center-tapped transformer is combined with a bridge (four diode) rectifier, it is possible to produce a positive and a negative voltage with respect to a ground at the tap. Dual voltage supplies are important for all sorts of electronics equipment.

• In early vacuum tube audio amplifiers, center-tapped transformers were sometimes used as the phase inverter to drive the two output tubes of a push-pull stage. The technique is nearly as old as electronic amplification and is well documented, for example, in "The Radiotron Designer's Handbook, Third Edition" of 1940. This technique was carried over into transistor designs also, part of the reason for which was that capacitors were large, expensive and unreliable. However, since that era, capacitors have become vastly smaller, cheaper and more reliable, whereas transformers are still relatively expensive. Furthermore, as designers acquired more experience with transistors, they stopped trying to treat them like tubes. Coupling a class A intermediate amplification stage to a class AB power stage using a transformer doesn't make sense any more even in small systems powered from a single-voltage supply. Modern higher-end equipment is based on dual-supply designs which eliminates coupling. It is possible for an amplifier, from the input all the way to the loudspeaker, to be DC coupled without any capacitance or inductance.

• In vacuum tube amplifiers, center-tapped transformers are used to couple a push-pull output stage to the speaker. This use is still relevant today because tubes and tube amplifiers continue to be produced for niche markets.

• In analog telecommunications systems center-tapped transformers can be used to provide a DC
  Direct current
  Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
  
   path around an AC
  Alternating current
  In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
  
   coupled amplifier for signalling purposes.

• Power distribution, see 3 wire single phase
  Split phase
  A split-phase electricity distribution system is a 3-wire single-phase distribution system, commonly used in North America for single-family residential and light commercial applications. It is the AC equivalent of the original Edison 3-wire direct current system...
  
  .

• The center-tapped rectifiers are preferred to the full bridge rectifier when the output DC current is high and the output voltage is low.

• Phantom power
  Phantom power
  Phantom power, in the context of professional audio equipment, is a method for transmitting DC electric power through microphone cables to operate microphones that contain active electronic circuitry....
  
   can be supplied to a condensor microphone using center tap transformers. One method, called "direct center tap" uses two center tap transformers, one at the microphone body and one at the microphone preamp. Filtered DC voltage is connected to the microphone preamp center tap, and the microphone body center tap is grounded through the cable shield. The second method uses the same center tap transformer topology at the microphone body, but at the microphone preamp, a matched pair of resistors spanning the signal lines in series creates an "artificial center tap".