In electronics, a center tap is a connection made to a point half way along a winding of a transformer or inductor, or along the element of a resistor or a potentiometer. 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 Colpitts oscillator. Inductors with taps also permit the transformation of the amplitude of alternating current (AC) voltages for the purpose of power conversion, in which case, they are referred to as autotransformers, since there is only one winding. An example of an autotransformer is an automobile ignition coil. 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 (VCT) describes the voltage output of a center tapped transformer. For example: A 24VCT 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 12VAC 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, a center-tapped transformer and two diodes 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, but has poorer utilization of the transformer windings. Center-tapped two-diode rectifiers were a common feature of power supplies in vacuum tube 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 chargers, where the extra transformer cost is offset by less costly rectifiers.
In an audio power amplifier center-tapped transformers are used to drive push-pull output stages. This allows two devices operating in Class B to combine their output to produce higher audio power with relatively low distortion. Design of such audio output transformers must tolerate a small amount of direct current that may pass through the winding.
Hundreds of millions of pocket-size transistor radios used this form of amplifier since the required transformers were very small and the design saved the extra cost and bulk of an output coupling capacitor that would be required for an output-transformerless design. However, since low-distortion high-power transformers are costly and heavy, most consumer audio products now use a transformerless output 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.
In electronic amplifiers, a center-tapped transformer is used as a phase splitter in coupling different stages of an amplifier.
Power distribution, see 3 wire single phase.
A Centre tapped rectifier is preferred to the full bridge rectifier when the output dc current is high and the output voltage is low.
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