Center tap

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 Hartley 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

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, 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. 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 path around an AC coupled amplifier for signalling purposes.

• Power distribution, see 3 wire single phase.

• 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 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".^[1]

See also

• Diode bridge.

References