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What Is a Center Tap?

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  • Written By: Geisha A. Legazpi
  • Edited By: Lauren Fritsky
  • Last Modified Date: 13 September 2016
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    Conjecture Corporation
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A center tap is usually a soldered electrical connection inside the transformer and at the midpoint of a winding. Electrical transformers usually have a single primary winding and most transformers have a single secondary winding. A connection may be made at the primary or at the secondary winding depending on the circuit configuration.

Like power transformers, the power inductor is usually wound on a bobbin installed on an E-I core made of laminated mild steel. The E-I core is a type of laminated transformer core that uses meshed layers of thin metal insulated sheets in the shape of E's and I's and is made of several laminates to reduce current flows internal to the core, which reduces efficiency. The center tap on transformers and inductors may be used to provide half voltages and half inductances for power and audio electronics, among others. In electronics, alternating current (AC) is usually converted to direct current (DC) using single-diode half-wave rectifiers or full-wave rectifiers, which could be a diode bridge or a center-tapped transformer output with dual-diode rectifiers. Depending on the isolation requirements, a transformer may or may not be used.

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In electrical circuits, the center tap is commonly used for splitting voltages and for neutral-ground points. For instance, some portable and transportable power generators may have an AC voltage output of 220 volts alternating current (VAC) instead of 110 VAC. If the 220 VAC output winding has a center tap, there will be two 110 VAC outputs available. The center tap can then be connected as neutral and grounded at the breaker panel.

The resistor is a component that is able to split voltages and power to a certain limit. A higher resistance will allow less current for the same voltage. The potentiometer is a special resistor that has two ends and a tap point in the form of a sliding contact. By moving the sliding contact by linear or angular displacement, a different resistance ratio is achieved between the two ends and the tap. At midpoint, the tap becomes a center tap and can be used for adjustable electronic equipment such as volume control for audio and voltage set for regulated power supplies.

For signal drivers and distribution equipment, the center tap is used to maintain signal-to-ground voltages at safe levels. Without it, the output of a secondary transformer may have predictable common-mode output. The latter is the signal level between the ends of the secondary winding. Without a connection, however, the signal-to-ground level may vary depending on external electromagnetic energy. To prevent possible damage, the midpoint of the secondary winding is grounded, and any extra energy that may interfere with the signal level is dissipated into the ground via the center tap.

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