center tap

A center tap is a wire lead brought out from the exact middle of a transformer or inductor winding, creating two equal halves. This contact sits between the full winding and ground or return, allowing the device to work in two different configurations without rewinding.

The most common use is in power transformers serving dual-voltage or bridge rectifier circuits. A center-tapped secondary winding of, say, 240 volts total will provide 120 volts from either half to the center point. This symmetry is essential in full-wave rectifiers, where two diodes work from the two halves to deliver direct current with lower ripple than single diodes can achieve. Audio output transformers also use center taps to split signal evenly between push-pull tube or transistor stages.

The physical construction requires the winding wire to be brought out and secured at the exact middle layer or turn count. On smaller transformers this is straightforward; on large industrial units, the tap position must be calculated precisely to avoid impedance imbalance between the two halves. Any asymmetry increases losses and can cause unequal current sharing in bridge circuits, heating one rectifier diode more than the other.

When the tap fails

Poor electrical connection at the center tap is a common field failure. Vibration, thermal cycling, and corrosion weaken solder joints, especially in older transformers. A weak center tap creates measurable voltage imbalance: the two halves no longer look identical to their respective loads. In bridge rectifiers this causes one diode pair to carry excess current, shortening their life. Testing requires a precision ohmmeter to confirm continuity from the tap to the winding, and an AC voltmeter to check the symmetry of output during operation.

Center taps are marked on schematic drawings as a dot or line on the secondary coil symbol, and on transformer data sheets as a numbered lead. Not all transformers have them; many are designed for single polarity or fixed voltage output only. When an engineer selects a transformer, the presence and position of center taps directly influence circuit topology, so this detail appears early in the specification.