Center-Tap Transformer Symbol
Definition: The Center-Tap Transformer symbol represents a transformer whose secondary winding has an additional connection brought out at its electrical midpoint, splitting the secondary into two equal, series-connected halves whose voltages are equal in magnitude and 180° opposed about the tap, drawn per IEC 60617 and ANSI Y32.2 / IEEE 315 as two windings with core lines and a third lead leaving the middle of the secondary.
Also known as: CT transformer, centre-tapped transformer, center-tapped transformer, 12-0-12 transformer, split-secondary transformer, dual-rail transformer, tapped secondary transformer.
What the Center-Tap Transformer symbol means
The Center-Tap Transformer symbol denotes a five-terminal transformer: a primary (Primary 1, Primary 2) magnetically coupled through a core to a secondary that presents three connections — Secondary 1, Center Tap, Secondary 2. Because the tap sits at the winding's electrical midpoint, the voltage from Secondary 1 to the tap equals the voltage from the tap to Secondary 2, and the two half-voltages are in antiphase (180° apart) when measured with the tap as reference. A '12-0-12' transformer, for example, delivers 12 V AC from each end to the tap, and 24 V AC end-to-end.
This one extra lead unlocks two classic circuits. First, the two-diode full-wave rectifier: with the tap grounded, each half of the secondary conducts through its own diode on alternate half-cycles, giving full-wave DC with only two diodes and one diode drop of loss. Second, the split (dual-rail) supply: rectifying both halves about the grounded tap yields symmetrical positive and negative rails (±12 V, ±15 V) for op-amps and audio amplifiers. Center taps also serve push-pull output stages, phase inverters, and — on the primary side — dual-voltage 120/240 V input selection.
How to identify the Center-Tap Transformer symbol
Start from the standard transformer symbol — two winding symbols facing each other across two parallel core lines (iron core) — then look for the third lead leaving the MIDDLE of one winding, usually the secondary. That midpoint lead is the center tap, often labeled CT, 0 V, or COM. Windings are drawn as a row of loops/humps (ANSI style) or as IEC's solid semicircular bumps; the two parallel lines between primary and secondary indicate a ferromagnetic core, and their absence indicates an air core.
IEC 60617 and ANSI Y32.2 / IEEE 315 differ mainly in winding style — IEC's smooth bumps versus the loopier ANSI coil — while the tap is drawn identically in both: a lead joining the winding midway. Phasing dots at the winding ends, where shown, indicate relative polarity: with the dot end of one half positive, the corresponding half-voltages measured from the tap are in antiphase, which is what the two-diode rectifier depends on.
Function in a circuit
The transformer transfers energy magnetically from primary to secondary with voltage scaled by the turns ratio; the tap merely provides mid-winding access. In the two-diode full-wave rectifier, the anodes of two diodes connect to Secondary 1 and Secondary 2, the cathodes join at the DC output, and the Center Tap is the return: on each half-cycle, whichever end is positive with respect to the tap conducts, so load current flows on both half-cycles at half the total secondary voltage. Each diode must withstand a peak inverse voltage of approximately TWICE the peak of one half-winding (both halves in series appear across the blocking diode).
For a dual-rail supply, a bridge rectifier across the full secondary with the tap grounded produces balanced positive and negative outputs. In push-pull audio and inverter service, the tap carries the DC feed while the two winding ends are driven (or driven from) alternately, each half handling one polarity of the waveform.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 defines transformer symbols as winding pairs with optional core lines; a tapped winding is drawn with the tap lead joining the winding symbol at its intermediate point. Reference designator T per IEC 81346-2. Power-transformer construction and safety are governed by IEC 61558 for supply transformers. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315 draws windings as coil loops with two parallel core lines for iron cores and phasing dots for polarity; the center tap is a lead from the winding midpoint. Reference designator T. UL 5085 / CSA class standards govern low-voltage supply transformers in North America. |
| Key difference | The graphical difference is winding style — IEC's smooth humps versus ANSI's loops — with tap and core notation effectively identical. Ratings language differs regionally: a European part sold as 12-0-12 (or 2×12 V) at 230 V primary corresponds to a North American 24 V CT part at 120 V primary; both describe the same five-terminal device. |
Terminals / pins
| Pin | Name |
|---|---|
| p1 | Primary 1 |
| p2 | Primary 2 |
| s1 | Secondary 1 |
| ct | Center Tap |
| s2 | Secondary 2 |
Typical values
Common small-power ratings: secondaries of 6-0-6, 9-0-9, 12-0-12, 15-0-15, 18-0-18, and 24-0-24 V AC at 0.5–10 A, with VA ratings from 5 VA (PCB mount) through 50–300 VA (chassis toroids) and up into kVA for power service. Primary voltages 120 V, 230 V, or dual 115/230 V. Regulation of small EI transformers runs 10–20% no-load to full-load. Rectifier design points: DC output of a two-diode full-wave stage ≈ 0.9 × Vrms(half) minus one diode drop before filtering; each diode's PIV requirement ≈ 2 × Vpeak(half) ≈ 2.83 × Vrms(half).
Where the Center-Tap Transformer symbol is used
- Two-diode full-wave rectifier power supplies, the classic pre-bridge-era rectification topology still used where diode drops must be minimized
- Split-rail (±12 V, ±15 V) linear supplies for op-amp, preamplifier, and audio power-amplifier circuits
- Push-pull audio output stages and vacuum-tube amplifiers, with the tap feeding B+ and the ends driving the tubes/transistors
- DC-AC inverters and DC-DC converters using push-pull switching into a center-tapped primary
- Phase-splitting and balanced (differential) signal drive, exploiting the antiphase half-windings
- North American 120/240 V split-phase utility service — the pole transformer's center-tapped secondary grounded as the neutral
Example
In a ±12 V bench-supply schematic, mains feeds the Primary 1 and Primary 2 pins of a 25 VA 12-0-12 transformer; Secondary 1 and Secondary 2 drive a bridge rectifier while the Center Tap pin is grounded as the 0 V rail, so the two filter capacitors charge to about +17 V and −17 V unregulated, which 7812 and 7912 regulators trim to clean ±12 V rails for the op-amp circuitry.
Key facts
- The center tap is the electrical midpoint of the winding: each half delivers equal voltage, and the two halves are 180° out of phase measured from the tap.
- A 12-0-12 transformer gives 12 V AC from each end to the tap and 24 V AC end to end — the naming states the two half-voltages about the 0 V tap.
- The two-diode full-wave rectifier needs a center tap; it wastes only one diode drop but each diode must block a PIV of about twice the half-winding peak (≈ 2.83 × Vrms of one half).
- Grounding the tap and rectifying both halves produces symmetrical dual rails (±V) — the standard architecture of linear op-amp and audio supplies.
- Each half-winding carries current only half the time in rectifier service, so a CT secondary is used less efficiently (copper utilization ≈ 0.57) than the same winding feeding a bridge.
- Phasing dots on the symbol show relative winding polarity; correct phasing is what makes the half-voltages antiphase and the rectifier full-wave.
- North American residential 120/240 V split-phase service is a center-tapped transformer secondary with the tap grounded as neutral.
- Reference designator is T in both IEC 81346-2 and ANSI/IEEE practice; supply-transformer safety standards are IEC 61558 and UL 5085.
Frequently asked questions
What does 12-0-12 mean on a transformer?
It describes a center-tapped secondary: 12 V AC between one end and the center tap, 12 V AC between the tap and the other end, hence 24 V AC across the whole winding. The middle 0 is the tap, normally used as the ground/common reference. The same part may be marked 24 V CT or 2×12 V depending on region and maker — all describe the identical five-terminal device.
How does a center-tap transformer make a full-wave rectifier with only two diodes?
Ground the tap and connect one diode from each secondary end to the common DC output. On one half-cycle, Secondary 1 is positive relative to the tap and its diode conducts; on the next, Secondary 2 is positive and its diode conducts. The load therefore receives current on both half-cycles — full-wave rectification — at the voltage of ONE half-winding, losing only a single diode drop per conduction path (versus two drops in a bridge).
Why do the diodes in a center-tap rectifier need double the PIV?
When one diode conducts, the full end-to-end secondary voltage appears across the OTHER, blocking diode: the conducting half pulls the output to +Vpeak while the blocking diode's anode sits at −Vpeak of its own half, stacking the two halves in series. Each diode must therefore withstand roughly 2 × Vpeak of one half-winding — about 2.83 × the half-winding RMS voltage — plus a safety margin.
How do I get a positive and negative supply from a center-tap transformer?
Ground the center tap and rectify the full secondary with a bridge (or two-diode pairs): the bridge's + output charges a capacitor positive with respect to the grounded tap, and its − output charges another capacitor negative. A 15-0-15 transformer yields about ±21 V unregulated peaks, which 7815/7915-style regulators turn into clean ±15 V rails — the standard supply for op-amp and audio circuitry.
Center-tap rectifier or bridge rectifier — which is better?
The bridge wins on transformer utilization (the whole secondary works on every half-cycle, no tap needed) and diode PIV (each diode blocks only one winding's peak), at the cost of two diode drops in series. The center-tap circuit wins where every diode drop matters — low-voltage, high-current, or high-efficiency outputs (it is universal in switch-mode secondary rectification with Schottky diodes) — at the cost of a tapped winding, poorer copper utilization, and double-PIV diodes.
Can I use the center tap to get half the voltage for another circuit?
Yes — tap-to-end gives half the full secondary voltage, and drawing a modest load from one half is common (for example, a 12 V relay supply from one half of a 12-0-12 winding that mainly serves a ±12 V supply). Keep the loading reasonably balanced between halves where possible: heavily loading only one half magnetizes the core asymmetrically in rectifier service and worsens regulation on that half.
Related symbols
- Autotransformer symbol
- Full-Wave Rectifier symbol
- Isolation Transformer symbol
- Low Voltage Transformer symbol
- Transformer symbol
- Variable Transformer (Variac) symbol
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