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Calculate transformer secondary voltage, current, turns ratio, and power transfer. Essential for power supply design, electrical engineering, audio transformers, and understanding how transformers step up or step down voltage.
Transformers have been the backbone of electrical power distribution for over 130 years. From the massive 500kV power transformers at transmission substations to the tiny 5V phone charger transformer on your desk, the fundamental physics is identical — electromagnetic induction transfers energy between windings with remarkable efficiency.
A transformer transfers electrical energy between two circuits through electromagnetic induction. It consists of two coils (primary and secondary) wound on a shared magnetic core. Transformers can step up or step down voltage while conserving power (in ideal transformers). They are fundamental to power distribution, electronics power supplies, and audio equipment.
The turns ratio (Np:Ns) determines voltage transformation: Vs/Vp = Ns/Np. A step-down transformer with 10:1 turns ratio converts 230V to 23V. A step-up transformer with 1:10 ratio converts 120V to 1,200V. The turns ratio is the most fundamental transformer specification.
Current transforms inversely to voltage: Is/Ip = Np/Ns. A step-down transformer that halves voltage doubles current. This is why power lines use high voltage (low current) for long-distance transmission — lower current means less I²R resistive losses in cables.
| Application | Primary V | Secondary V | Turns Ratio | Current Ratio |
|---|---|---|---|---|
| Wall adapter (5V/2A) | 230V | 5V | 46:1 | 0.043A |
| Audio output transformer | 120V | 8Ω load equiv. | — | Impedance matching |
| Distribution transformer | 11,000V | 230V | 47.8:1 | ×47.8 |
| Microwave oven | 120V | 2,000V | 1:16.7 | ×16.7 |
| Welding transformer | 240V | 40V | 6:1 | ×6 |
Transformers are critical components in power supplies, audio equipment, and electrical distribution. Selecting the right transformer requires understanding VA rating, regulation, core material, and frequency. A mismatch can cause overheating, excessive voltage drop under load, or audio hum.
Transformer capacity is rated in VA (volt-amperes), not watts. A 100VA transformer delivers 100VA regardless of load power factor. For resistive loads (VA = W), 100VA = 100W capacity. For a 12V secondary, 100VA = 12V × 8.33A maximum current. Always size the transformer at 1.2-1.5× your expected load to account for regulation and inrush current.
Voltage regulation = (Vno-load - Vfull-load) / Vfull-load × 100%. A poorly regulated transformer may show 14V with no load but drop to 11V under full load. Regulation depends on winding resistance and leakage inductance. High-quality toroidal transformers: 3-5% regulation. Standard EI-core: 8-12%. For precision power supplies, use a voltage regulator after the transformer.
Toroidal transformers (donut-shaped core) have 50% less stray magnetic field, lower audible hum, higher efficiency (98% vs 95%), and better regulation than EI-core (stacked lamination) transformers. EI-core is cheaper to manufacture. Toroidal is preferred for audio amplifiers, medical equipment, and noise-sensitive applications. Both work at 50/60 Hz mains frequency.