Transformer Calculator

Transformer voltage conversion is governed by the turns ratio — the ratio of wire coil windings on the primary (input) side to the secondary (output) side. Transformers transfer electrical energy between circuits through electromagnetic induction with minimal loss.

Turns ratio formula: Turns Ratio = N₁ ÷ N₂ = V₁ ÷ V₂

Where:

• N₁ = number of windings on the primary (input) coil
• N₂ = number of windings on the secondary (output) coil
• V₁ = primary (input) voltage
• V₂ = secondary (output) voltage

Solving for output voltage: V₂ = V₁ × (N₂ ÷ N₁)

Solving for turns ratio needed: N₁/N₂ = V₁ ÷ V₂

Current relationship (conservation of power): In an ideal transformer (100% efficient): P₁ = P₂ → V₁ × I₁ = V₂ × I₂

Therefore: I₂ = I₁ × (N₁ ÷ N₂)

When voltage steps up, current steps down proportionally. When voltage steps down, current steps up.

Real transformer efficiency: Efficiency (η) = (P₂ ÷ P₁) × 100% High-quality power transformers: 95–99% efficient. Small consumer transformers: 85–95%.

Step-up vs step-down:

• Step-up transformer: N₂ > N₁ → V₂ > V₁ (used in power transmission, radio transmitters)
• Step-down transformer: N₂ < N₁ → V₂ < V₁ (used in phone chargers, wall adapters, doorbells)

What each variable means:

• Primary voltage (V₁) — the input; typically grid voltage (120V or 230V AC)
• Secondary voltage (V₂) — the output; the target voltage for the connected device
• VA rating — apparent power capacity of the transformer in volt-amperes; must exceed the load’s wattage

Worked example: Step-down transformer: 230V primary, 12V secondary needed.

Turns ratio = 230 ÷ 12 = 19.17:1 (N₁/N₂)

If primary current = 0.5A: Primary power = 230 × 0.5 = 115W Secondary current = 0.5 × (230 ÷ 12) = 9.58A Secondary power = 12 × 9.58 = 114.96W (≈ 115W, ideal transformer)

For a 20W load at 12V: secondary current = 20 ÷ 12 = 1.67A → primary current = 1.67 × (12 ÷ 230) = 0.087A. Choose a transformer rated at minimum 25 VA for this load with safety margin.