Transformer Calculator

Transformer voltage conversion is governed by the turns ratio, meaning the ratio of wire coil windings on the primary (input) side to the secondary (output) side.
Transformers move energy between two circuits through electromagnetic induction, with no electrical connection between them and very little 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) is the input, typically grid voltage: 120V or 230V AC
• Secondary voltage (V₂) is the output, the voltage the connected device needs
• VA rating is the transformer’s apparent power capacity in volt-amperes, and it 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.