Transformer Turns Ratio

In an ideal transformer, the same changing magnetic flux links both coils, so the induced voltage in each winding is proportional to how many turns it has. That is why the ratio of primary to secondary voltage matches the ratio of primary to secondary turns. In practice this is a very good approximation when losses are small and the transformer is operating normally with alternating current.

Use this relation when you know a transformer’s turns on each winding and want to find the output voltage, or when you know the voltages and need the turns ratio. It is commonly used in power supplies, adapters, and step-up/step-down transformer problems in circuits and physics.

• Swapping primary and secondary values in the ratio
• Using the formula for DC circuits instead of AC transformer operation
• Forgetting that the ratio applies to an ideal or nearly ideal transformer, so real devices may differ slightly