Isolation Transformer Transformation Ratio: Usually 1:1
A — 1 : 1. A usual isolation transformer has equal primary and secondary turns so that it transfers approximately the same AC voltage while providing galvanic separation between the circuits.
Apply the transformer voltage ratio
For an ideal transformer, V₂/V₁ = N₂/N₁. Equal turns give N₂ = N₁, hence V₂/V₁ = 1. For example, a 230 V primary would ideally produce 230 V at the secondary. A real transformer may show a small voltage difference because of its losses and voltage regulation.
What is achieved if the voltage stays the same?
The primary and secondary are separate insulated windings with no direct conductive connection through the transformer. Energy passes through their shared magnetic field by electromagnetic induction. This galvanic isolation is the useful function even when the voltage ratio is unity.
An autotransformer uses a common winding section and therefore does not provide the same primary-to-secondary galvanic separation. A 1:1 voltage ratio alone is not proof of isolation; the winding construction matters.
Why the question says “usually”
Electrical isolation and voltage conversion are different properties. An isolating two-winding transformer can also be designed to step voltage up or down. However, when an exam asks for the usual ratio of an isolation transformer without another rating, the conventional answer is 1:1.
Isolation does not make the secondary voltage zero or harmless. A hazardous voltage can still exist between the secondary terminals.
Useful Formula
Ideal transformer: V₂/V₁ = N₂/N₁. For equal turns, N₂ = N₁ and V₂/V₁ = 1.
Why the Other Options Are Not the Answer
- B — 1 : 10: is a non-unity transformation ratio, so it does not describe the usual same-voltage isolation transformer.
- C — 2 : 1: also changes voltage rather than giving the usual unity ratio.
- D — 10 : 1: is likewise a non-unity ratio. For non-unity ratios, step-up or step-down interpretation depends on which winding is written first.
Remember for the Exam
Select 1:1 when “usual isolation transformer” is asked. Separately check for electrically separate windings when galvanic isolation is the issue.
Common mistake: Do not assume that equal voltages mean the circuits are electrically connected, or that every transformer providing isolation must have a 1:1 ratio.
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