Isolation Transformer Theory

Written by christopher donahue
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Electrical transformers have several useful properties. They allow the step-up or step-down of AC voltages and the circuits attached to the primary and secondary windings are DC-isolated. For isolation transformers, the second feature is the more important. By isolating the primary from secondary circuits, ground loop and other noise problems may be reduced or eliminated from high-performance audio circuits.

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What Is a Transformer?

In its basic form, a transformer is a primary winding (inductor) wrapped around a core with a secondary winding wrapped around the same core.

These insulated windings do not conduct DC current. An ohmmeter should show open (near-infinite resistance) between the primary and secondary windings.

When an AC signal is applied to the primary winding, the changing magnetic flux around the primary will cut through the secondary windings, inducing current in the secondary. This action allows the transfer of AC signals from the primary to the secondary circuit without a direct-current connection.

Value of Isolation

Because the primary circuit has no DC connection to the secondary circuit, ground loop noise between the primary and secondary circuits is eliminated. The secondary circuit is able to amplify, filter or otherwise process only the AC signals passed to it through the isolation transformer.

Step-Up and Step-Down Features

The ratio of windings (number of turns of the wire around the core) for the primary compared with the secondary allows a step-up or step-down action. If the primary is 100 windings and the secondary is also 100 windings, a 10 VAC signal applied to the primary will result in a (nearly) 10 VAC signal on the secondary, regardless of the DC voltage level of the primary.

If the primary is 100 windings, but the secondary is 200 windings, the same 10 VAC signal on the primary will give (nearly) 20 VAC on the secondary.

The difference between theoretical secondary and actual secondary results will be based on the tightness of the windings, the material of the transformer core and other factors, but the rough ratios will hold true.

Isolation Transformers

Most isolation transformers will be a one-to-one primary to secondary ratio because signal amplification is not the main value.

When mounting or replacing an isolation transformer, care must be taken to avoid any contact between the transformer case and the ground planes of the primary and secondary circuits. Contact between the usually metallic case of the transformer and both planes will create an electrical bridge and defeat the purpose of the isolation step.

Sources

Isolation transformers are manufactured or supplied by most upper-end audio equipment companies. These devices are chosen for their high isolation values and tight coupling for optimal transmission of undistorted signals between primary and secondary windings.

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