Isolation Transformer

An isolation transformer is a transformer used to transfer electrical power from a source of alternating current (AC) power to some equipment or device while isolating the powered device from the power source, usually for safety. Isolation transformers provide galvanic isolation and are used to protect against electric shock, to suppress electrical noise in sensitive devices, or to transfer power between two circuits which must not be connected. Isolation transformers block transmission of DC signals from one circuit to the other, but allow AC signals to pass.

Suitably designed isolation transformers block interference caused by ground loops. Isolation transformers with electrostatic shields are used for power supplies for sensitive equipment such as computers or laboratory instruments.

Strictly speaking any true transformer, whether used to transfer signals or power, is isolating, as the primary and secondary are not connected by conductors but only by induction.

However, only transformers whose primary purpose is to isolate circuits (opposed to the more common transformer function of voltage conversion), are routinely described as isolation transformers.

Given this function, a transformer sold for isolation is often built with special insulation between primary and secondary, and is tested, specified, and marked to withstand a high voltage between windings, typically in the 1000 to 5000 volt range.

Sometimes the term is exceptionally used to clarify that some transformer, although not primarily intended for isolation, is a true transformer rather than an autotransformer (whose primary and secondary are not isolated from each other). Even step-down power transformers required, amongst other things, to protect low-voltage equipment from mains voltage by isolating the secondary and primary such as are used in older "wall warts", are not usually described specifically as "isolation transformers".

Some very small transformers—e.g. 4 transformers in one tiny dual in-line (DIP) chip package—used to isolate high-frequency low-voltage (logic) pulse circuits (e.g., 500V RMS primary–secondary for one second), are described as isolation transformers

Isolation transformers are commonly designed with careful attention to capacitive coupling between the two windings. The capacitance between primary and secondary windings would also couple AC current from the primary to the secondary. A grounded Faraday shield between the primary and the secondary greatly reduces the coupling of common-mode noise. This may be another winding or a metal strip surrounding a winding.

Differential noise can magnetically couple from the primary to the secondary of an isolation transformer, and must be filtered out if a problem.