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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.
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.
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.
In electronics testing and servicing an isolation transformer is a 1:1 (under load) power transformer used for safety. Without it, exposed live metal in a device under test is at a hazardous voltage relative to grounded objects such as a heating radiator or oscilloscope ground lead (a particular hazard with some old vacuum-tube equipment with live chassis). With the transformer, as there is no conductive connection between transformer secondary and earth, there is no danger in touching a live part of the circuit while another part of the body is earthed.
Electrical isolation is considered to be particularly important on medical equipment, and special standards apply. Often the system must additionally be designed so that fault conditions do not interrupt power, but generate a warning.
Isolation transformers are also used for the power supply of devices not at ground potential. An example is the Austin transformer for the power supply of air-traffic obstacle warning lamps on radio antenna masts. Without the isolation transformer, the lighting circuits on the mast would conduct radio-frequency energy to ground through the power supply.
Metal boats are subject to corrosion if they use earthed power from shore when moored, due to galvanic currents that flow through the water between shore earth and the hull. This can be avoided by using an isolation transformer with the primary and case connected to shore earth, and the secondary "floating". A metal safety screen between primary and secondary is connected to shore earth; in the event of a fault current in the primary (due, e.g., to insulation breakdown) it will cause the fault current to return and trip a shore-based circuit breaker rather than making the hull live.
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