An isolation transformer is a

transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...

used to transfer

electrical power Electric power is the rate at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt, one joule per second. Standard prefixes apply to watts as with other SI units: thousands, millions and billions of ...

from a source of

alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...

(AC) power to some equipment or device while isolating the powered device from the power source, usually for safety reasons or to reduce transients and harmonics. Isolation transformers provide

galvanic isolation Galvanic isolation is a principle of isolating functional sections of electrical systems to prevent current flow; no direct conduction path is permitted.John Huntington ''Show Networks and Control Systems: Formerly Control Systems for Live E ...

; no conductive path is present between source and load. This isolation is used to protect against

electric shock Electrical injury is a physiological reaction caused by electric current passing through the body. The injury depends on the density of the current, tissue resistance and duration of contact. Very small currents may be imperceptible or produce ...

, to suppress electrical noise in sensitive devices, or to transfer power between two circuits which must not be connected. A transformer sold for isolation is often built with special insulation between primary and secondary, and is specified to withstand a high voltage between windings. Isolation transformers block transmission of the DC component in signals from one circuit to the other, but allow AC components in signals to pass. Transformers that have a ratio of 1 to 1 between the primary and secondary windings are often used to protect secondary circuits and individuals from electrical shocks between energized conductors and earth ground. 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, medical devices, or laboratory instruments. Some specifications require that Isolation transformers be a part of the lightning protection on the AC circuits.

Terminology

Sometimes the term is used to emphasize that a device is not an

autotransformer An autotransformer is an electrical transformer with only one winding. The "auto" (Greek for "self") prefix refers to the single coil acting alone, not to any kind of automatic mechanism. In an autotransformer, portions of the same winding act as ...

whose primary and secondary circuits are connected.Website of typical electronics distributor
/ref> Power transformers with specified insulation between primary and secondary are not usually described only as "isolation transformers" unless this is their primary function. Only transformers whose ''primary'' purpose is to isolate circuits are routinely described as isolation transformers.

Operation

Isolation transformers are designed with attention to

capacitive coupling Capacitive coupling is the transfer of energy within an electrical network or between distant networks by means of displacement current between circuit(s) nodes, induced by the electric field. This coupling can have an intentional or accidental ...

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 A Faraday cage or Faraday shield is an enclosure used to block electromagnetic fields. A Faraday shield may be formed by a continuous covering of electrical conductor, conductive material, or in the case of a Faraday cage, by a mesh of such mat ...

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 occurs.

Applications

Pulse transformers

Some small transformers are used for isolation in

pulse In medicine, a pulse represents the tactile arterial palpation of the cardiac cycle (heartbeat) by trained fingertips. The pulse may be palpated in any place that allows an artery to be compressed near the surface of the body, such as at the nec ...

circuits.

Electronics testing

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 A device under test (DUT), also known as equipment under test (EUT) and unit under test (UUT), is a manufactured product undergoing testing, either at first manufacture or later during its life cycle as part of ongoing functional testing and calibra ...

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 primary, only a small leakage current will flow if the exposed live metal is connected to earth. 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.Hugh Nash et al., (ed), ''IEEE Recommended Practice for Electric Systems in Health Care Facilities, IEEE Standard 602-1996'',

Supply of equipment at elevated potentials

Isolation transformers are also used for the power supply of devices not at ground potential. An example is the

Austin transformer An Austin ring transformer is a special type of isolation transformer with low capacitance and high isolation between the primary and secondary. It is formed of two interlocking rings: one with the transformer core and primary windings, much like ...

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.

References

External links

Transformer Isolation
{{Electric transformers Electric transformers Electric power conversion Electrical safety