An isolation transformer is a transformer used to transfer electrical
power from a source of alternating current power to some equipment or
device while isolating the powered device from the power source, usually
for safety reasons. 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. 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. 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. Terminology
Sometimes the term is used to emphasize that a device is not an autotransformer
whose primary and secondary circuits are connected. 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 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. Applications
= Pulse transformers = Some small transformers are used for
isolation in pulse circuits. = Electronics Testing =
In electronics testing and servicing an isolation transformer is a 1:1 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. 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. = 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 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. = Supplying power to ships =
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 it will cause the fault current
to return and trip a shore-based circuit breaker rather than making the hull
live. References
^ a b Website of typical electronics distributor
^ Specification of a typical line of high-frequency electrically isolating
pulse transformers in 16-pin DIL package The manufacturer describes them as
"data-bus isolators", but some distributors describe them as isolation
transformers. ^ Hugh Nash et. al,, IEEE Recommended
Practice for Electric Systems in Health Care Facilities, IEEE Standard 602-1996,
ISBN 1-55937-772-0 ^ Boat installation, connected to shore
earth and secondary "floating" External links
Transformer Isolation See also
Austin transformer Balun
Galvanic isolation Transformer types
Power quality