Isolation transformer
Encyclopedia
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 together.
Suitably designed isolation transformers block interference caused by ground loop
s. 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 wart
s", 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.
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 boat
s 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.
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
used to transfer electrical power from a source of alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
(AC) power to some equipment or device while isolating the powered device from the power source, usually for safety. Isolation transformers provide galvanic isolation
Galvanic isolation
Galvanic isolation is a principle of isolating functional sections of electrical systems, thus preventing the movement of charge-carrying particles from one section to another, i.e. no direct current flows between the sections. Energy or information can still be exchanged between the sections by...
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 together.
Suitably designed isolation transformers block interference caused by ground loop
Ground loop (electricity)
In an electrical system, a ground loop usually refers to a current, almost always unwanted, in a conductor connecting two points that are supposed to be at the same potential, often ground, but are actually at different potentials. Ground loops created by improperly designed or improperly installed...
s. 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
Induction
-General use:* Induction , induction of childbirth* Rite of passage** Introduction of an individual into a body such as the armed forces** Formal introduction of a priest into possession of the position to which she or he has been presented and instituted...
.
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
Autotransformer
An autotransformer is an electrical transformer with only one winding. The auto prefix refers to the single coil acting on itself rather than any automatic mechanism. In an autotransformer portions of the same winding act as both the primary and secondary. The winding has at least three taps where...
(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 wart
Wall wart
The AC adapter, AC/DC adapter or AC/DC converter is a type of external power supply, often enclosed in what looks like an over-sized AC plug. Other names include plug pack, plug-in adapter, adapter block, domestic mains adapter, line power adapter, or power adapter...
s", are not usually described specifically as "isolation transformers".
Some very small transformers—e.g. 4 transformers in one tiny dual in-line (DIP) chip
Dual in-line package
In microelectronics, a dual in-line package is an electronic device package with a rectangular housing and two parallel rows of electrical connecting pins. The package may be through-hole mounted to a printed circuit board or inserted in a socket.A DIP is usually referred to as a DIPn, where n is...
package—used to isolate high-frequency low-voltage (logic) pulse
Pulse
In medicine, one's pulse represents the tactile arterial palpation of the heartbeat by trained fingertips. The pulse may be palpated in any place that allows an artery to be compressed against a bone, such as at the neck , at the wrist , behind the knee , on the inside of the elbow , and near the...
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
Capacitive coupling
In electronics, capacitive coupling is the transfer of energy within an electrical network by means of the capacitance between circuit nodes. This coupling can have an intentional or accidental effect...
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
Electronic filter
Electronic filters are electronic circuits which perform signal processing functions, specifically to remove unwanted frequency components from the signal, to enhance wanted ones, or both...
if a problem.
Applications
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 testDevice under test
Device under test , also known as unit under test , is a term commonly used to refer to a manufactured product undergoing testing.-In semiconductor testing:...
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
Austin transformer
An Austin transformer is a special type of an Isolation transformer used for feeding the air-traffic obstacle lamps and other devices on a mast radiator antenna insulated from ground. As the electrical potential difference between the antenna and ground is high , feeding the lamps directly is...
for the power supply of air-traffic obstacle warning lamps on radio antenna masts
Mast radiator
A mast radiator is a radio mast or tower in which the whole structure itself functions as an antenna. This design is commonly used for transmitting antennas operating at low frequencies, in the VLF, LF and MF ranges, in particular those used for AM broadcasting. The metal mast is electrically...
. Without the isolation transformer, the lighting circuits on the mast would conduct radio-frequency energy to ground through the power supply.
Metal boat
Boat
A boat is a watercraft of any size designed to float or plane, to provide passage across water. Usually this water will be inland or in protected coastal areas. However, boats such as the whaleboat were designed to be operated from a ship in an offshore environment. In naval terms, a boat is a...
s are subject to corrosion
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
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.
See also
- Austin transformerAustin transformerAn Austin transformer is a special type of an Isolation transformer used for feeding the air-traffic obstacle lamps and other devices on a mast radiator antenna insulated from ground. As the electrical potential difference between the antenna and ground is high , feeding the lamps directly is...
- BalunBalunA balun is a type of electrical transformer that can convert electrical signals that are balanced about ground to signals that are unbalanced , and the reverse. They are also often used to connect lines of differing impedance...
- Galvanic isolationGalvanic isolationGalvanic isolation is a principle of isolating functional sections of electrical systems, thus preventing the movement of charge-carrying particles from one section to another, i.e. no direct current flows between the sections. Energy or information can still be exchanged between the sections by...
- Transformer typesTransformer typesA variety of types of electrical transformer are made for different purposes. Despite their design differences, the various types employ the same basic principle as discovered in 1831 by Michael Faraday, and share several key functional parts.-Laminated core:...
- Power qualityPower qualityPower quality is the set of limits of electrical properties that allows electrical systems to function in their intended manner without significant loss of performance or life. The term is used to describe electric power that drives an electrical load and the load's ability to function properly...