Isolation Transformers

Electrical isolationis necessary to protect: –

  • Circuits
  • Equipment
  • People from shocks and short circuits
  • To make accurate measurements.

Isolation transformersare used to transfer electrical power from a source of alternating current power to a device, where the powered device is isolated from the power source for safety measures.

An isolation transformer allows an AC signal or power to be taken from one device and fed into another without electrically connecting the two circuits. Isolation transformers block transmission of DC signals from one circuit to the other, but allow AC signals to pass. They also 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.

They provide galvanic isolation, which is the principle of isolating different sections of electrical systems to prevent current flow. There is no direct conduction path but, the energy is still exchanged between sections by capacitance, induction or electromagnetic waves. However, these transformers block the transmission of the DC component in signals from one circuit to another, while letting the AC components to pass.

Isolation transformers that have a ratio of 1:1 between the primary and secondary windings are used:-

  • Toprotectagainstelectric shock between the ground and energized conductors.
  • to suppress electrical noise
  • For supplying power in sensitive devices like computers, medical devices and laboratory instruments.

There is special insulation between the primary and secondary in such transformers, and high voltage ranging from 1000 to 4000 volts can be borne between the windings.

When equipment under test is powered by an isolating transformer, its ground (equipment ground) is separated from earth ground; the transformer isolates the device under test from the common supply ground.  A person working on the equipment (standing at earth ground) can’t accidentally provide a path to ground should they make contact with the circuit. This makes the set up safer for users, eliminating the possibility of a shock. Should they touch a live part of the circuit by accident, there’s no conductive connection to earth ground.

Sometimes the term “isolation transformer” is applied to transformers that isolate AC spikes, transients and noise, but maintain an earth ground connection. This type of transformer would not provide electrical isolation. If the transformer has inter-winding screen, the screen should be connected to earth ground; also it’s common practice to connect the transformer frame to earth ground as well.

Isolation Transformer Construction
The schematic representation shows the primary and secondary coils; the electric source is connected to the primary, the isolated output is taken from the secondary. The coils are physically separate from each other and the core.

Michael Faraday first used an early transformer during his experiments investigating electromagnetism.

Faraday found that a wire carrying a current induces a magnetic field surrounding the wire and that when two separate wires were coiled around a toroid of soft iron, a current in one induced a magnetic field, and the changing flux in turn induced a voltage in the other.  Now known as mutual induction, Faraday is credited with discovering that an electromotive force is induced in a circuit by a changing magnetic flux according to the formula:


The negative indicating the electromotive force opposes the current.

Sometimes this is shown using the absolute value of E:


Because Faraday was working with DC voltage, he only saw the effect of electromagnetic induction when a battery was initially connected or disconnected to the circuits, when the magnetic flux was changing.

  • With AC power connected to the primary, the varying current creates a varying magnetic field, the magnetic flux is realized in the core, and that in turn induces a voltage the secondary, with no electrical path between the two coils.
  • The inductive coupling provided by the changing magnetic flux between the two coils allows communication across transformer.
  • The magnetic field induced by a transformer depends on the number of turns/unit length of the windings, the permittivity of the magnetic core, and the current magnitude.

The first commercially viable transformer was invented by William Stanley, working for George Westinghouse in the 1880s.

Benefits of isolation transformers

  • Isolation transformers can separate the primary and secondary windings.
  • The separation of direct current components from the signal to control every amplifier on the line.
  • Prevent the risk of electric shock by uniting a vessel to the electric power source.
  • Separation of the person from the resource in a manner that the electric wirings do not touch the power line directly.
  • To provide safety.
  • All kind of noise and sound that is created by connecting the signal of the audio amplifier to the speaker output circuit is reduced with the help of isolation transformers.

Separate the amount generated by a radio frequency on large devices of the circuit from the transmitter line.

Isolation transformers uses

  • In small sizes for isolation in pulse circuits.
  • To provide electrical isolation in medical equipment.
  • For power supply of devices that are not at ground potential.
  • In electronics testing and servicing to provide safety, without which touching a live part of the circuit with hazardous voltage can cause severe damage.
  • Protect users from faulty equipment
  • Enable safe and accurate measurements
  • Avoid ground loops
  • Physically separate one part of an electrical system from another


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