What is an Isolation Transformer?
About Transformer (for a Laymen):
To understand the Isolation Transformer, you must know what is a transformer. A transformer is a device that converts one voltage to another. There are different types of transformers.
Some transformers have single coil with various fixed or movable tappings. These are non-isolated transfomers. They are generally called variac or auto-transformer.
Such non-isolated transformers are becoming rare these days. They are generally used within other electrical equipment like voltage regulators or voltage stabilizers.
And there is another type of transformer that contains two coils. Both these coils are isolated from each other by non-conductiong material called lamination. One coil functions as input (or primary) and the other coil functions as output (secondary). Usually there will be no direct electrical path (connection) between the two coils. The primary coil of a transformer converts (or transforms) the electrical power into magnetic field and the secondary coil in turn converts this magnetic field (inducted by primary coil) into electrical power.
Usually most of these transformers are designed in such a way that both the windings are wound on top of each other with insulation (called lamination) in between.
Technically speaking any transformer with two coils that are not directly connected through a conductor (wire) is an Isolation Transformer.
About Isolation Transformer
However, the term "Isolation Transformer" is used to differentiate the special type transformers that are used to protect electrical equipment from spikes and surges. The term "isolation" is generally used to specify that the transformer is used for isolation of electricity between the input and output of the transformer.
The devices that are known as Isolation Transformers, use separate bobbins for the primary and secondary coil windings. The secondary winding is kept isolated (floating) from the primary.
Ultra Isolation Transformer
Even when the primary and secondary coils are separated, the differential noise can magnetically couple from the primary to the secondary of an isolation transformer. The excessive capacitance could couple AC current from the primary to the secondary.
To prevent this, an electrostatic shield (also known as Faraday Shield) is interposed between the primary and the secondary. This reduces the coupling of common-mode noise. Such isolation transformers are called Ultra Isolation Transformers or Shielded Isolation Transformers.
The shield that isolates the primary and secondary is also connected to earth ground. This performs two functions:
- Filters or attenuates voltage transients (spikes or surges, i.e.
thousands of volts supplied on the primary side fo the transformer)
- Filters common mode noise (an attenuation of upto 30 decibels)
The secondary of an isolation transformer is usually kept floating from earth ground to avoid the earth bondage. Practically, the input phase, neutral and earth are not at all connected to the output (completely isolated).
The isolation transformers used for commercial purposes have one leg of the secondary coil connected to the ground. This forms a new neutral that eliminates common mode noise and differential ground noise.
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.
When more than one shielded isolation transformer is installed between the source and the load, this is referred to as "cascading" and vastly improves power quality.
The Ultra Isolation transformers with electrostatic shields are used to protect sophisticated/expensive electrical or electronic equipment. Isolation transformers are also used in the medical industry, to protect patients hooked up to monitoring instruments that are powered by utility mains.
By using an isolation transformer, the shock hazard (due to differential ground noise) is almost completely prevented.
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