Transformers are electromagnetic devices which transform alternating current (AC) electrical energy from primary to secondary side. The energy is transformed with equal frequency and approximately equal power by means of the transformer core magnetic field. Thus they provide galvanic isolation in the electrical system. The isolation transformers operate in the same way as other transformer types. But the main task is to provide the galvanic isolation in the electrical system. They can work as step-up transformer or step-down transformers but often operate with turns ratio . This means that the primary and secondary voltage values are equal. This is obtained with an same number of turns on the primary and secondary windings.
The isolation transformers are used in many electrical devices as computers, measurement devices or specific industry power electronic devices. It is very important to use isolating transformers when an oscilloscope measures signals in an electrical circuit which is not galvanically isolated from the network. Because the current circuit can be closed (short-circuited) between oscilloscope common point and grounding. The main purpose of the isolation transformer is safety and protection of electronic components and the persons against electrical shock. It physically separates the power supplying from primary side and a secondary side circuit connected to electronic components and grounded metal parts which are in contact with the person. Basically, the transformer secondary side is isolated from the grounding.
This means that the isolation transformer secondary side must not be grounded. It would create a physical connection between the primary and secondary transformer side. The auto transformer with common winding can not be used as isolation transformer because it has a connection between primary and secondary side. Isolation transformer provides available supplying even if the device is broken. The primary side remains under voltage which can be used to supply some alarm or warning beep circuits when the device is broken. The transformers suppress the electrical noise from supplying or electromagnetic induction. That is very important incase of sensitive devices as measurement or medical devices. This transformer is built with electrostatic shields which additionally increase the electrical noise suppression. The proper isolation transformer design avoids ground loops. Ground loops create an additional current path where the current created by electromagnetic induction can flow. This is the main reason for noise and interference in the signal.
When the isolation transformer is designed it is very important to pay attention to windings capacitance values which create capacitive coupling. This enables AC signal to pass from primary to the secondary side which significantly increased the noise level. For this purpose, the windings are surrounded by a metal strip which is grounded (creating a Faraday shield). The isolation transformers are used as instrument transformers when the high voltage should be measured. The high voltage is dangerous for the person who tries to measure high voltage but it can also harm the measurement circuits. In this case, the step-down isolation transformer is used to reduce the high voltage to the safe level and for measurement range.