A line isolation transformer for telephone use is a special type of
impedance matching transformer that is used to couple
telephone land lines to subscriber equipment to prevent dangerous common mode voltages from appearing on the line. It has a specific 600Ω
input impedance which matches the telephone equipment.
A "wet" type will maintain its characteristics while passing the DC current needed to seize the telephone line (signal off-hook to the central office). It is also made with a gapped
magnetic core
A magnetic core is a piece of magnetic material with a high magnetic permeability used to confine and guide magnetic fields in electrical, electromechanical and magnetic devices such as electromagnets, transformers, electric motors, generators, in ...
that is not used in normal
transformers. A "dry" isolation transformer need not carry DC current and an alternative DC path would be provided.
''AN-5 TRANSFORMER-BASED PHONE LINE INTERFACES (DAA, FXO)''
by Joe Randolph, retrieved 2018 March 20
See also
* Isolation transformer
References
Electric transformers
Telephony equipment