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A line isolation transformer for telephone use is a special type of
impedance matching In electronics, impedance matching is the practice of designing or adjusting the input impedance or output impedance of an electrical device for a desired value. Often, the desired value is selected to maximize power transfer or minimize signal ...
transformer that is used to couple
telephone A telephone is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most efficiently the human voice, into e ...
land lines to subscriber equipment to prevent dangerous common mode voltages from appearing on the line. It has a specific 600Ω
input impedance The input impedance of an electrical network is the measure of the opposition to current ( impedance), both static ( resistance) and dynamic ( reactance), into the load network that is ''external'' to the electrical source. The input admittance (the ...
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
transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
s. 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

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Isolation transformer 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 reasons or to re ...


References

Electric transformers Telephony equipment