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Tuesday, January 26, 2010

Telephone line

A telephone line is usually connected to the telephone exchange by about three miles(4.83 km) of a twisted pair of 0.54mm copper wires known by phone company as "the loop". Although a copper is a good conductor of, it does have resistance. the resistance of No 22 AWG wire is 16.46 ohms over 1000 ft at 77 degrees F(25 degree C). Because telephone apparatus is generally considered to be current driven, all phone measurements refer to current consumption not voltage. the length of the wire connecting the subscriber to the telephone exchange effects the total amount of the current that can be drawn by anything at subscribers end of line.

When the telephone is in on-hook stage the "TIP" is about 0 Volt, while the "RING" is about -28 Volt with respect to ground. when in offf-hook state and current is drawn, TIP goes negative and RING goes positive. A typical off-hook condition is TIP is about -20 V and RING is about 28 V. This means there are about 8 V voltage between the wires going to telephone in normal operation condition. The DC resistance of typical telephone equipment is about 200-300 ohms range and current flowing through the telephone is in 20-50 mA range.

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