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Telephone company circuits
November 30th, 2009 by admin
To ring your telephone, the telephone company momentarily applies a 90 VRMS 20 Hz AC signal to the line. The remaining voltage drop occurs over the copper wire path and over the telephone company circuits where there is usually 200 to 400 ohms of series resistance to protect from short circuits and decouple the audiocircuits. An off-hook telephone typically draws about 20 milliamps of DC current to operate, at a DC resistance around 180 ohms. The POTS phone line, with all phones on-hook reverse cell phone directory, should measure around 48 volts DC. This is a closed loop, balanced system not referenced to earth ground. These two wires provide: DC current to power the telephone electronics, AC current to ring the telephone bell or electronic ringer, full duplex balanced voice path. The POTS line consists of two wires called tip and ring.