TELEPHONE LINES OVERVOLTAGE PROTECTION APPARATUS

摘要

Overvoltage protection circuits (10 or 50 or 52 or 54 or 56 or 58 or 60 or 62 or 318) are provided for a pair of telephone lines and effective for suppressing noise and transient voltage signals occurring above data and voice signals carried on the telephone lines. The circuits provide common mode signal rejection and avoid current flow between the telephone lines responsive to common mode voltage signals on the lines so that audible hum is avoided. The circuits include first voltage clamping circuitry (11 or 311) for clamping voltage signals on the lines at a first predetermined voltage potential and second voltage clamping and filter circuitry (22 or 50 or 52 or 54 or 56 or 58 or 60 or 62 or 318) for clamping voltage signals on the lines at a second predetermined voltage potential and for filtering noise and transient signals from the voltage signals only when the voltage signals exceed the second predetermined voltage potential. The second voltage clamping circuitry provides common mode signal rejection and substantially avoids current flow between the lines responsive to common mode voltage signals on the lines.