| 摘要 |
<p>A protection circuit is provided for a telecommunications device which has first and second conductors (11, 13) for transmitting and receiving signals to and from the communications device, signal circuitry for processing the signals and switch means (28) for selectively connecting the conductors to the signal circuitry. The circuit comprises a protection circuit means (112, 116a, 116b) connectable between the first and second conductors (11, 13) and comprising first and second switch means (116a, 116b, 118a, 118b, 116b; 120, 126, 122, 124; 132, 134; 140, 138) having first and second voltage threshold values. Current monitoring means (R) are provided for monitoring the current through one of the conductors (11, 13). The first and second switch means (116a, 116b; 118a, 118b, 116b; 120, 126, 122, 124; 132, 134; 140, 138) area arranged to switch form a first, non-conducting state to a second, conducting state in response to the voltage across the conductors (11, 13) exceeding the first and second combined threshold voltage values, thereby to provide overvoltage protection at a first voltage magnitude. The monitoring means (R) is also operable to switch the first switch means (116a; 118a, 118b; 120, 122; 132; 138) from a non-conducting to a conducting state in response to current through the one of the conductors (11, 13) exceeding a preselected threshold value, thereby to enable the second switch means (116b; 118a, 118b; 120, 122; 134; 140) to provide overvoltage protection at a second voltage magnitude less than the first voltage magnitude during operation of the telecommunications device.</p> |
