Telephone protection circuit
AssigneeStandard Telephone and Cables Pty, Ltd.
InventorFrederick D. Montano
Life of this patent
Estimating the term…
Abstract
A protection device is provided for a telephone having a two wire subscriber line and an earth line. The protection device comprises a single surge protection device connected between a bridge rectifier polarity guard and the remainder of the telephone circuitry, which is connected to the output of the polarity guard. Three diodes are used to interconnect the earth line and the pair of output lines from the polarity guard to the surge protection device in such a way as to prevent surges from being fed to the remainder of the telephone circuitry. The surge paths including one or two arms of the polarity guard.
Claims8
A protection circuit for telephone equipment including a transmission circuit connected to a telephone line having an earth wire and a subscriber line comprising first and second wires, the protection circuit comprising
- a polarity guard having a pair of input terminals connected to the subscriber line and a pair of output terminals connected to the transmission circuit,
- a first diode forward biased with respect to a first said output terminal,
- a protection device in parallel with the transmission circuit and in series with the first diode,
- a second diode connecting the earth wire to a first terminal of the protection device adjacent the first diode to pass, via a first surge path which includes the protection device but which does not include the first diode, surges of a first polarity between the earth wire and the polarity guard, and
- a third diode connecting the earth wire to a second terminal of the protection device remote from the first diode to pass, via a second surge path between the earth wire and the polarity guard which includes both the protection device and the first diode, surges of the opposite polarity from those surges passed by the second diode.
A telephone subset comprising
- a protection circuit for equipment connected to a telephone line having an earth wire and a subscriber line comprising first and second wires, the protection set further comprising
- a polarity guard having a pair of input terminals connected to the subscriber line and a pair of output terminals connected to the transmission circuit,
- a first diode forward biased with respect to a first said output terminal,
- a protection device in parallel with the transmission circuit and in series with the first diode,
- a second diode connecting the earth wire to a first terminal of the protection device adjacent the first diode to pass, via a first surge path which includes the protection device but which does not include the first diode, surges of a first polarity between the earth wire and the polarity guard, and
- a third diode connecting the earth wire to a second terminal of the protection device remote from the first diode to pass, via a second surge path between the earth wire and the polarity guard which includes both the protection device and the first diode, surges of the opposite polarity from those surges passed by the second diode, and
- an earth recall switch in parallel with the third diode.
Dependencies are inferred from the claim text.