Telephonic communications – Subscriber line or transmission line interface – Protective circuit
Patent
1988-10-19
1990-08-07
Ng, Jin F.
Telephonic communications
Subscriber line or transmission line interface
Protective circuit
379387, H04M 174
Patent
active
049474260
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to an electronic protection circuit for a telephone subset.
BACKGROUND ART
Subsets are now utilizing integrated circuits which are not as robust in regard to line surges as many conventional components, though surge protection has been applied to the conventional subsets. Because of the layout of the various components of a subset and the possibility of transverse or longitudinal faults it has been found that two or three protection devices have been required to protect the various subset blocks, e.g. the ringing circuit and the rectifier bridge and the rest of the subset circuitry. In subsets which have an earth line, e.g. phones connected to PMBX's or some PABX's or in some Telecom administrations where it is necessary to make an earth contact to seize a line, there is also a requirement to have protection on the earth line. Some administrations require all subsets to be adapted to be used with either earth recall or timed loop break.
Surges which may occur include transverse lightning surges of 2 kv between the wires of the subscriber line, or longitudinal surges of 5 kv between the line and the earth. Other faults, include mains cross, spikes generated by signalling and induced currents.
Normally protection devices are provided between L1 and E, L2 and E and between L1 and L2.
DISCLOSURE OF THE INVENTION
This specification discloses the use of a single protection device connected by diodes to steer the surges from the various line configurations through the protective device. The diodes need to have a rapid response time and a high surge rating. An acceptable rating has been found to be 30 A for 8 ms. Diodes capable of this are cheap and readily available, offering considerable saving over the cost of the protective devices.
This specification discloses a protection circuit for a telephone having an earth wire and a subscriber line comprising first and second wires, wherein there is a polarity guard, a pair of input terminals connected to the subscriber line and a pair of output terminals connected to the telephone transmission and reception circuit, wherein, in parallel with the telephone circuit there is a protection device in series with a first diode, the first diode being forward biased with respect to the polarity of the polarity guard output, the earth wire is connected to the junction of the protective device and the first diode by a second diode connected to pass surges of a first polarity between the earth wire and the output of the polarity guard the second diode and the protection device being shunted by a third diode, connected to pass surges between the earth wire and the polarity guard output of the opposite polarity from those surges passed by the second diode, the latter surge path including the first diode.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described with reference to the drawing, in which:
FIG. 1 shows the layout of a circuit embodying the protection systems.
BEST MODE OF CARRYING OUT THE INVENTION
In FIG. 1 the subscriber line wires connect to terminals L1 and L2 which feed into a polarity guard comprising bridge rectifier D1, D2, D3, D4. The output of the bridge is connected to the subset circuit 1 in parallel with protective device 2 which is in series with diode D5. The earth line E is connected to the protective device 2 by diode D6 and, for surges of the opposite polarity by diode D7.
The operation of this protection circuit is described below.
For a transverse surge between L1 and L2 the bridge ensures that the surge is of the correct polarity to pass through the protective device. The path of the surge is via D5 which is poled to be forward biased with reference to the bridge output. The protective device 2 may for instance be a metal oxide varistor (MOV), one or more power zener diodes, or thyristor clamps, which appear as an open circuit at normal operating voltages and exhibit a breakdown characteristic at a specified overvoltage.
For a surge between L1 and E the path may be E, D6, 2, D2, L1. If the p
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patent: 2960328 (1960-11-01), Tate
patent: 4079211 (1978-03-01), Janssen
patent: 4297875 (1981-11-01), Kuhnle et al.
patent: 4408248 (1983-10-01), Bulley et al.
patent: 4794640 (1988-12-01), Yeh
patent: 4809322 (1989-02-01), Riesmeyer
patent: 4815126 (1989-03-01), Goode et al.
Ng Jin F.
Standard Telephone and Cables Pty, Ltd.
Vaas Randall S.
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