WO2002021658A1

WO2002021658A1 - Method and apparatus for protecting multiple telephone lines

Info

Publication number: WO2002021658A1
Application number: PCT/US2001/026602
Authority: WO
Inventors: David L. Rawlings
Original assignee: Porta Systems Corporation
Priority date: 2000-09-01
Filing date: 2001-08-24
Publication date: 2002-03-14
Also published as: AU2001285289A1

Abstract

A method and apparatus for protecting multiple telephone lines in a cost-effective manner. The apparatus includes a plurality of diode bridges (10) and a zener diode (12) or thyristor. The plurality of diode bridges is electrically coupled in parallel with each other, and the zener diode is electrically coupled in parallel across and shared by the plurality of diode bridges.

Description

METHOD AM) APPARATUS FOR PROTECTING MULTIPLE TELEPHONE LINES

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates generally to telephone equipment, and more particularly to protecting such equipment from excessive voltages and/or currents that may occur on telephone lines during normal operation.

Description of the Prior Art

It is known to provide one or more protective devices connected to a communication line, such as a balanced, two-wire telephone subscriber line, to protect equipment &om excessive voltages and currents that may occur on the line. These conditions are typically caused by lightning strikes, power line crosses, and currents induced from adjacent power lines.

Protection from surges due to lightning is commonly provided by surge arresters, while protection from power line crosses and induced currents is typically afforded by so-called "heat coils". Heat coils include two resistances, each preferably of about 4 ohms. These resistances are connected in series with the telephone lines and resistively heat in response to sustained excessive currents. The heat thus generated activates a thermally-sensitive shorting mechanism, which shunts the current to ground.

These protective devices introduce various well-known disadvantages and problems. For example, heat coils are not resettable, so that a triggered heat coil must be identified and replaced after it has effected a protective operation. In addition, since heat coils are thermal devices, they are typically slow to operate. Further, heat coils introduce an undesirable resistance into the line. Nevertheless, such devices continue to be used extensively for protective purposes in view of their low cost, relatively small size, and the lack of an economically viable alternative. Various solid state devices have been proposed for protective purposes. For instance, semiconductor transient voltage protectors have been proposed for replacing carbon block and gas tube surge arresters. In addition, a solid state transient overvoltage suppressor and overcurrent arrestor, which includes a gate or trigger electrode, operates in a similar manner to a silicon controlled rectifier (SCR). Such a device becomes conductive either in response to an excessive voltage between its anode and cathode or a voltage drop caused by an excessive current flowing through a series resistor connected between its cathode and gate.

The SCR automatically resets itself (i.e., becomes non-conductive) upon removal of the excessive voltage or current. However, the need for the series resistor remains. In addition, although such a protective arrangement is responsive to absolute current it can be insensitive to common mode currents (longitudinal currents flowing in the same direction on both wires of a two-wire line) and semi-longitudinal currents (currents flowing on only one wire of a two-wire line) that arise from lightning strikes and power line induction.

Another conventional protective circuit for telephone lines is shown in Figure 1. This circuit includes a diode bridge 10 and a zener diode 12. The zener diode 12 is electrically connected in parallel across two nodes of the diode bridge 10. A node is defined as the electrical connection between two diodes forming the diode bridge 10. The two remaining nodes of the diode bridge 10 are electrically connected in parallel across a telephone circuit 15. Each telephone circuit 15 requires a separate protective circuit.

However, one of the primary disadvantages of the protective circuit shown in Figure 1 is that a dedicated zener diode 12 is required for each telephone circuit. Since the zener diodes 12 are typically able to withstand both high power and high voltage, their cost becomes a significant disadvantage in utilizing an otherwise efficient and straightforward method of protecting telephone circuits from excessive voltages and/or currents. OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method and apparatus, which significantly reduces the cost, size, and failure rate of protection circuits for telephone equipment.

It is a further object of the present invention to provide a method and apparatus, which protects telephone equipment from excessive voltages and/or currents normally occurring on telephone lines, which are caused by lightaing strikes, power line crosses, and currents induced by adjacent power lines.

It is still a further object of the present invention to provide a method and apparatus, which substantially increases the reliability of telephone equipment.

In accordance with the present invention, an apparatus for protecting multiple telephone lines includes a plurality of diode bridges, and a zener diode or a thyristor. The diode bridges are electrically coupled in parallel with each other, and the zener diode or thyristor is electrically coupled in parallel across the plurality of diode bridges.

In further accordance with the present invention, a method of protecting multiple telephone lines includes the steps of coupling a plurality of diodes to form a plurality of diode bridges, and coupling the plurality of diode bridges electrically in parallel across a zener diode. A thyristor may be substituted for the zener diode.

These and other objects, features, and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic diagram showing a conventional protection circuit applied to three independent telephone circuits.

Figure 2 is a schematic diagram showing a protection circuit, which is formed in accordance with the present invention, applied to three independent telephone circuits. Figure 3 is a schematic diagram showing a second embodiment of the protection circuit formed in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following is provided to briefly describe the electrical connectivity of the protection circuit shown in Figure 1. The protection circuit preferably includes four diodes 14, 16, 18, 20, and a zener diode 12. The cathode of diode 14 is preferably coupled to the cathode of diode 16 at node A, and the anode of diode 16 is preferably coupled to the cathode of diode 18 at node B of the diode bridge 10. The anode of diode 18 is preferably coupled to the anode of diode 20 at node C, and the cathode of diode 20 is preferably coupled to the anode of diode 14 at node D of the diode bridge 10.

The cathode of the zener diode 12 is preferably coupled to node A, and the anode of the zener diode 12 is preferably coupled to node C. In addition, the protection circuit preferably includes diodes 22 and 24. The cathode of diode 22 is preferably coupled to the cathode of the zener diode 12 at node A, and the anode of diode 24 is preferably coupled to the anode of the zener diode 12 at node C. Both the anode of diode 22 and the cathode of diode 24 are preferably coupled to ground.

The operation of the protection circuit will now be described under various overvoltage conditions. If an excessive positive voltage appears from the tip signal 26 (positive) to the ring signal 28 (negative), then diodes 14, 12, and 18 are biased on, a d the current through these diodes reduces the voltage between the tip and ring signals. If an excessive positive voltage appears from the tip signal 26 (positive) to ground, then diodes 14, 12, and 24 are biased on, and the current through these diodes reduces the voltage between the tip signal and ground. If an excessive positive voltage appears from the ring signal 28 (positive) to ground, then diodes 16, 12, and 24 are biased on, and the current through these diodes reduces the voltage between the ring signal and ground.

Similarly, if an excessive negative voltage appears from the tip signal 26 (negative) to the ring signal 28 (positive), then diodes 16, 12, and 20 are biased on, and the current through these diodes reduces the voltage between the tip and ring signals. If an excessive negative voltage appears from the tip signal 26 (negative) to ground, then diodes 22, 12, and 2-0 are biased on, and the current through these diodes reduces the voltage between the tip signal and ground. If an excessive negative voltage appears from the ring signal 28 (negative) to ground, then diodes 22, 12, and 18 are biased on, and the current through these diodes reduces the voltage between the ring signal and ground.

It is anticipated that the zener diode 12 can readily be replaced by a semiconductor device 58 in the so-called generic "thyristor family", such as a reverse blocking diode thyristor or a bidirectional diode thyristor (diac), as shown in Figure 3. The zener diode 12 shown in Figure 2 is preferably described as "clamping" the voltage to a specified value, whereas the thyristor 58 shown in Figure 3 is preferably described as a "crowbar" for the excessive current.

Further details concerning the operation of the diode bridge 10, zener diode 12 and semiconductor device 58 can be found in N. Veley, "Benchtop Electronics Handbook", McGraw-Hill Book Company, pp. 288-294, (1998), and G. Deboo and C. Burrous, "Integrated Circuits and Semiconductor Devices: Theory and Operation", McGraw-Hill Book Company, pp. 365-458, (1977), which are incorporated herein by reference.

As shown in Figure 1, a telephone circuit 15 is preferably connected in parallel across the tip signal 26 and the ring signal 28. The tip signal 26 is coupled to node D of the diode bridge 10, and the ring signal 28 is coupled to node B of the diode bridge 10. The functional elements of the telephone circuit 15 include a carbon transmitter 30, which converts acoustic energy to an electrical voice signal, and an electromagnetic receiver 32, which converts the electrical voice signal back into acoustic energy. The telephone circuit 15 also includes a switch hook 34, which turns the telephone off and on in response to lifting the receiver 32, rotary dial contacts, which make and break loop circuits, a loop-equalizer circuit 38 to compensate for loop resistance, and an electromechanical ringer 40.

The ringer 40 is shown bridged across the tip signal 26 and the ring signal 28. A capacitor 42 blocks the flow of loop current through the ringer 40. A resistor 44 and a varistor 46 form a loop-equalizer circuit 38. On long loops with low loop current, the varistor 46 maintains a high resistance and preferably uses a relatively small amount of current in comparison with the remaining components in the telephone circuit 15. On short loops, higher levels of loop current result in a lower resistance across the varistor 46, which reduces transmit and receive voltage levels.

The combination of a three-winding hybrid transformer 48 and impedance balancing circuitry provide a means for coupling the fransmitter 30 and receiver 32 to the loop independently. This is called an anti-sidetone network. The sidetone is that portion of the transmitted signal that is heard in the receiver while tall ng. The sidetone is subjectively desirable because it provides the live quality of a face-to-face conversation. The anti-sidetone network is designed to provide a sidetone signal at about the same level as the received speech. If the sidetone level is too high, the talker tends to speak softly to keep the sidetone level pleasant, which results in signal strength too low for effective transmission. If the sidetone level is too low, the talker perceives the telephone as dead or inoperative.

Incoming voice signals from the tip and ring signals 26 and 28 are transformer coupled to the receiver 32. The induction voltages are such that most of the incoming signal power is delivered to the receiver 32 with little power to a balance network, which includes a resistor 50, a capacitor 52, and a varistor 54. Outgoing voice signals generated by the transmitter 30 induce voltages in two of the transformer 48 windings that cancel each other, so that most of the signal power is divided between the resistor 50 and loop impedances with little to the receiver 32. The choice of the impedance and turns ratio of the transformer 48 provides a compromise in sidetone balance and impedance matching for the tip and ring signals 26, 28. The capacitor 52 prevents direct current (dc) power from being dissipated in the resistor 50. The varistor 54 helps match the balance circuit impedance to the loop impedance. Further details concerning telecommunications equipment can be found in D. Christiansen, "Electronics Engineers' Handbook", 4^th Ed., IEEE Press, pp. 26.1-26.121, (1997), which is herein incorporated by reference.

A protection circuit 56 formed in accordance with the present invention is shown in Figure 2. The protection circuit 56 includes a plurality of diode bridge circuits 10, which are electrically connected in parallel to each other and across the zener diode 12. Each of the plurality of diode bridge circuits 10 is also connected in parallel across a separate telephone circuit 15. The remaining connectivity between circuit components is substantially similar to that described in connection with Figure 1. It is to be noted that the zener diode 12 as well as diodes 22, 24 are shared by each of the diode bridge circuits 10 and are preferably not replicated to protect each of the telephone circuits 15 individually.

Thus, the protection circuit 56 formed in accordance with the present invention advantageously reduces the number of components required to protect multiple telephone circuits. Further, since the zener diode 12 is expensive, the protection circuit 56 results in a significant cost reduction over the conventional protection circuit shown in Figure 1.

From the foregoing description, it will be appreciated that the method and apparatus in accordance with the present invention protect telephone equipment from excessive voltages and/or currents normally occurring on telephone lines, which are caused by lightning strikes, power line crosses, and currents induced by adjacent power lines. It will also be appreciated that the method and apparatus significantly reduce the cost, size, and failure rate of protection circuits for telephone equipment.

Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention.

Claims

WHAT IS CLAIMED IS:

1. An apparatus for protecting multiple telephone lines, the apparatus comprising:

a plurality of diode bridges, the plurality of diode bridges being electrically coupled in parallel with each other; and

a zener diode, the zener diode being electrically coupled in parallel across each of the plurality of diode bridges.

2. An apparatus for protecting multiple telephone lines as defined by Claim 1, wherein the diode bridge includes a first diode, a second diode, a third diode, and a fourth diode, each of the diodes including an anode and a cathode, the cathode of the first diode being coupled to the cathode of the second diode and defining a first node, the anode of the second diode being coupled to the cathode of the third diode and defining a second node, the anode of the third diode being coupled to the anode of the fourth diode and defining a third node, the cathode of the fourth diode being coupled to the anode of the first diode and definin -Όg a fourth node.

3. An apparatus for protecting multiple telephone lines as defined by Claim 2, wherein the zener diode includes an anode and a cathode, the anode of the zener diode being coupled to the second node, the cathode of the zener diode being coupled to the first node.

4. An apparatus for protecting multiple telephone lines as defined by Claim 2, wherein the each of the plurality of diode bridges are electrically coupled in parallel across a telephone circuit, the telephone circuit including a tip signal and a ring signal, the fourth node being coupled to the tip signal, the second node being coupled to the ring signal.

5. An apparatus for protecting multiple telephone lines as defined by Claim 2, further including a fifth diode and a sixth diode, each of the fifth and sixth diodes including an anode and a cathode, the cathode of the fifth diode being coupled to the cathode of the zener diode, the anode of the fifth diode being coupled to ground, the anode of the sixth diode being coupled to the anode of the zener diode, the cathode of the sixth diode being coupled to ground.

6. A method of protecting multiple telephone lines, the method comprising the steps of:

coupling a plurality of diodes to form a plurality of diode bridges; and

coupling each of the plurality of diode bridges electrically in parallel across a zener diode.

7. A method of protecting multiple telephone lines as defined by Claim 6, wherein the diode bridge includes a first diode, a second diode, a third diode, and a fourth diode, each of the diodes including an anode and a cathode, the method further including the steps of:

coupling the cathode of the first diode to the cathode of the second diode at a first node;

coupling the anode of the second diode to the cathode of the third diode at a second node;

coupling the anode of the third diode to the anode of the fourth diode at a third node; and

coupling the cathode of the fourth diode to the anode of the first diode at a fourth node.

8. A method of protecting multiple telephone lines as defined by Claim 7, wherein the zener diode includes an anode and a cathode, the method further including the steps of:

coupling the cathode of the zener diode to the first node; and

coupling the anode of the zener diode to the third node.

9. A method of protecting multiple telephone lines as defined by Claim 7, the method further including the steps of : coupling each of the plurality of diode bridges electrically in parallel across a telephone circuit, the telephone circuit including a tip signal and a ring signal;

coupling the fourth node to the tip signal; and

coupling the second node to the ring signal.

10. A method of protecting multiple telephone lines as defined by Claim 7, further including a fifth diode and a sixth diode, each of the fifth and sixth diodes including an anode and a cathode, the method further including the steps of:

coupling the cathode of the fifth diode to the cathode of the zener diode;

coupling the anode of the fifth diode to ground;

coupling the andoe of the sixth diode to the anode of the zener diode; and

coupling the cathode of the sixth diode to ground.

11. An apparatus for protecting multiple telephone lines, the apparatus comprising:

a thyristor, the thyristor being electrically coupled in parallel across each of the plurality of diode bridges.

12. An apparatus for protecting multiple telephone lines as defined by Claim 11, wherein the diode bridge includes a first diode, a second diode, a third diode, and a fourth diode, each of the diodes including an anode and a cathode, the cathode of the first diode being coupled to the cathode of the second diode and defining a first node, the anode of the second diode being coupled to the cathode of the third diode and defining a second node, the anode of the third diode being coupled to the anode of the fourth diode and defining a third node, the cathode of the fourth diode being coupled to the anode of the first diode and defining a fourth node.

13. An apparatus for protecting multiple telephone lines as defined by Claim 12, wherein the thyristor includes a first terminal and a second teπninal, the first terminal of the thyristor being coupled to the second node, the second teπninal of the thyristor being coupled to the first node.

14. An apparatus for protecting multiple telephone lines as defined by Claim 12, wherein the each of the plurality of diode bridges are electrically coupled in parallel across a telephone circuit, the telephone circuit including a tip signal and a ring signal, the fourth node being coupled to the tip signal, the second node being coupled to the ring signal.

15. An apparatus for protecting multiple telephone lines as defined by Claim 12, further including a fifth diode and a sixth diode, each of the fifth and sixth diodes including an anode and a cathode, the cathode of the fifth diode being coupled to the second teπninal of the thyristor, the anode of the fifth diode being coupled to ground, the anode of the sixth diode being coupled to the first terminal of the thyristor, the cathode of the sixth diode being coupled to ground.

16. A method of protecting multiple telephone lines, the method comprising the steps of:

coupling a plurality of diodes to form a plurality of diode bridges; and

coupling each of the plurality of diode bridges electrically in parallel across a thyristor.

17. A method of protecting multiple telephone lines as defined by Claim 16, wherein the diode bridge includes a first diode, a second diode, a third diode, and a fourth diode, each of the diodes including an anode and a cathode, the method further including the steps of:

coupling the anode of the second diode to the cathode of the third diode at a second node; coupling the anode of the third diode to the anode of the fourth diode at a 1-bird node; and

18. A method of protecting multiple telephone lines as defined by Claim 17, wherein the thyristor includes a first terminal and a second terminal, the method further including the steps of:

coupling the second terminal of the thyristor to the first node; and

coupling the first terminal of the thyristor to the third node.

19. A method of protecting multiple telephone lines as defined by Claim 17, the method -further including the steps of

coupling each of the plurality of diode bridges electrically in parallel across a telephone circuit, the telephone circuit including a tip signal and a ring signal;

coupling the fourth node to the tip signal; and

coupling the second node to the ring signal.

20. A method of protecting multiple telephone lines as defined by Claim 17, further including a fifth diode and a sixth diode, each of the fifth and sixth diodes including an anode and a cathode, the method further including the steps of:

coupling the cathode of the fifth diode to the second teπninal of the thyristor;

coupling the anode of the fifth diode to ground;

coupling the anode of the sixth diode to the first terrriinal of the thyristor; and

coupling the cathode of the sixth diode to ground.