Telephone equipment protection circuit

Details Telephone equipment protection circuit Telephone equipment protection circuit

1998-06-16

2000-12-19

Leja, Ronald W.

Electricity: electrical systems and devices

Safety and protection of systems and devices

High voltage dissipation

H02H 900

Patent

active

061634476

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to a circuit for protecting front end circuitry in telephone equipment from damage due to damaging signals, such as short circuits or lightning strikes, on the telephone line to which they are attached.
Present electronic equipment may include telephone equipment, such as modem circuitry, to access outside computers via the subscriber's telephone line. Such equipment is relatively expensive. However, a subscriber telephone line is subject to various signal conditions which can either damage telephone equipment attached to it, or seriously degrade the performance of that equipment. For example, electromagnetic interference (EMI) may be picked up by the telephone lines acting as antennae; or, if the telephone lines at the subscriber location are not grounded properly, a high voltage static charge may build up on those lines which will discharge when circuitry is connected to the telephone lines; or various other circuitry also coupled to the telephone line may malfunction and generate a signal having too high a voltage or a short circuit; or lightning may strike the telephone lines. In all of these cases, the telephone lines pick up a signal which, if transmitted to the telephone equipment connected to them, will damage that circuitry or degrade its performance.
In order to ensure protection for both telephone equipment and telephone lines against damage, various tests have been mandated in order to ensure that telephone equipment will continue to operate in the presence of such damaging signals. In the United States, for example, Underwriter's Laboratory has specified a series of test set forth in UL specification 1459, and the Federal Communications Commission has specified other tests in the FCC telephone equipment regulations, part 68. All equipment which is to connect to telephone lines must pass the tests set forth in these documents before they may be sold in the United States.
The tests set forth in these documents a re designed to protect telephone equipment from the various damaging signals which can occur on telephone lines. Furthermore, manufacturers, in order to minimize repair and warrantee repair expenses, and maximize consumer satisfaction with their equipment, desire to minimize damage to their equipment caused by such damaging signals. Furthermore, manufacturers include circuitry which will prevent signals which can degrade the operation of their telephone equipment from reaching that equipment.
Various protection circuits have been developed to protect telephone equipment from damaging or operation-degrading signals. This protection circuitry is coupled between a subscriber's telephone lines and the telephone equipment, and is designed to pass the required tests, as described above, and to protect the telephone equipment from damage or degraded operation. For example, in prior art protection circuits, inductors are coupled directly to the subscriber telephone lines. The inductors minimize or eliminate the EMI from entering the telephone equipment. In addition, the intent is that inductors prevent the damaging signals from passing through them. However, it has been found that inductors will not completely prevent high voltage spikes from passing through them and into the telephone equipment.
In order to prevent such high voltage spikes from passing through to the telephone equipment, some protection circuits further include a gasfilled spark gap between the inductors and the telephone equipment to transfer the energy in the high voltage spikes to ground, and thus prevent it from reaching the telephone equipment. However, once a high voltage spike has reached the gasfilled spark gap, some portion of that spike is able to adversely affect the telephone equipment.
It is desired to provide protection from both high voltage spikes and from EMI, and other damaging or operation-degrading signals. Preferably, protection circuitry should be provided which, though it may be destroyed when a damaging signal occurs, prevents the more expensive telephone equipment which it

REFERENCES:
patent: 4254442 (1981-03-01), Dijkmans et al.
patent: 4586104 (1986-04-01), Standler
patent: 5392349 (1995-02-01), Elder, Jr.
patent: 5493469 (1996-02-01), Lace
patent: 5621602 (1997-04-01), Winkelmann
WESCON '95 Conference Record. Microelectronics, Communications Technology, Producing Quality Products, Mobile and Portable Power, Emerging Technologies, San Francisco, Nov. 7-9, 1995, Institute of Electrical and Electronics Engineers, M. Williams et al. "Circuit Protection For Telephone Networks--Needs and Co-ordination", pp. 189-194.

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