Electricity: electrical systems and devices – Safety and protection of systems and devices – Voltage regulator protective circuits
Patent
1991-07-09
1993-05-04
DeBoer, Todd E.
Electricity: electrical systems and devices
Safety and protection of systems and devices
Voltage regulator protective circuits
361111, H02H 904
Patent
active
052087182
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to a protective circuit arrangement for protecting circuit elements connected to a line against the damaging effects of transient abnormal high voltage surges on the line caused by, for example, lightning or fluctuations in adjacent power transmission lines. The circuit arrangement of the present invention is particularly, though not exclusively, suitable for protecting line associated elements of a telephone subset connected to an exchange line via a solid state line switch, the line switch providing a hook-switch function and loop-current interrupt functions.
Protection circuits for telephone subsets of the aforementioned kind are known and comprises a voltage limiting circuit and a current limiting circuit. The voltage limiting circuit comprises a voltage dependent nonlinear resistor connected across the subset's line terminals to substantially absorb transient high voltage surges on the line, thus protecting circuit elements in the subset that are associated with the line terminal end of the subset's circuit. In addition to the voltage limiting circuit, a current limiting circuit is also provided. The current limiting circuit comprises an arrangement of a resistor in series with and on the line side of the conductive path of the line switch, and a diode arrangement connected between the line end of the resistor and the line switch's control element. The value of the resistor is chosen so that when a current threshold is exceeded, the voltage drop across the resistor exceeds the forward drop across the diode arrangement thus limiting the current through the conductive path of the line switch.
A disadvantage of the known protection circuit is that a proportion of the current produced by the transient voltage surge passes through the line switch and therefore the line switch's power rating must be sufficient to accommodate this transient current and the power rating chosen accordingly. This is reflected in the cost of the transistors used in the line switch.
It is therefore an object of the present invention to provide an improved protection circuit for protecting electrical equipment connected to a line via a solid state switch from the damaging effects of the transient abnormal voltage surges on the line, and thereby permitting the power rating of the transistors used in the line switch to be lowered.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention there is provided a protection circuit for protecting circuit elements connected to a line via line terminal means from the effects of a transient abnormal high voltage surge, said circuit comprising a controllable semiconductor switch having a conductive path is serially connected between said line terminal means and said circuit elements for selectively connecting said circuit elements to said line, said protection circuit comprising a first circuit comprising a first resistance means of a predetermined value connected in series between one terminal of the line terminal means and the conductive path of the said semiconductor switch, and a first voltage reference means connected in parallel with said first resistance means and the junction of the said semiconductor's conductive path its control element, and a second circuit comprising the first voltage reference means in series with a second voltage reference means, said second circuit being connected in parallel with said first resistance means, said second circuit further including a voltage dependent second resistance means having a predetermined threshold value connected between the junction of the first and resistance means and the conductive path of the said semiconductor switch and another terminal of said line terminal means, the resistance value of said first resistance means being chosen so that with a normal magnitude of line current the sum of the voltage drops across the first resistance means and the said junction of the semiconductor's conductive path and its control element is less than the reference voltage of the first
REFERENCES:
patent: 3582713 (1971-06-01), Till
patent: 4530023 (1985-07-01), Brown
patent: 4531173 (1985-07-01), Yamada
ALCATEL N.V.
Deboer Todd E.
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