Foldback current limiting of a telephone line switch

Details Foldback current limiting of a telephone line switch Foldback current limiting of a telephone line switch

1994-03-28

1997-08-26

Gaffin, Jeffrey A.

Electricity: electrical systems and devices

Safety and protection of systems and devices

With specific quantity comparison means

361119, 361101, H02H 318

Patent

active

056616244

ABSTRACT:
A telephone having a semiconductor line switch TR1 is protected from excess line voltage or current by the use of current foldback techniques. A current sensing resistor R6, in series with the line switch provides a measure of the line current and this operates a bypass transistor TR4 which reduces the drive current for the line switch TR1 causing it to come out of saturation and begin to block line current. A voltage sensing resistor R7 provides a measure of the voltage across TR1 and this further increases the bypass current through TR4. This ensures that the voltage and current applied to TR1 remain within permitted limits.

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Patent Abstracts of Japan, vol. 12, No. 127 (E-602) Apr. 20, 1988 & JP-A-62 253 252 (NEC Corp) Nov. 5, 1987.
Patent Abstracts of Japan, vol. 12, No. 127 (E-602), Apr. 20, 1988 and Japanese Patent Application JP-A-62 253 252 (NEC Corp). Nov. 5, 1987.
"Adding foldback resistor provides overload safety" by W.J. Riley, Electronics Designer's Casebook, vol. 14D, approx. 1973, McGraw-Hill Inc., New York, NY, p. 20.

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