Intrinsically safe telephone systems

Details Intrinsically safe telephone systems Intrinsically safe telephone systems

1976-01-15

1977-12-20

Claffy, Kathleen H.

179 2BC, 79 81R, H04M 118

Patent

active

040643662

ABSTRACT:
An intrinsically safe telephone system for use in mines or similar hazardous areas including a battery charging circuit arranged to supply charging current of a predetermined low value to the battery of an operational unit located in the hazardous area during on-hook periods. A circuit is provided for modulating the charging current below the predetermined value to generate both the incoming and outgoing call signals during signaling periods.

REFERENCES:
patent: 3909547 (1975-09-01), Day

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