GB1350015A - Battery chargers - Google Patents

Abstract

1350015 Substation equipment STANDARD TELEPHONES & CABLES Ltd 12 Jan 1972 1443/72 Heading H4K A battery B in a telephone substation instrument is charged from the exchange over the telephone line only when the instrument is "on-hook" through circuitry which includes a full wave rectifier RB and a transistor VT3 which is made conductive by a time delay circuit that is enabled when the instrument goes "on-hook". When the instrument goes "on-hook" the exchange battery is connected to terminals L1, L2 and capacitor C2 is charged through transistor VT1. As capacitor C2 charges the base voltage of transistor VT2 rises until the Darlington triplet connected transistors VT2, VT2a, VT3 conduct to charge battery B. Resistor R3 in the emitter circuit of VT3 causes transistor VT4 to conduct if the voltage across it rises and then limits the base current of transistor VT2 to give constant current charging of the battery. When the instrument goes "off-hook", the potential across R4 drops and transistors VT6 and VT5 conduct to discharge capacitor C2. When ringing is applied the Zener diode DZ1 conducts to reduce the current in the Darlington triplet transistors and the capacitor C2 is discharged by transistors VT5 and VT6.