Power circuit for telephone accessory devices

Claim 1

In a power circuit for supplying power to a device connected through a telephone line to a central power source which normally supplies power to said device, the improvement wherein said power circuit comprises batteries and switching means for detecting a voltage drop in said central power source and thereupon automatically causing said batteries to serve as a power source for said device, said switching means comprising a diode bridge having input terminals connected to said telephone line and output terminals connected to said device, a first diode connected between said device and said diode bridge, a first transistor, the collector and the emitter thereof being connected respectively to said device and to said batteries, a second transistor having the base and emitter therof so connected to said diode bridge through a voltage divider as to be switched off by a voltage drop in said central power source, a third transistor, the base thereof being connected to the collector of said second transistor and the emitter thereof being grounded, a second diode connected between said device and the collector of said first transistor, said first and second diodes being connected head-to-head relationship with respect to each other, a first resistor connecting the emitter and the base of said first transistor, said voltage divider being formed as a series connection of a second resistor and a third resistor at a junction, the base of said second transistor being connected to said junction. a fourth resistor connected between the base of said third transistor and said device, and d first a fifth resistor connected between the base of said transistor and the collector of said third transistor.