PURPOSE: To constitute a feeding circuit with use of a miniature transistor, by protecting the transistor which feeds a DC power supply when a telephone circuit is short-circuited.
CONSTITUTION: First the base currents of transistors TR103 and 108 flow to turn on these TRs. Then a DC current is supplied to a telephone 1. While a transformer 5 transmits an AC signal. In this case, circuits 102 and 107 have no AC loss for their feeding circuits since they have high impedance in terms of AC. If a telephone circuit is short-circuited, the voltage generated across a resistance 109 increases. Then a signal of H level is transmitted from a comparator 12. This signal turns on a thyristor 113 and turns off the TR108 respectively. As a result, the current flowed out of a plus power supply is supplied to a minus power supply through the TR103, the telephone 1, a resistance 110 and the thyristor 113. Therefore the TR108 is turned off, and the resistance 110 of high resistance value is connected in series to the TR103. Thus both TRs can be protected.
SUGIYAMA FUMIO
TAKIGUCHI MASAHISA
