Telephone ancillary equipment supply limiter - is fitted between line and any subsidiary equipment and monitors derived line voltage and regulates supply

摘要

An incoming telephone line is connected to a series of circuits which is connected between the line and a telephone set and any subsidiary equipments. The circuits consist of an over voltage protection circuit, a reverse polarity protection circuit, a voltage regulator and a current limiter. In the current limiter, a current measuring circuit (23) controls a MOSFET emitter folower (12) which is connected in parallel with a current limiter circuit (22). When the voltage is high the emitter follower is prevented from conducting and the subsidiary equipment is fed via the limiter. When the voltage is low the emitter follower conducts thus bypassing the limiter. A thermister is included in the limiter circuit to makde it independent of ambient temperature. ADVANTAGE - Simple, temperature independent.