GB1045629A - Circuit arrangement for supervising a plurality of signal circuits with respect to interruptions - Google Patents

Abstract

1,045,629. Transistor switching circuits. STANDARD TELEPHONES & CABLES Ltd. March 12, 1964 [March 14, 1963], No. 10468/64. Heading H3T. [Also in Division G4] An alarm circuit for monitoring a number of signalling devices, e.g. traffic lights H, connected to a common point E comprises a relay B which is normally held energized by the current following through its winding 1, 2 and the emitter collector path of transistors T1, T2. An A.C. supply is connected between P and N and during the half cycle when P is negative relay B is held in by the decay current flowing through diode G1. If a lamp H fails during the time that its associated switch s is closed, the potential at E tends towards that of P during the half cycle when P is positive, and the transistors are cut off releasing relay B to complete an alarm circuit. Any tendency for residual current to hold the relay in is counteracted by a small opposing winding 3, 4. Excessive voltage drop across the transistors when blocked is prevented by Zener diode G2. A further Zener diode prevents damage to the transistors should one of the diodes G1 or G3 fail. In the case of the latter failing a fuse Si is also tripped. If, however, one of the transistors should go short-circuit, the system will still function with the remaining one. A switch T p is provided for testing the system. Transistor T2 and winding 3, 4 may be omitted, Fig. 1 (not shown).