GB1399097A - Flashing lamp circuits - Google Patents

Abstract

1399097 Lamp failure alarms CAV Ltd 11 July 1972 [17 July 1971] 33692/71 Heading G4N [Also in Division H3] In a lamp flasher circuit of the kind in which the lamp 15 is switched on by a relay 12 energized when a capacitor C becomes charged to a predetermined voltage, means are provided for sensing the current through the lamp and maintaining the relay energized in the event of failure of the lamp. Closure of switch 16 illuminates warning lamp 17, and allows capacitor C to charge through resistor R4, Fig. 1, until the voltage across it is sufficient to turn on T1 and energize relay 12. Contacts 14 then close to extinguish lamp 17, and illuminate lamps 15. Capacitor C now discharges until T1 turns off and the cycle recommences cuasing warning lamp 17 and lamps 15 to flash alternately. Under normal conditions, closure of contacts 14 will turn off transistor T2, but if one or more of lamps 16 fail, the change in current through resistor R5 changes the bias conditions of transistor T2 such that it does not turn off. T2 nows provides a hold current for relay 12 and warning lamp 17 is extinguished. In a modification, Fig. 2, the transistor T1 is replaced by a Darlington pair T3 which is protected against break down by Zener ZD 1 and against reverse voltages by diode D4. Transistor T4 maintains lamp 17 extinguished during the time the contacts 14 are closed, its base voltage being compensated by Zener ZD2 against variation of lamp current with supply voltage. D6 compensates the base-emitter voltage of T4 for temperature variations, and D5, D7 protect if for reverse voltages.