Railway signal lamp safety cct. - has dual-filament lamp switched without contacts from failed to standby filament

摘要

The signal light safety circuit, for railway systems, has the dual-filament signalling lamp switched over without make/break contacts from its failed, i.e. burnt out, main filament to its second, standby filament. The standby filament is bridged by a transformer secondary winding. The transformer primary winding is connected between the first end of the secondary and the main filament. The A.C. supply is passed over two fuses and a variable resistor to the secondary winding second end, i.e. the end connected to the common filament terminal, and to the primary/main filament junction. When the main filament fails, the increased current through the primary causes a sufficiently large voltage in the secondary to light the standby filament.