Dual filament current limiting and status indicating circuit

Abstract

A status indicating and current limiting circuit includes a pair of lamp filaments connected in parallel to provide a high reliability for status indication of a monitored switch. The circuit indicates the status of the lamps as well as the open or closed status of the switch. In one embodiment, the pair of lamp filaments are serially connected in parallel between a source of current and the switch so that both the lamp filaments emit bright light when the switch is closed. The lamps are also connected to ground potential through a resistive element to maintain current flow through the lamp filaments when the switch is open, the current being at a level just below the current flow necessary to dimly light the filaments. In the event of a failure of one of the filaments, the other filament will begin to emit a dim but visible light even though the monitored switch is open indicating the failure of the one lamp filament. The lamp filaments may thus limit the current that will flow through the switch when the switch is closed, and indicate the status thereof since the lamp filaments are in a preheated condition as a result of the current flow through the resistive element, the resistance of the filaments to mechanical shock and vibration are improved, the surge current shock upon closure of the switch is reduced, and the switch contacts are relieved of high surge current. The longevity of the lamp filaments and the switch contacts is thus enhanced.