Disconnection detecting circuit of double-filament spherical lamp

Claim 1

In combination with a double-filament lamp having a first and a second filament connected to a power source by respective circuits which include respective first and second control switches and an indicator device for indicating failure of either filament, an apparatus for detecting the failure of a filament in the lamp and activating said indicator, said apparatus comprising: a first voltage-dividing circuit connected to said first filament circuit and said indicator which includes said second filament as a voltage-dividing resistor in said first voltage-dividing circuit; a second voltage-dividing circuit connected to said second filament circuit and said indicator which includes said first filament as a voltage-dividing resistor in said second voltage-dividing circuit; a first comparator connected to said voltage-dividing circuit and said indicator; a second comparator connected to said second voltage-dividing circuit and said indicator; a third voltage-dividing circuit connected to said first filament circuit and said first comparator for setting a reference voltage; and a fourth voltage-dividing circuit connected to said second filament circuit and said second comparator for setting a reference voltage; whereby with both filaments intact the heat from the energization of either filament increases the resistance of the remaining filament to increase the voltage potential sensed by the comparator in the circuit of the energized filament to prevent activation of said indicator, and when an energized filament has failed the resistance of the remaining filament is not increased and the voltage potential sensed by the comparator in the circuit of the energized filament remains low to cause said comparator to activate said indicator to signal a failed filament, said respective third and fourth voltage-dividing circuits provide a voltage drop to prevent the non-energized filament from being energized should the energized filament fail.