Tungsten lamp circuit for current limiting and fault indication

Abstract

A current limiting and fault indicating circuit for connection between a load and a source of electrical current including a pair of tungsten lamp filaments connected in parallel. The lamp filaments are chosen so that the current drawn by the load during the normal operation of the load produces only a small voltage drop. In the event of an overload or a short circuit, however, the increased current flow through the lamp filaments will heat the filaments to incandescence and the non-linear increase in impedence will absorb the source voltage and indicate the circuit to malfunction by the emission of bright light. The current flowing as a result of the fault will be limited to the rated current of the lamps. Since the rated current of the lamps may be high relative to the normal load current to minimize the voltage drop across the lamps under normal conditions, further limiting the flow of current through the circuit in the event of a short circuit may be achieved through the use of two lamp filaments of different characteristics, i.e., a high wattage-low voltage filament and a low wattage-high voltage filament. The high wattage-low voltage filament can be automatically switched out of the circuit in the event of a fault to force all of the fault current through the low wattage filament. The fault current can thus be limited to the rated current of the low wattage filament. Even this current will be sufficient to raise the filament to incandescence indicating the existence of the fault. The two lamp filaments may be enclosed within a single glass envelope which may be filled with an inert gas at atmospheric pressure to effect cooling of the filaments under normal current conditions. A resettable remote status indicator lamp may be included in the circuit to indicate the existence of an intermittent short circuit.