Current flow indicator

Abstract

An apparatus connectable between a current source and an electrical load for indicating current flow through the load above a threshold level. The indicator includes two negative temperature coefficient (NTC) thermistors, and a neon indicator lamp. The first thermistor is connected directly in series with the electrical load and the second thermistor is connected in series with the neon lamp. The thermistors are packaged together by an epoxy resin, which thermally couples the thermistors so that both thermistors operate at substantially identical temperatures. Current flow above the load's operational threshold level causes the first thermistor to heat to an equilibrium temperature well above the ambient temperature. At this elevated equilibrium temperature, both resistors have extremely low resistance values. The low resistance value of the second thermistor permits maximum current flow to the neon lamp, which illuminates the lamp. As the current flow through the load drops, the self-heating experienced by the first thermistor decreases and the resistance value of the second thermistor increases. When the current flow through the load falls below the load's operational threshold level, the resistance value of the second thermistor is sufficiently high to limit the current flow to the neon lamp and prevent illumination. The failure of the neon lamp to illuminate provides a visual indication that the current flow to load is below the load's operational threshold.

Dependencies are inferred from the claim text.