Patent ID: 8035938

Claim:
An active current surge limiting apparatus for reduction of inrush current to an electrical load in response to detection of a disturbance in an input AC power supply coupled to the electrical load, comprising: a current-limiting circuit coupled between the input power supply and the electrical load, the current-limiting circuit comprising a parallel arrangement of (a) a negative temperature coefficient (NTC) component and (b) a relay having contacts that are in the open position during start-up of power provided to the electrical load and close in response to a control signal to couple the input power supply to the electrical load and bypass the NTC component; a relay activator circuit responsive to the control signal for activating the relay to place the relay into a closed position; and a load modeling circuit coupled to the input AC power supply for simulating characteristics of the electrical load based on the waveform of the AC power supply and for (a) providing the control signal to maintain the relay in the closed position during normal operation of the power supply and (b) terminating the control signal in response to detection of the disturbance in the input power supply to open the relay and couple the input power supply to the electrical load through the NTC component, said load modeling circuit further operative to detect resumption of normal conditions on the input power supply and close the relay between the input power supply and the electrical load, whereby inrush current that would result from removal of the disturbance is resisted by the NTC component when the relay is in the open position and wherein the load modeling circuit comprises a diode bridge, capacitor, and charging resistor having a time constant chosen to be similar to that of the rectifier/capacitor circuit of the electrical load to automatically simulate the operation of the electrical load.