Monitoring device for airfield lighting system

An electrical circuit for monitoring the operation of a plurality of lights, comprising:

• power source means for supplying constant current AC power;
• a plurality of isolation transformer means, each of said transformers plurality having serially interconnected primary circuitry, said interconnected primary circuitry coupled to said power source means forming a circuit loop, each of said transformer plurality having secondary circuitry electrically coupled to a corresponding one of the plurality of lights;
• monitoring current transformer means having a magnetic core with a primary circuit serially coupled to said circuit loop such that said core magnetically saturates in response to current flowing through said circuit loop, said monitoring current transformer means having a secondary circuit in a substantially open circuit condition for generating a monitoring signal in response to each inversion of current in said circuit loop;
• enabling circuit means coupled to receive said monitoring signal for providing an output window signal having a duration corresponding to the time said monitoring signal amplitude is within a predetermined range;
• means coupled to said circuit loop for providing signals indicative of the voltage across said serially interconnected isolation transformer means primary circuitry;
• integrating circuit means receiving said voltage indicating signals and said output window signals, for providing an integrated voltage signal during the presence of said output window signal;
• and
• subtracting circuit means coupled to receive said integrated voltage signal and a reference voltage signal indicative of current loop voltage when all of the plurality of lights are operative, for providing a difference signal indicative the number of defective lights in the loop.