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The current transformer used in a conduction detector switch circuit for lighting up an light emitting diode (LED) for detection of Load current in distribution lines has to be small and of Low excitation current. Since field- annealed Fe-based amorphous cores have high saturation magnetization and the squareness ratio of a hysteresis loop and low coercive force, current transformers that are superior to ferrite cores can be designed. Furthers more, in cases where reliability is required over long periods such as in LED lighting circuits, for the purpose of design, it is necessary to predict what influence is exerted on the circuit characteristics by the aging variations of the amorphous magnetic characteristics [1, 2]. Therefore, recently, based on a prediction method of dc coercive force researched by the present authors [3], aging variation prediction was carried out on ac (60 Hz) coercive force. Next, the ac magnetic characteristics of Fe-based amorphous cores were approximated by Potter's model [4], LED circuit analysis was carried out by application in a Spice program, and good agreement with experimental results was confirmed. hi addition, using the prediction values of aging variation of the cores, on deducing the effect of influence on circuit characteristics it was found that the degradation of LED circuit characteristics at 100°C and 106 hours of elapse was minimal, and it is possible to predict the maintenance of excellent characteristics better than in the case of ferrite cores. [ABSTRACT FROM AUTHOR]